Link 2-MELANIN 84
www.tightrope.it/nicolaus/index.htm
Pigment
cell research is rich in perspiration but poor in inspiration.
Recently the
study of melanins has greatly intensified and has brought new results
especially in the physical field. For this reason it appeared useful to me to
publish a bibliographic adjournment of the book edited in the long ago 1968.
The work, which the Accademia Pontaniana has accepted for its Quaderni, also
wishes to illustrate the contribution of Neapolitan chemistry to the better
understanding of the melanins and how, after the discovery of the
Cystenyldopas, biological interest in melanocyte has grown.
BIOLOGY
The study
of melanocyte, the cell which produces melanin has developed greatly in the
last 25 years (1). The progress made covers various branches of science
like physics, biology and medicine.
Melanocyte
produces organelles called melanosomes which contain an enzyme called
tyrosinase. The enzyme, a protein containing copper, catalyses the oxidative
transformation of the DOPA (tyrosine) (2) into a pigment called melanin
(melaV = black).
Pigments
of red-brown colour can be produced by the cell. The pigments which are, among
other things, responsible for the colouration of so-called red hair, take the
name of the pheomelanin (jaioV = brown). A
precursor of pheomelanin is the 5-S- cysteinyldopa which is formed by reaction
between cysteine ( glutathione) with dopaquinone (3). For this reason
the various colourations of hair and skin are due to two different chemical
pigments: melanin and pheomelanin. (4). The physiological colour of skin
is produced by a combination of different pigments, among which melanin,
haemoglobin and carotinoids. Among these melanin is certainly the pigment which
mainly determines the colour (5) of the skin and which is seen when the
granules of the pigment transfer from the melanocytes to the keratinocytes. In
the epidermis every melanocyte is associated to 36 keratinocytes in a
characteristic biological unit (6). The colour of the skin, controlled
genetically, may be modified by the action of physical agents (e.g. tan through
the action of ultraviolet rays), or by the action of chemical agents (7).
Besides, in pathological cases one can find features having different colours
to the physiological ones. Addison’s syndrome produces a hyperpigmentation due,
perhaps, to an alteration of the catecholic metabolism .Tyrosinase (8),
the enzyme with a somewhat misleading name, should operate in the first phase
with transformation of tyrosine into DOPA by consuming oxygen;(For the 1978 Enzyme Nomenclature see
the International Union of Biochemistry, Academic Press, Inc. New York 1979) .
As is
noted the enzymes which oxidise the o. phenols to o. quinones, and which are
very widespread both in the animal and vegetable kingdoms, are called
polyphenoloxidases. Therefore the passage from dopa to dopaquinone is under the
control of a polyphenoloxidase. Some authors limit the use of the name
tyrosinases to the enzymes of animal origins but one also finds names like
phenolases and dopaoxidases. The enzyme is proteic by nature and contains
copper irrespective of the animal or vegetable source from which it is
extracted. It should be noted that copper is necessary in the pigmentation of
the mammals and that several complexes of copper oxidise the monophenols or the
diphenols into o.quinones in the presence of oxygen. The enzyme does not seem to
be very specific in its action (136): one cannot distinguish the
stereoisomers of the DOPA, it is active on a great number of o. diphenols and
oxidises with a velocity 1000 times greater than the catechol DOPA (enzyme
extracted from the potato). On the other hand the enzyme extracted from human
melanoma does not seem to have an appreciable action on the catechol (136):
further investigations of the specificity of differing sources would be useful.
Many
substances inhibit the activity of the enzyme in vitro.
2-mercaptoethylamine and N-(2-mercaptoethyl)dimethylamine cause depigmentation
of the areas of the skin where they are applied (9) and in general the
thiols are considered inhibitors of tyrosinase and the phenomenon of tanning is
also explained by the oxidation of the thiols with the consequent activation of
the enzyme. On this note one should observe that the thiols like cysteine or
glutathione are biological metabolites of pheomelanogenesis (Link 14).
Fig. 1 -
DOPA and tyrosine metabolism
According
to several authors the enzyme is first activated by the reducing action of a
catechol and then there is ortho oxhydrilation of the phenol (tyrosine) while
according to others (10) the quinone produced by oxidation of the
catechol (DOPA) is the agent which introduces the hydroxyl into the phenol
(tyrosine).
More
recently the classic role of the tyrosinase has been contested (11). The
enzyme which, in mammals, converts tyrosine into DOPA and therefore into
melanin would be the peroxidase, the enzyme of iron which separates the
peroxides forming active oxygen. Confirming this there is the demonstration of
the incapacity of the tyrosinase of the melanoma to transform the tyrosine into
DOPA (11). The conversion of the tyrosine into DOPA is slow and requires
an induction period which can be accelerated with the addition of small quantities
of DOPA and for this reason the name which is given to the enzyme (12)
seems little correct. Recently three factors which regulate melanogenesis have
been identified (4, 24) .
According
to Pawelek (4, 24) these factors could be in some way correlated to the
action of melatonin. Pawelek has observed that cultures of non-pigmented
melanoma cells possess an elevated expression of blocking factor while the
pigmented cells have large quantities of indolic conversion factors.
The
passage of the blocking factor to the conversion factor is stimulated by the
exposure of the cells to MSH (melaoncyte stimulating hormone)(84), which
also increases the activity of the tyrosinase. The dopachromic conversion
factor is present in almost equal quantities both in the pigmented and in the
non-pigmented cells and does not seem to be under the control of the MSH.
Pawelek considers the new factors as being associated to the tyrosinase (4,
24) of the melanosome and therefore melanogenesis must be considered a more
complex biological system, and different to how it has been considered up to
now. It would be very interesting to isolate these factors from different
biological sources and to know their chemical nature so as to be able to
correctly insert them in the various schemes of melanogenesis which are
fashionable.
Recently
Ortonne (14) has given a very interesting picture of melanogenesis in
the cephalopods Sepia officinalis, Loligo vulgaris and Octopus
vulgaris. The sack containing the ink is composed of two distinct zones
having different stages of maturation. The larger apical part of the gland is
composed of pigmented tyrosinase-positive cells in different stages of
maturation. In the caudal part of the gland the epithelial cells associated to
the Golgi apparatus together with the small vesicles show tyrosinasic activity
but do not produce melanin which suggests that the production of the tyrosinase
is not necessarily linked to the production of melanin.
In fact
the ink of the cephalopods contains notable amounts of free tyrosinases (14,
2), so that today it may be considered the most convenient biological source
for the extraction of the enzyme. The presence of an active principle in the
ink of the cephalopods is particularly interesting in relation to the defensive
function of the secretion and it is probable that the tyrosinase shall be
recognised as the composite which acts on the olefactive sense of the predator
(15) with the production of quinones both in the sack (14, 2) and
by oxidising phenolic composites of the predator. Besides, one should remember
that insects can also produce quinones (p. benzoquinones) with a defensive
purpose(4, 8).
