5.6- Dihydroxyindole polymerization

Alessandro Pezzella received his Ph.D. in 1997 under the direction of Prof. G. Prota at Naples University Federico II. Since 1999 he holds a permament position as researcher in the Department of Organic Chemistry and Biochemistry of Naples University. He has carried out research mainly in the field of 5,6-dihydroxyindole polymerization and oxidative behavior of phenolic compounds. 

The oxidative polymerization of 5,6-dihydroxyindole (1) and related tyrosine-derived metabolites is a central, most elusive process in the biosynthesis of eumelanins, which are the characteristic pigments responsible for the dark color of human skin, hair, and eyes. Despite the intense experimental research for more than a century,36 the eumelanin structure remains uncharacterized because of the lack of defined physicochemical properties and the low solubility, which often prevents successful investigations by modem spectroscopic techniques. The starting step of the oxidative process is a one-electron oxidation of 5,6-dihydroxyindole generating the semiquinone 1-SQ (Scheme 2.7). 

Since the oxidative polymerization of phenols is the industrial process used to produce poly(phenyleneoxide)s (Scheme 4), the application of polymer catalysts may well be of interest. Furthermore, enzymic, oxidative polymerization of phenols is an important pathway in biosynthesis. For example, black pigment of animal kingdom "melanin" is the polymeric product of 2,6-dihydroxyindole which is the oxidative product of tyrosine, catalyzed by copper enzyme "tyrosinase". In plants "lignin" is the natural polymer of phenols, such as coniferyl alcohol 2 and sinapyl alcohol 3. Tyrosinase contains four Cu ions in cataly-tically active site which are considered to act cooperatively. These Cu ions are presumed to be surrounded by the non-polar apoprotein, and their reactivities in substitution and redox reactions are controlled by the environmental protein. 

Oxidative polymerization of phenol derivatives is also important pathway in vivo, and one example is the formation of melanin from tyrosine catalyzed by the Cu enzyme, tyrosinase. The pathway from tyrosine to melanin is described by Raper (7) and Mason (8) as Scheme 8 the oxygenation of tyrosine to 4-(3,4-dihydro-xyphenyl)-L-alanin (dopa), its subsequent oxidation to dopaqui-none, its oxidative cyclization to dopachrome and succeeding decarboxylation to 5,6-dihydroxyindole, and the oxidative coupling of the products leads to the melanin polymer. The oxidation of dopa to melanin was attempted here by using Cu as the catalyst. 

Catechol melanin, a black pigment of plants, is a polymeric product formed by the oxidative polymerization of catechol. The formation route of catechol melanin (Eq. 5) is described as follows [33-37] At first, 3-(3, 4 -dihydroxyphe-nyl)-L-alanine (DOPA) is derived from tyrosine. It is oxidized to dopaquinone and forms dopachrome. 5,6-Dihydroxyindole is formed, accompanied by the elimination of C02. The oxidative coupling polymerization produces a melanin polymer whose primary structure contains 4,7-conjugated indole units, which exist as a three-dimensional irregular polymer similar to lignin. Multistep oxidation reactions and coupling reactions in the formation of catechol melanin are catalyzed by a copper enzyme such as tyrosinase. Tyrosinase is an oxidase con-... 

Kinetically slow steps in the formation of melanin from DOPA are the formation of dopaquinone from DOPA (step 1, kD), the reaction of dopachrome to dihydroxyindole (step 2), and the polymerization to form melanin (step 3, kM). Step 1 and step 2 proceed with about the same rate in the oxidative coupling polymerization catalyzed by tyrosinase. However, step 1 becomes remarkably slow when a macromolecule-metal complex is used as a catalyst. The copper complex in poly(l-vinylimidazole-co-vinylpyrrolidone) has been found [38] to act as an excellent catalyst and to exhibit the highest activity for melanin formation. The ratio of the rate constants ( m/ d) is approximately 3 (tyrosinase... 

A conducting, polymeric film of poly(indole-5-carboxylic acid) has been employed for covalent immobilization of tyrosinase, which retains catalytic activity and catalyzes oxidation of catechol to the quinone <2006MI41>. Poly(l-vinylpyrrole), polyfl-vinylindole), and some methyl-substituted compounds of poly(l-vinylindole) are of potential interest as photorefractive materials with a relatively low glass-transition temperature and requiring a lower quantity of plasticizer in the final photorefractive blend <2001MI253>. Polymers of 5,6-dihydroxyindoles fall within the peculiar class of pigments known as eumelanins and their chemistry has been reviewed <2005AHC(89)1>. 

