Melanins definition

Melanine, ammelide and cyanuric acid were not detected in the HCOOH extracts and the identification of s-triazines previously reported does not prove that s-triazines are indigenous. This kind of compound can easily be formed during the experimental and analytical procedures. Nevertheless, this last conclusion is not definitive. As pointed out by Stoks and Schwartz33, the carbonaceous chondrites are far from homogenous. Important variations from sample to sample exist, even if all these samples are fragments of the same chondrite. Moreover, and as previously reported, the sample treatment is of prime importance. It can lead to the formation of secondary products or liberate molecules unobservable after milder treatment. To conclude, it seems clear that nitrogen heterocycles are present in carbonaceous chondrites and that pyrimidine derivatives are less abundant than purine derivatives. 

The subsequent conversion of these intermediates to eumelanins is still far from being well understood and represents a major focus for further work aimed at a definite understanding of the structure of both natural and synthetic melanins. 

Dihydroxyphenylalanine (for structure see diagram 11) is an amino acid isolated (from the pods and sprouts of Vida fd>a) and first definitely identified in 1913 by Guggenheim (323), who showed it to be identical with synthetic material previously prepared (280). It is widely distributed in certain types of plants (beans, etc.) but is not a normal protein constituent. However, it plays an important part in mammalian metabohsm of tyrosine, as it is the precursor of adrenaline (epinephrine), noradrenaline (arterenol, norepinephrine), and melanin. In this review these substances will be considered only insofar as they account for a portion of normal tyrosine metabolism. 

Tyrosine is a (crystalline) amino add with the formula HOQH4CH2CH(NH2) COOH. It results from the hydrolysis of proteins, that is, the reaction of proteins with water. The -C6H4- group within the compoimd denotes a ring-type stmcture. Another definition provided is that tyrosine is found in most proteins and can be synthesized metabolically from phenylalanine. Furthermore, tyrosine is a precursor for thyroid hormones, melanin (the dark pigment of body parts, notably, the skin), and catecholamines (that affect the sympathetic nervous system). 

The most salient point of melanin chemistry is the isolation of the pigment polymer, occurring in vivo or produced in vitro, as a single chemical compound of definite composition. The intractable nature of the material, the concomitant protein residue, metal ions, and the hydrated state of the sample are the main difficulties in obtaining the pigment in pure form. 

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