It is
probable that the tyrosinases have a different function, besides that of
producing pigment in biological situations differing from the cephalopods. Chen
and Chavin (16) have used a precise radiometric method for measurement
of enzymatic activity in the skin and have applied it to the study of
tyrosinase in the vertebrates. The authors have often observed the presence of
tyrosinases in soluble fractions, like in the guitarfish, in dogfish, in the
eel of the Congo, in serpents, in frogs. In anurous (amphibian) the tyrosinase
activity is concentrated in non-pigmented zones. In melanomas in general the
enzymatic activity is very high but it is also high in non-pigmented tumours,
as occurs for example in the crab Xiphophorus helleri.
The
function of the tyrosinases could also be understood through the chemical mechanism
with which it works and the properties of the melanin by studying the
microorganisms and fungi.
In fungi
tyrosinases, through the pigment, play a clear cytoprotective role. According
to Liach and Ruban (4, 15) this role has two explanations:
a)
protecting the cells from different types of electromagnetic radiation, so
common in nature, and which in many cases are a severe ecological selection
factor.
b)
protecting the cells from the lythic action of the microbic enzymes, allowing
the existence of a biologically highly active microflora like that on earth.
In
general strongly pigmented fungi are found in zones rich in radiation, in the
western Turkmenia (hyphomicetes of the families Dematiaceae, Stemphylium,
Macrosporium, Alternaria, Cladosporium), in the desert terrain of
Tadzikistan and in the uplands of central Asia (4, 15). Fungi of the
uplands of eastern Pamir territory are principally represented by imperfect
highly pigmented forms of the Dematiaceae family, most of the time one
finds the species Cladosporium and Hormodendrum, and these forms
are sometimes the only ones which are found in high mountain territory (4,
15). From the desert terrain of Nevada mainly fungi with brown coloured spores
or mycelium have been isolated (Alternaria tenuis, Clodosporium herbrum,
Phoma sp., Pullularia pullulans, Rhizopus nigricans, Stemphylium ilicis,
Stachybotrys atra) while in the desert terrain of the Sahara the strongly
pigmented species of Helminthosporium, Curvularia, Alternaria, and Stemphlium
prevail (4, 15). Generalising the bibliographic data, Liach and Ruban
conclude that the pigments present in the hyphomicytes allow life in the
limiting conditions created by various ultraviolet coloured, thermic and
ionized radiations, in that the spores with dark coverings are much more
resistant than the non-coloured ones. Using glycosidase of basidiomycetes, the
chitilinases of streptomycetes and proteases of Bacillus subtilis,
Alexander (quoted in (4, 15)) found that while the coverings of the
non-melanised fungi broke easily the enzymes did not act on the structures or
parts containing melanin (121). On the other hand it should be
remembered that the quinones produced by the enzyme are antibiotics with a wide
range.
According
to some authors tyrosinase participates in morphological and functional
differentiation in some fungi.
In Neurospora
crassa the enzyme is not absolutely necessary to life while for Stemphylium
sar. it is indispensable, for which reason it has been suggested that it
acts as a terminal oxidase (4, 15).
Parting
from the fact that the appearance of the tyrosinasic activity in the mycelium
of Glomerells cingulata coincides with the end of the growth period and
with the period of autolysis of the mycelium, one presumes that the activity of
the tyrosinase is induced by the accumulation of substances which appear as the
result of autolysis (4, 15). Sometimes the activity of the enzyme is
connected to the pathogenicity of the fungus, imagining, in the case of Fusarium
vasinfectum, that it is employed for disintoxication of the phenolic
composites of the host plant (4, 15).
Disintoxication
from the phenols as a function of the tyrosinase is also suggested for the
fungi Stemphtlium sarcinaeforme and Monilia (Sclerotina) fructicola.
It is probable that also for other fungi the enzyme oxidises the phenols to
quinones making them inoffensive by polymerisation. It is worth remembering
that polymerisation is the essential part of disintoxication since mere
oxidation of the phenols would bring about the formation of quinones which are
highly toxic for the organism because of their reactivity. It is interesting to
note that more recently these concepts have been transferred to melanogenesis
in mammals (46) (4, 22) (49).
The
enzyme is active towards a large number of different mono and diphenols (4,
1, 4, 8, 12, 14, 21, 24) (136) and it is so in a particular way in the
fungi; perhaps it formed in evolution under the action of the large variety of
toxic phenols, esogenes for the organism, with which it entered into contact (4,
15). According to Blois (4, 8) melanins, among which one prepared by
photo-oxidation of phenylalanin, must be considered as matrices of chemical
evolution (120) more than the mineral lattices to which the
evolutionists always refer. According to this concept, in the course of
evolution the melanin would have appeared before the proteins and DNA, and
therefore with a different role to that sustained by evolutionist
microbiologists. The study of Blois (5, 2-7) shows that the melaninic
pigments possess several properties which are interesting for evolutionist
molecular chemistry like, for example, the distribution of unpaired electrons
in the melaninic particles which are stable and chemically highly reactive.
Ascorbic acid and the oxygen do not manage to arrive in contact with the
unpaired electrons while the copper ions, after saturation of the other binding
sites of the "macromolecule", do manage, thereby profoundly
influencing relaxation times. Therefore melanin has the property both of a molecular
sieve and of an ionic exchange resin. It is still possible that the pigment
exercises a catalytic activity or, as has been shown, functions as a capturer
of O2- cells. (74) (108).
The participation
of the tyrosinase in the respiratory chain seems to be excluded in the case of
the fungi Glomerella cingulata while it seems accepted in the case of Aspergillus
nidulans (Sussmann, Markert (1953) and Carter, Bull (1969) cited in (4,
15)). The authors also report a hypothetical scheme of the participation of
tyrosinase in the joint transport of the electrons. The tyrosinase of the
fungus introduces an hydroxyl in the ortho position to that already existing in
the tyrosine (p. oxyphenilalanine), with the formation, that is, of DOPA which
is oxidised to dopaquinone. The DOPA/dopaquinone redox potential is highly
elevated and in relation to this the dopaquinone seems a very favourable
vehicle for the electron in that, at least theoretically, because a
contemporaneous variation of the free energy, the phosphorilation of 3
molecules of ATP can occur. One presumes that the DOPA system ¤ dopaquinone, in Aspergillus nidulans may
function by regeneration of NADP or another oxidant (17).
Also in
man, through the products of melanogenesis, tyrosine seems to exercise a
disintoxicating activity via melanocyte-keratinocyte. It has been found, in
fact, that a large number of chemical substances (Tab. 1) and drugs accumulate
in the pigmented tissues. This accumulation can also provoke chronic skin
lesions, lesions to the eyes, internal parts of the ear and in certain cerebral
nervous cells. Such manifestations may be, perhaps, explained by the
denaturation of some properties of the melanins like that of amorphic semiconductors
and "electron transfer" molecules. Besides, the pigment possesses a
notable complexing capacity which makes it similar to an ionic exchange resin.
Many of
the chemical and physical properties, common to all the melanins, from fungus
to man, could justify the very different functions. More rigorous proofs and a
differentiation between melanins with differing chemical structures, however,
are still lacking.
In man
the melanin is generally considered as an agent protecting from solar radiation
even if it does not produce complete protection. The cytotoxicity (49)
of the intermediaries and as a consequence the hypothesis of disintoxication
should be considered: a function which has been attributed to the melanin by
researchers of prebiological systems and of their molecular matrices. The
various functions attributed to the melanins would therefore be seen as a
memory of the functions of the preenzymatic era.
Chemically
the accumulation of so many different substances ( Table 1 ) in melanins can be interpreted in two ways with the high
reactivity of quinones, by linking effect or clatratisation .Unfortunately the
process which occurs, both in vivo and in vitro has not yet been studied in depth, neither
for its chemistry nor for its biology.
The
melanins are among the most widespread common pigments in nature. They are
responsible for the brown and black colour of skin, hair and eyes in mammals,
in feathers, the eggs and skin of birds, the scales and the eggs of reptiles,
the amphibians, of the cuticle of insects, the ink of the cephalopods, for
several microorganisms and fungi (4, 19). In man the pigment is also
found located in the Substantia nigra, in the Locus coeruleus, in
the internal parts of the ear, in the hepatocytes, in the muscular cells of the
heart, in the leucocytes, in the pineal tissue and in the surrenal gland.
Yellow, red and brown colourings are due to a particular class of melanins the
so-called pheomelanins (hair human, feathers of birds, etc).
The
melanins are also responsible for blue and green coloured characteristics
(schemochromes), very common in nature, and in man appear in the eye: these
colours are due to a physical effect called the Tyndall effect (4, 5,
19).
www.tightrope.it/nicolaus/index.htm (link 9)
These
pigments are also found in several pathological forms (melanoma, vitiligo, Acanthosis
nigricans, Schizophrenia, plant parasites etc).
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Tab. 1.
Substances which are copolymerized or linked to melanins both in vitro and in
vivo.
Tryptophan
(4, 7, 14) (151), Tyramine (4, 7, 14), Phenylalanine (4,
7, 14), Histhidine (4, 7, 14), Phenol (4, 7, 14), Valine (18),
Chloropromazine (19-28), Phenothiazine (40), Aminoacids (16)
(4, 7, 14) (18), Thiouracil (36) (46) (47),
Iodoquine, Chloroquine (37-39), Inorganic Ions (4, 14) (41-42)
(83), Nucleides (41), Dyes (43), Amphetamine (44),
Epinephrine, Ephredine, Octopamine, Norepinephrine, Cocaine (44),
Atropine (44) (147), Porphyrin (45), Nicotine (46)
(128), Aniline (43) (46), Paraquat (46), Dopamine (48),
Diethylamine (81), Ametryn (125), Amanitin (126), Vitamin
C (127).
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The
melanins give the terrain its characteristic colour and contribute to its
fertility with their property of being resins with ionic exchange and gas agent
transport. These melanins, often of microbic origin, taking the name of humic
acids, while many pigments of artificial origin are given the generic name of
melanoidins.
An
attempt to describe the pigments correlated to them on a prevalently chemical
basis is presented.
The
natural melanins have been attributed different properties and functions like
filters of radiation in the skin and the eye, mimetic function, antimicrobic
function, disintoxicantifiers, protection from noise in the ear, fixers of free
radicals, eliminators of oxygenated water, ionic exchange resins with a
particular importance in the skin and in the coroide, in hair, in soils, and
finally the function of redox and of semiconductors with a charge transfer
effect at physiological electrical potentials.
The only
generally accepted function is that of filter (partial) of light and especially
of the dangerous ultraviolet rays. For the other functions, certainly worthy of
the utmost attention, a deeper examination is necessary extending to
philogenetically different species. For example is the antimicrobic action of
the melanins (or intermediaries) of the fungi also carried out in the skin of
man? Different research is currently under way to give an adequate and not
hypothetical explanation to the biological functions of the melanins and of
melanogenesis.
The
"colour" of the melanins has always been a puzzle for chemistry. In
fact, for a black substance which absorbs all the wavelengths, it is necessary
to imagine having an electrical conductor (metallic dust, amorphous carbon) or
a complex mixture of different chromophores. A radical polymerisation of the
precursors of melanin would seem an ideal condition for the formation of such a
"chromophore" complex.
Effectively
one should also think that the spacial positioning of the molecules or the
atoms plays an important role for colour: its enough to think of amorphous
carbon and diamond. Important research of Blois in California has shown, with
X-rays, (see) that a special packaging of the melaninic macromolecules can
contribute to the characteristic "colour". In other words if the
distance between the various molecular strata of the melanins themselves could
be increased they would no longer be black.(4,19) (5)
CHEMISTRY
The study
of the melaninic pigments, as almost always happens with substances not easily
reconductible to crystalline and homogeneous products, has given scant
information about their chemical structure (4, 14, 18) (50).
Essentially
research has turned in two directions: a) the study of the nature and
reactivity of the precursors of the pigments; b) the direct structural study of
the natural melanins according to the classical lines of organic chemistry.
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Table
2. - Melanin classification.
Eumelanins
Pigments
derived from DOPA (3,4-dihydroxyphenylalanine): like Sepiomelanin (50,
3). Pigments derived from Dopamine like Neuromelanin (140).
Pheomelanins ( Link 14 )
Brown,
red, yellow pigments derived from 5-S-Cysteinyldopa from hair and chicken feathers
-1 (50, 33, 36, 43, 45- 47, 50- 52) (4, 21) (www.tightrope.it/nicolaus/pheomelanin.htm). Brown, red, yellow,
pigments derived from tryptophan (58) (95) (113) (151)
like those extrated from Australian kangaroo furs (65).
Allomelanins
Brown
pigments from catechol and others polyphenols like Alternaria-melanin (61),
Ustilago-melanin (4, 14), Daldinia-melanina (4, 14) (149),
microrganism-melanin (4, 15), humic acids (141) (137)
(4, 14) (64) (142), fungi (63),
Aspergillin (4, 1, 14).
Melanogens
Black
pigments are formed in different conditions from indole (82) and pyrrole
(4, 1). The structure of these polymers was not yet elucidated but a
trimer was isolated by Pieroni (143). Brown pigments form in oxidation
of tryptophan and derivatives (113) (151) (144), hystmine,
phenol, phenylalanine (black material is also formed from aromatic amine and
heterocyclic systems), tyramine (132). The existence in Nature of such
melanogens is not known.
The first
type of approach is described in the
monograph of Swan (4, 18). The identification of some
intermediaries of melanogenesis from the work of Raper brought about the
construction of the following scheme of biogenesis:
Tyrosine----->
DOPA-----> Dopaquinone-------> Dopachrome--------> Leucodopachrome
-------> 5,6-dihydroxyindole (DHI)------> 5,6-Indolequinone-------->
melanochromes (oligomers of the preceding) ---------> melanin.(51)
(compare to Link 20 ).
This
scheme, constructed in an arbitrary way both from a biological and a chemical
point of view, was given the name Raper. According to such a scheme the passage
from tyrosine to dopaquinone is under the control of the tyrosinase while the
successive phases are represented by an ordered series of spontaneous oxidative
transformations which lead to a regular polymer of the 5,6-indolequinone.Even
though reference to this scheme is still made today melanogenesis must be
thought of as a much more complex semi-enzymatic biological process (4,
7) not yet describable in exact terms because any intermediates may contribute
to melanin formation.Furthermore no eumelanin has ever been found to correspond
to DHI-melanin or its corresponding indolequinone.
The
various products described in the scheme of melanogenesis have been isolated in
the pure state or in forms of derivatives, or characterised by spectroscopy (52)
and each of them converts into melanin either by oxidation or in the presence
of inorganic catalysts (57) (104) (135). The 5,6-dioxyindol
easily oxidises even in the presence of atmospheric O2 (50, 28) (59)
and the pigment which forms, with yields which do not correspond with the
theoretical values, shows a centesimal composition not corresponding to the
theoretically calculable value for a polymer of 5,6-indolquinone; such a
discrepancy is generally explained by the supporters of Mason’s biological
scheme by the presence of molecules of water strongly bound to the melanin (59).
Also the
fact that the oxidation of the tyrosine, dopa, dopamine, 5,6-dioxyindol brought
about physically and chemically different melanins, while according to Mason’s
scheme of melanogenesis they should be identical, has not yet found an
explanation. From the start, these contrasting data were the object of a scientific
dispute (50, 30), based especially on the theory of H. S.Mason , that
is, whether the pigment were or were not formed of only units of
5,6-indolequinone, briefly whether it was a poikilopolymer or a homopolymer.
Interesting studies with models made in Great Britain by Bu'lock, Kirby,
Harley-Mason, and Swan (4, 18) showed that the melanin was composed of
different units and that given the negative chemical results obtained with the
melanins it was not possible to write a formula of structure (4, 7).
In 1926
Raper, pioneer of melanogenesis, isolated DOPA, 5,6-dihydroxyindole and its
acid 5,6-dihydroxyindole-2-carboxylic acid (DHICA), under the form of
ethers/methylic esters, after enzymatic oxidation of the tyrosine. Later
Piattelli isolated 5,6-dihydroxyindole, in very low yield, by simple extraction
with ether from the DOPA solution, oxidised enzymatically at pH 6.8 (50,
14).
The
possibility that, in the course of melanogenesis, there is the formation of
different composites to those foreseen in Mason’s scheme (53) was
discussed by Graham in the case of the composite II (54) (97) (92).
The composites III, obtained by synthesis, are interesting products for
melanogenesis (55) and composite IV is isolated from the tapetum lucidum
of the catfish (56): these two composites, very interesting in relation
to the biogenesis of the melanins, were not studied any further. The
5-hydroxy-tryptophan V, which can supply the 5,6-dihydroxyindole by oxidation,
has been suggested as a precursor of the melanins (58) (151). The
recent discovery of melanogenes and pheomelanogenes biogenetically correlated
with tryptophane suggests that the role this aminoacid plays in melanogenesis
is less hypothetical than it has been considered up to now.
Recently the
differences between the natural melanins (melanosomes) which had already been
demonstrated (4, 14, 18) were quantified, by Wyler at Lausanne, with the
nomination of the acid functional groups present in the
"macromolecule" (4, 23) (60). One finds, within the
limits deriving from working in heterogene phases, that the proportions among
the moderately acidic (carboxyls) groups and weakly acidic (like phenols)
groups is the following:
natural
melanin 1.1:1
dopa-melanin
(self-oxidative) 1.7:1
dopa-melanin
(enzymatic) 5.5:1
which
means that melanins obtained by self-oxidation have a higher number of
carboxyls, while the number of carboxyls of the natural melanins is different
to that of the enzymatic melanins. This difference may, in fact, give rise to
different hypotheses like: the presence of a pheomelanic fraction in the
natural melanins , the nature of the enzyme, size of the particles
(granules),oxidative processes etc.
In
summarising the study of the nature and the properties of the precursors of the
pigments (the melanins) it is usual to indicate that they possess a complex
structure, from a point of view of the monomers, even if characterised by a
certain order in which the heterogeneous "macromolecules" are aligned
and packaged. The enzyme, often little specific, transforms the orthodiphenols
in the pigment and only operates in the first phase, even if later the final
result is the synthesis of melanin. In our opinion, this would deserve the
denomination of: Polyphenoloxidases poikilopolymerases. (with sub-enzyme
factors)
Direct
chemical study of the brown and black pigments aimed at obtaining
structuralistic information of some relevance from the degradation products was
completely negative up to 1952 (4, 1, 2, 5). Dealing with substances of
great molecular complexity which are insoluble, infusible, incrystalisible and
non-hydrolisible made their mixing and isolation in states of chemically
defined comosites practically impossible. Also the terms molecule and
macromolecule which are used in organic chemistry to indicate a definite and
unitary structure cannot be used in this field.
The
melaninic pigments are more realistically definable in terms of particles or
granules instead of molecules or macromolecules, even though this is more than
a little disturbing for chemists. A different meaning is almost always possible
after the obligatory step in the structuralistic study of every natural
substance, that of purification, as it is possible to follow two paths, either
the drastic treatment with acids and bases to remove the soluble or
solubilisable products, or the bland treatment which is limited to simply
washing the pigment with solvent.
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Table 3-
Elemental Analysis
C H N S
Bakunin (50, 1) melanoma C% 52.5 H%
3.6 N% 7.9
Panizzi (50, 3) sepia C% 57.5
H% 2.9 N% 10.5
Piattelli (50, 11) sepia C%
64.7 H% 2.5 N% 9.7
Ortonne (14) sepia C% 54.3 H%
2.9 N% 8.7
Benathan
(4, 23) sepia C% 57.7 H% 2.6 N% 6.2
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In the
drastic treatment, which can lead to not easily observable structural
modifications, every result obtained must be controlled on native product which
has not undergone chemical treatment. The differences which are seen in the
elementary analysis (Tab. 3) are probably to be put down to the different
conditions used in the purification of the pigments. The differences seen, at
that time, in the melanins extractable from different sources could be
explained today by the different type and grade of copolymerisation of the
intermediates . These and other unfavourable properties and the scarce success
obtained in the structure investigation with physical methods make it very
difficult if not impossible to reach the ultimate goal of organic chemistry:
the formula of the structure of the molecule.
The most
studied melanin to date is that obtainable from the ink sack of the squid Sepia
officinalis (50, 3) (4, 14) .
In 1952
Panizzi isolated the acid 2,3,5-pyrroltricarboxylic XI (50, 3) from the
mixture of oxidation of the pigment with H2O2 (KMnO4)
which at that time represented the first fragment of a certain structural
interest obtained by degradation. Successively, with a very low yield,
different products of degradation among which the acids VII, VIII, IX, X, XII (4,
14, 23, 75) (62) were obtained by different techniques. The acid XI is
also formed by alkaline treatment of the sepiomelanin (50, 23, 30).
For some
of these acids it is difficult to explain the genesis from a polyindolequinone
polymer via 3-7 as foreseen in the Raper-Mason scheme and from the study of
models (4, 14, 18). The tetracarboxylic acid XII, for example, seems to
form from preexisting polycyclic structures which can originate from the
intermediate heterocyclic enamines, which form by cyclisation of the
dopaquinone according to a chemical process already clarified in the case of
the alkaloids (111). It is worth pointing out how the formation of the
polycarboxylic acids is a characteristic of the melanins.
An attack
with H2O2
conducted at room temperature transforms the sepiomelanin (50, 3)
into sepiomelanic acid soluble in the carbonated alkalis and insoluble in the
acids. The soluble fraction both in the acids and in the alkalis already
contains the acid XI. It is interesting to note that Wolfram (138)
managed, with a bland attack with oxygenated water, to solubilise the pigment
and to show the proteic matrix to which it is linked. According to other
experiments of Wolfram the pigment (hair) should have a molecular weight of
11,400 (osmometry) and of 15,000 (gel-filtration). The proteic matrix which
makes up 8-10% of the weight of the melanin has a composition in similar
aminoacids, even if one operates with melanins from different biological
sources, which suggests a specific type of protein.
The
study of degradation products has also provided evidence for the presence of
dopachrome units in the polymer ( 50,11,18,21 ). The participation of
dopachrome in melanogenesis does not occur in vitro to any extent, probably
owing to the fact that conditions of pigment formation in vivo are not
reproducible (50,18 pag 946)
The
oxidation of the sepiomelanin always shows a fraction more resistant than the
others to degradation but if this is to be attributed to the residue proteic
fraction or to a polycyclic "heart" as has been shown for the
melanins of soil by Haworth (64) (137) it has not been studied.
The acid
XI which is formed with the total yield (successive oxidations) of about 3% may
originate either by rupture of an "indolic" unit of the macromolecule
or from a unit already open, of the type XIII or of the type VI, found by Wyler
among the products of selfoxidation of the DOPA and which most probably comes
from a sequence of oxidative transformations of the dopachrome (60).
Further structuralistic information can be found from the study of the esters
and the ethers of the sepiomelanin and of their degradation products: the
carboxyl is also present in the pigment of phenolic groups (weakly acidic
groups). Among the degradation products it is possible to show mere traces of
indolic substances, for this reason it is not possible to have direct chemical
proof of a polyindolquinonic structure for the melanin.
The
direct chemical study was then extended to a large number of natural melanins (50,
8, 16, 20, 22, 25 - 27, 49) and biosynthetic melanins (50, 18, 24) which
allowed identification, in some cases, of the precursor chemical of the pigment
and showed the complex heterogenity common to all the melanins.
As is
well known the first objective in the study of the structure of natural substances
is the isolation in the crystalline state of the substance which one wants to
study. The appearance of crystals is, on the psychological plane, a source of
great pleasure for the researcher but this joy was for a long time denied to
people who worked in Naples on the melanins, until studies on the pheomelanins,
classes of pigments little known chemically,were started (4, 5).
The study
of the pheomelanins started with the extraction of the pigments of the feathers
of the New Hampshire or Rhode Island hen, (130) (Link 14). Even with repeated
extraction it is not possible to decolour the feathers. The yields which are
generally reported are therefore relative. The pigment is collected in oval or
globular granules somewhat smaller than the black ones (0.5-1.3 mu). The yellow
pigments are contained in even smaller granules.
Unlike
for the melanins the contribution of the pheomelanins to the so-called
structural colours or schemochromes which are very common in birds is not known
(4 ,2, 19). Other modifications of colour are due to the co-called dust
which covers the barbules of the feathers (4, 5) and which seem due to a
process of keratinization or disintegration of the cells.It seems that this
dust contributes to creating the soft colours (viola, green-yellow etc). The
chemical nature of these pigments or of
the schemochromes is, for most of the cases, still unknown.
The
pigments of the feathers were given the name gallopheomelanins (50, 33,
36, 43, 45, 46, 52). In the course of extraction of the gallopheomelanins,
(mixtures of pigments mixed with proteins with a continuity of molecular
weights (from 1800 upwards)), a compound called C1 was isolated and was
assigned the structure XIV. This and other correlated compounds or isomers were
given, in chronological order, the names of tricosiderine by Flesch (85)
(4, 17), of pyrrotricole
by Boldt
(86) (4, 17), and of tricochromes (pheochromes Link 14 ) by Prota
(4, 17). (www.tightrope.it/nicolaus/pheomelanin.htm ).
The
synthesis of the pigments strictly correlated to the trichromes
(tricosiderines) was realised in 1974: Kaul prepared (115) the cis-D2,2’-Bis(2H-1,4-benzothiazine)-3(4H)-one
showing its characteristic photochromism.(Link 14)
A
derivative of this was found to be the colouring pigment XIV of human red hair.
It was also the only crystallizable pigment obtained from red hair and New
Hampshire chicken feathers.
In
precedence a series of model composites, which were very useful for
understanding the chemical and physical behaviour of the pigments, were
realised by Sica (50, 37, 39) and by Santacroce (50, 38, 41,58) (146).
A notable contribution to the understanding of the pheomelanogenetic process
was also given by Crescenzi (40, 59) in the synthesis of the D2,2’-Bis(5-oxy-7-methyl-2H-1,4-benzothiazine).
Unfortunately
the biological significance of the tricochromes (pheochromes) (87) is
still uncertain because it has not yet been demonstrated that they preexist in
exactly the same way in the various biological sources from which they, or
their artefacts, are extracted (133) (93).
The
chemical study of the pigments of feathers was carried out mainly on the
gallopheomelanin - 1, fraction without proteins. For action of the acids or of
the different oxidants characteristic products of degradation were obtained. Link
14 .(50, 35, 36),
In
parallel with the direct chemical studies, synthetic and biogenetic research
were also carried out in the laboratory. Fattorusso (50, 46) found that
the enzymatic oxidation of DOPA in the presence of cystein gave a pigment (134)
very similar in chemical and physical properties to gallopheomelanin - 1.
Besides in 1961 it was found that the 5-S-Cysteinyldopa synthesised (50,
35) and its isomers coming from the cysteinic residue in 2 and in 6,
synthesised by Fattorusso (50,46), also yielded a product chemically and
physically similar to gallopheomelanin - 1, for which reason it was clear that
the natural pigment was a product of a complex oxidative reaction
(polymerisation) which involved these aminoacids (50, 46) (88).
Successively experiments carried out in vivo by Misuraca (50, 48)
demonstrated that the 14C cystein and the 2-14C tyrosine
(precursors of DOPA) were incorporated in the papillae of the feathers of the
embryo of the New Hampshire hen. Another experiment conducted at Naples (50,
54) gave information about the first stages of pheomelanogenesis with the
isolation of the cyclocysteinildopa (Fig. 3) from the mixture of enzymatic
oxidation of the 5-S-Cysteinildopa (89) (107).
The
interpretation of the successive stages of pheomelanogenesis or the structure
of the pigments is more difficult, seeing the creation, that is, of situations
similar to that of the eumelanogenesis where some intermediaries (see Fig. 3)
are known and others supposed present, but how polymerisation occurs and what
the structure of the final product is, are not proved.
According
to Minale (50, 45, 50) the analytic data available allow hypothesising
both a partial structure of the pigment of type XXVII in which there are
present "benzothiazolic" units forming through a Mannich and a XXVI
structure with "benzothiazinic" rings forming by 2-8 polymerisation.
Chioccara, instead (50, 65), following the biogenesis of the alkaloids (111)
and on the basis of experiments carried out on 2H-1,4-benzothiazine, thinks that
the pheomelanic pigments can form through a process of aldolization which would
lead to cyclic trimer intermediaries as represented in the hypothetical
compound XXVIII. The work, though, does not state how the uncoloured composite
XXVIII becomes the pigment. It is interesting to note that the model polymers (50,
65) easily convert into the artifact tricochromes which would lead to the
conclusion that, effectively, the natural tricochromes are the artefacts or
products of degradation of the pheomelanin. Another hypothesis on the first
stages of pheomelanogenesis is that the composite I
forms: the closure of the
"thiazolic" ring is made between N1 and C2, that of the isoquinolinic
ring between C3 and C4 and finally with N1 and C3 and C4 the closure of the
"thiazinic" and "isoquinolinic " rings occurs with the
eventual formation of a spiranic system. The hypothetical structure I can also be considered a potential source of
tricochromes. The action of the acids on I could, in the presence of O2 , bring about a
series of transformations like hydrolysis of the base of Schiff, N1-C3
cyclization and dimerization to tricochromes. The closure of the
"isoquinolinic" rings by the action of acids on the bases of Schiff
has been known for time in the laboratory (reaction of Pictet-Spengler) and was
used by Minale (50, 51) in the study of pheomelanic models. Perhaps it
is worth recalling that the isoquinolinic compounds XXII and XXIII are obtained
by acid degradation of the gallopheomelanin. Even these degradation products
leave doubts about the preexistence of isoquinolinic structures in the pigment.
The
degradation products of gallopheomelanin - 1 have been used by Fattorusso (50,
49) to characterise the pheomelanin in various biological sources. These
studies, given that the structures of the pigments are not known, also
represent a chemical way even if approximative of differentiating the melanins
among themselves, and of identifying the precursor or the precursors. The
observation that the melanin which can be extracted from the hair of deer
supplies both the degradation product of the sepiomelanin and that of the
gallopheomelanin is interesting and suggests that one is dealing with a mixed
melanin (150).
More
recent work by Iso (50, 70) (90) and Rorsman (91) (92)
has demonstrated that the phenomenon of copolymerisation between the
intermediates of melanogenesis of the DOPA and that of the 5-S-cisdopa and
2-S-cisdopa is perhaps more vast than previously believed. These fundamental
works assign a new role, still to be clarified, to the cystein (glutathione) in
the area of melanogenesis and in the cellular metabolism (4, 22) (50,
68, 69). From a chemical point of view the sulphur present in many eumelanins,
for example in the melanoma melanin, and attributed to a cysteinilproteic link
(50, 17) (13) is probably included in "thiazolic" or
"thiazinic" systems.
From 1968,
the year in which Prota synthesised 5-S-Cysteinildopa and Fattorusso
2-S-Cysteinildopa, interest in these aminoacids has been slowly growing thanks
especially to the research of Rorsman and Lund (91) (4, 14) (92).
The 5-S-Cysteinildopa (5-cisdopa) which had been seen spectroscopicly by
Bouchilloux (98) in 1960 has been found again today in the feathers of
the red hen (91), in the skin, in the bull’s eye (114) (101),
in some sea anemones, in crustaceans (99), in hair of mice (129),
in the siero and urine of man . A derivative of 5-S-CISDOPA, the aminoacid
3-(2,5-S,S-dicysteinyl-3,4-oxyphenyl)alanine has been isolated from tapetum
lucidum of the catfish (lepidosteidi) by Ito (100).See recent papers.
The
5-S-cisdopa and its methylic ethers are found in the urine of patients with
melanoma next to 6-S-cysteinildopa and to 2-S-cysteinildopa (50, 62), to
2,5-S,S-dicysteininildopa (50, 62), and to the B and C tricochromes (50,
61) (102). One should stress that the measure of the 5-S- cisdopa and
its derivatives, realised with refined analytic methods by Rorsman (91),
by Hansson (102), by Crescenzi (50, 67), by Ruffo (131),
in the urine and siero of carriers of melanoma is of a diagnostic value in the
initial and therapeutic phases of the tumour. The 5-S-cisdopa, which is found
in physiological conditions in the urine and siero of mammals increases not
only in pathological cases but also after the exposure to ultraviolet radiation
(106).
Recently
Hagström (103) has shown the effect of 5-cisdopa both on the fertilisation
and in the successive stages of embryo development in the eggs of the sea
urchin Paracentrotus lividus. The property of this aminoacid,
demonstrated by Palumbo (104), of forming reversible complexes with
metals and their role in the metabolism in vivo is interesting.
All these
studies show that perhaps this new catecholic aminoacid has a role and a
biological meaning which goes beyond a purely pheomelaninic interpretation. The
discovery of dopa (105) and of glutathiondopa (91) in malign
tumours poses, then, a series of different problems relative to the role of the
glutathione in the cells (6 ,6) (4, 22) (50, 68) (109).
In
relation to the cytotoxicity of the catechols and to the genesis of the
neuromelanin, Ito (108) has studied the oxidative process of DOPA in the
presence of cystein with different systems of biological interest: tyrosinase,
peroxidases, hypoxantin-xantin, O2 + Fe/EDTA, H2O2 + Fe/EDTA. One
reaches the result that the Cisdopas can also form without tyrosinasic activity
and that their synthesis can be mediated in vitro by peroxydases, by
superoxide radicals, by oxygen in the presence of kelated Fe, and by
oxyhydeinic radicals. Ito (110) has also found that considerable
quantities of DOPA and 5-S-cisdopa are found in the hair of white mice which
completely lack follicular melanocytes and which give a negative tyrosinase
test.
From the
proceeding it is also possible, in a rapid summary of the literature like this,
to note that after the discovery of 5-S-cisdopa and 2-S-cisdopa, chemical
research has served to develop biological studies inherent to pigmentation,
with very interesting results from all points of view.
Chemical
knowledge on the melanins which results from work carried out in the recent
years can be summarised as follows.
The
chemical precursors of the natural melanins identified to date are DOPA,
5-S-CISDOPA, 2-S-CISDOPA and most probably catechol and 1,8-dioxynapthalin; probable precursors of the pheomelanin of the
kangaroo and the melanins of some invertebrates (sponges, leeches etc) are
derived from tryptophane. In general the diphenols suffer different known
transformations, among which the fundamental one into quinonic systems, but the
mechanism and the way in which the various units link among themselves, and the
type of units which concur in forming a particle granule are still uncertain.
A large
number of chemical substances can be involved in polymerisation or link to the
melanin and it is difficult to say if, in the world, there are two equal
melanins. The mechanism of copolymerisation which leads to the mixed melanins
is not known.
Synthetic
melanins can be obtained also from the non-phenolic precursors (Tab. 2) like
indole and pyrrole. Also for these pigments the chemical structure is unknown
but the physical properties of all the melanins, either of biological or of
synthetic origin, appear very similar to each other (Raggi X, IR, etc). On the
basis of the deluding chemical results to date obtained two structuralistic
hypotheses seem to be the possible for the natural melanins:
a)
melanins with polycondensate cyclic systems.
b)
linear polyindolquinone melanins.
The
formation of polycarboxylic acids among the oxidative degradation products of
different natural melanins speaks in favour of the presence of condensated
aromatic nuclei in the pigments. The 2,3,4,5-pyrroltetracarboxylic acid of the
sepiomelanin, the 2,4,5,6-pyridintetracarboxylic and 2,4,5-thiazoltricarboxylic
acids of the gallopheomelanin, the mellitic acid of the Aspergillus niger
melanin are indicative of the polycondensated polycyclic structure. The
benzenpolycarboxylic acids which are obtained from Aspergillin or from humic
acids . The interpretation of data is to be made with great caution and great
care because it is now knownthat melanin samples are always altered.
Physics
Solubility
Little
soluble in water even as salts, insoluble in organic solvents. The solubility
varies with the treatment undergone. In some cases they can be solubilised with
"Solvene 100" (116) or liberate proteic material with Triton
X-100 + SDS (117).Sonication may be useful.The pheomelanins or the mixed
melanins are more soluble especially in the alkalis. In the melanoma melanin
there is also a part that is soluble in piperidine (50, 1). When the
solubility is sufficient it is possible to purify the melanins for
chromatography (4, 23) (50, 49) (60) (132). The use
of spectroscopic methods is often limited by scarce solubility. In the case of
the pheomelanins it is possible to make them more soluble in organic solvents
under the form of salts of TEA (triethylamine) or after treatment with BSA
(bis-sylilacetamide) or with acetoacetic esters (94). The acetoacetic
esters, already used as N-group protectors of the aminoacids (E. Dane et al,
Angew. Chemie Vol. 74, 873, 1962), can be removed by simple acidification. The
same DOPA becomes soluble in alcohol and other solvents which makes more
controlled study of the oxidation products possible.According to Das (145)
the solubility of the melanins in water increases notably after reduction with
sodiumborohydride
UV
Spectrophotometry
Spectra
without definite bands with high absorption which grows from the visible to low
UB (4, 14, 18) (132)
NMR
Spectroscopy
Badly
defined spectra (94) (119) (142). The scarce solubility, the
presence of free radicals, the molecular complexity are the causes which limit
the structuristic information to some function.
IR
Spectrophotometry
The
spectrum shows wide bands with a difficult interpretation (4, 14, 18) (5,
7) (132) (139) (145).
ESR
(electron spin resonance or EPR) Spectroscopy
The
signals which are obtained seem due to free radicals trapped in the
"macromolecule" (66). All the melanins, from the humic acids
to those of the skin show a signal (122) (4, 14, 18) (70)
(71) without hyperfine structure which is present instead for the
precursors (68).According to Mason (53) (67) the melanins
contain one radical for about 1000 units indolquinonic. Blois (5, 2, 6,
7) makes a higher estimate of one radical per about 200 units. Considering that
the particle of melanin has a volume of about 10-3 m3, with about 106 monomeric units
there would be about 104 spin in the particle (5,
7).According to Mason (67) there are differences in the quantity of free
radicals of indolesemiquinone type, in the various melanins. The sepiomelanin
loses many of its radicals after reduction with ascorbic acid which is not
confirmed for other melanins. The paramagnetism of the melanins is explained by
Longuet-Higgins (Arch. Biochem. Biophysics Vol. 86, 225, 1960) with the
property of a monodimensional conductor with the protons which act as a trap
for the electrons.Zanotti has obtained simple ESR spectra from both white and
black hair and pelts (bovine) while albinos present a very weak spectrum or it
is completely absent (69). Compared to black ones the ESR spectra of red
hair and red animal hair are different and more structured (69) and
besides all the red hairs (different bovine races) possess a structured
spectrum. Recently Sealy (72) (73) found a new, pH dependent free
radical, of o. semiquinonic structure, studying the synthetic pheomelanin
obtained from 5-S-Cysteinildopa. The ESR study of the natural eumelanins and
pheomelanins (72) (73) shows that copolymers are in question .
Sarna has applied ESR united to Mossbauer spectroscopy to study the ion
exchange properties of the melanins (83).Constant dielectric,
conductivity, ESR, of the water-melanin mixtures are sensible to pH, to the
ionic force, to the grade and time of hydration (75).
X-Rays
Allow
establishing that in all the melanins the plane structures tend to align in a
parallel way with the distance between the different strata of 3.4 Å (5).
The total lack of crystallinity, even if in a certain "order", is
necessary to package the various strata. for the study with synchrotron
radiation see (5, 10).
XPS
(X-ray photoelectron spectroscopy)
This
relatively new technique allows having, among other things, information about
the state of oxidation of the nitrogen and of the sulphur in the melanins and
in other biological material in the solid state. Williams-Smith (76)
have examined different natural and synthetic melanins. Typical spectra found
N1s and S2p for the melanoma-melanin (horse) and for the melanins of the human
skin. This technique can be used with great advantage for differentiating
eumelanin and pheomelanin and their grade of copolymerisation, besides
establishing the grade of oxidation of N and S.
Electrical
characteristics
All
black material are electroactive. Conductivity is strongly increased by doping.
Electrical and sound conductivity show by melanins are of biological interest.
Superconductivity may be present.Both the natural and the synthetic melanins
behave as amorphous semi-conductors with threshold switching. (77-81), (152),
(50,81) ( www.organicsemiconductors.com ).The method of
isolation may produce artifacts which affect the electrical behaviour of the
melanin.
...The
melanosome has long been considered a passive cellular organelle. Its
considered role as a photoprotective agent in the skin and other illuminated
areas, could not explain its presence and function in the non-illuminated areas
(for example, the midbrain). A single hypothesis was developed, on the basis of
a quantum mechanical model of disordered materials (amorphous semiconductivity)
to explain the functional role of the melanosome in both illuminated and
non-illuminated areas. This hypothesis was based on electron-phonon
interactions, which seems to be particularly strong in melanins, and on the
large density of available energy states. A particularly useful probe for
determining the nature of these states is a measurement of low temperature
specific heat. The measurements presented here include two anomalies, a
transition and an unusually high linear contribution. The observed anomalies
probably arise as a result of the electronphonon coupling and high density of
unpaired spins, which until now were difficult to correlate. further
experimental measurements at near the transition temperature may yield a
detailed quantum mechanical description of the states, which will then afford a
more precise understanding of the biological functions of melanosomes than has
been possible to date....
U. Mizutani, T. B. Massalski, J. E. McGinness, P. M. Corry, Nature 259,
505-507 (1976).
...Threshold switching in hydrated melanin was first reported by
McGinness at al. Their discovery opened a new area for switching studies by
showing that low electric field switching occurs in organic semiconductors.
They, and others, have discussed some of the ramifications of this discovery to
living systems. No attempt was made to explain the mechanism of the melanin
threshold switch. Switching has been studied in amorphous semiconductors for
several years. It has been shown that in some bulk amorphous semiconductors the
switching action can be explained on the basis of a thermal model. Two types of
switching experiments were done on bulk melanin samples to distinguish between
electronic and thermal effects. Measurement of the sample current vs voltage as
a function of time provides information on the delay time and on the time to
traverse the negative-differential-resistance (NDR) segment of the
current-vs-voltage trace. The delay time is the time between the application of
a voltage greater than the threshold voltage and the attainment of low
resistance in the sample. Thermal models imply a much slower time to traverse
the NRD segment than electronic models. A more definitive experiment to
distinguish between electronic and thermal thermal effects is a double-pulse
experiment, which reverses the polarity of the applied voltage before the
sample switches. Voltage reversal cannot alter the Joule heating which is
responsible for the thermal switching effect but would be expected to reverse
electronic effects. Threshold-switching measurements of synthetic melanin
confirm that this organic semiconductor switches to a low-resistance state in
low electric fields as reported by McGinness at al. Time-dependent
current-vs-voltage curves show that the time to traverse the
negative-differential-resistance (NDR) segment is much slower than would be
expected from electronic-switching mechanisms. Double-pulse measurements add to
the evidence that thermal effects dominate electronic effects in melanin. A
pseudomemory effect was found in melanin....
C. H. Culp, D. E. Eckela, P. H. Sidles, J. Appl. Phys. 46, 3658-3660
(1975).
...Basic semiconductor characteristics of natural melanins isolated from
bovine eye, human dark hair, and banana peel were obtained by means of the dc
dark conductivity experiments and optical absorption measurements. The results
were compared with results obtained for synthetic melanin. Specific
conductivity in natural melanins is of the order 10-11 W
-1 cm-1 and in synthetic melanin 10-8
W -1 cm-1
. Thermal activation energies in the range 298-333° K are eye melanin,
0.93 eV; hair melanin, 1,01 eV; banana melanin, 1,04 eV; whereas synthetic
melanin has two values of activation energy: up to 311° K, 0,1 eV; above 313°
K, 0,78 eV. Optical gaps are: in eye melanin, 1,73 eV:in hair melanin, 1,35 eV;
in banana melanin, 1, 55 eV; and in synthetic melanin, 1,40 eV. The observed
differences between natural melanins and the synthetic one could be explained
by either the presence of protein residues in natural melanins or the influence
of the isolation method on their electrical properties....
In amorphous semiconductor the optical absorption coefficient follows
the relation (ahv)1/n ~ (hv-Eo), n=1,2, or 3 where a=optical absorption coefficient, hv=photon energy of incident light,
Eo=optical gap Optical absorption were performed with melanin disolued in NaOH
in the region of 250 to 840 nm. Dependance of (ahv)1/4 on hv plotted yields a straight line which gives an
optical value Eo= 1,4 ± 0,1 eV.
T. Strzelecka, Physiol. Chem. Phys. 14, 219-236 (1982).
TSDC
(Thermally stimulated depolarisation current)
Both the
natural and the synthetic melanins are able to conserve electrical charge
and/or polarisation. This effect (electret effect) depends on the content in
water (80).The pigment cell research on the behaviour of melanins with
respect to the electromagnetic spectrum were not much considered until now. All
black materials are or may be electric or sound wave conductors.
Superconductivity in melanins is to be taken in account.The concept of the
melanosome as an inert biopolymer is still largely accepted..Now at least one biological
material has been shown to have a strikingly large conductivity when correctly
excited. Melanins can be made to switch from a poorly conducting to a higly
conducting state at fairly low electric fields ( from 10K ohm-cm to 100 ohm-cm
with a field of 300 V cm-1 ). This remarkable
phenomenon occurs both in melanin made synthetically from tyrosine and in that
extracted from human melanoma ( Nature Vol. 248, April 5, 1974, pag. 475.
''News and Views").Many results are obtained from artifacts and are to be
revisited. Conductivity, like other physical parameters, must be measured in
physiological conditions, that is, in the presence of water, and on samples
which have not been altered by heating. Some difficulties also arise in the
study of pigment conductivity due to the lack of elemental analysis of the
material under study. It is surprising that many scientifical journals continue
to accept papers without the analytical data.(elemental composition).