Ty initiates melanin synthesis by the hydroxylation of L-tyrosine to 3,4-dihydroxyphenylalanine (Dopa) and the oxidation of dopa to dopaquinone. In the presence of L-cysteine, dopaquinone rapidly combines with the thiol group to form cysteinyldopas, which undergo nonen-zymatic conversion and polymerization to pheomelanin via benzothiazine intermediates. In the absence of thiol groups, dopaquinone very rapidly undergoes conversion to dopachrome, which is transformed to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) by dopachrome tautomerase. Alternatively, dopachrome is converted nonenzymatically to 5,6-dihydroxyindole (DHI). Oxidation of DHICA and DHI to the corresponding quinones and subsequent polymerization leads to eumelanins. It is still questionable if Ty is involved in this step. 

Scheme 3. Mechanism of polymerization of 5,6-dihydroxyindole according to Bu lock...

As far as the mechanism of polymerization of 5,6-dihydroxyindoles is concerned, kinetic experiments (I84J94) and pulse irradiation studies (190) suggest that coupling proceeds via oxygen-centered semiquinone radicals. If confirmed, such reactive intermediates may account for the complexity of the later stages of melanogenesis and the heterogeneity of natural and synthetic melanins. 

Polymerization of 5,6-dihydroxyindole probably (112) occurs by oxidation to the quinone, which then polymerizes through the 3-, 4-, 7-, and occasionally 2-, positions to give a polymer as in diagram 13. 

Although the exact structure of eumelanin has still not been determined, there is strong evidence that it has a polymeric structure built up of 5,6-dihydroxyindole 1, 5,6-indolequinone 6 and semiquinone 7 monomers. Eumelanin has exceptionally good photoprotective properties and an absorption spectrum spanning a large part of the solar spectrum. These special properties have prompted a number of theoretical studies of 5,6-dihydroxyindoles, their oxidation products and dimers, and oligomeric models of eumelanin. 

As mentioned above, the direct investigation of the reactivity of 5,6-indolequinones 149 is a difficult task and most of what is known derives from studies of the oxidation of 5,6-dihydroxyindoles in the presenee of nucleophiles as trapping agents. These studies form the core of this seetion. In faet, the oxidative polymerization of 5,6-dihydroxyindoles itself might be taken as reflecting the reactivity of 5,6-indolequinones versus 5,6-dihydroxyindoles, which has been addressed in a previous section. 

In the Raper-Mason scheme of melanin biosynthesis (Fig. 5) 216, 217), tyrosine is enzymatically converted via dopa to dopaquinone. The subsequent oxidation steps leading to melanin formation depend upon the biochemical environment of the reaction site. However, the melanization process in vitro or in vivo has two important features the rearrangement of dopachrome and the oxidative polymerization of 5,6-dihydroxyindoles leading to melanochrome. 

For many years the biosynthesis of melanin was thought to result from the spontaneous oxidation and polymerization of dopachrome produced by the tyrosinase-catalyzed hydroxylation of tyrosine to dopa and subsequent oxidation (5 ). In addition to tyrosinase, however, several enzymatic factors have been recently identified in mammalian tissues that appear to regulate melanogenesis at intermediate steps distal to those involving tyrosine and dopa. The factors include dopachrome conversion factor, dihydroxyindole blocking factor, dihydroxyindole conversion factor and dopachrome oxidoreductase (54-59). 

Indeed, the biosynthesis of the biopolymer melanin involves the oxidative cyclization of dihydroxyphenylala-nine (DOPA) to phenylalanine-3,4-quinone (dopaquinone), which eventnally forms 5,6-dihydroxyindole (DHI). Polymerization of DHI affords melanin [1], Lim and Patil have exploited this biochemical transformation using commercial mushroom tyrosinase in a synthesis of 5,6-dihydroxyindoles protected as the diacetates (2) (Scheme 1) [2], The parent indole (R = R =H) is obtained in less than 10% yield. Carpender has reported a similar oxidative cyclization using manganese dioxide to give 2 (R =Me) in 80% overall yield from epinine (1, R =Me, R =H) [3]. Other oxidants (H, Hp /FeSO, O, NaOCl, NaClOj/ VjOj) gave little or no product. Choi, Nam, and colleagues have effected an electrochemical oxidation of dopamine (1, R = R =H) to 5,6-dihydroxyindole that polymerizes to form films of polydopamine suitable for neural attachment and function [4]. 

The reaction of peroxynitrite with the melanin precursor 5,6-dihydroxyindole-2-carboxylic acid (DHICA) produces polymeric products. The reaction proceeds through unstable intermediates including dimers and trimers of DHICA in addition to indole-5,6-quinone-2-carboxylic acid thus peroxynitrite, which nitrates phenolic compounds. 

Melanomas are among the deadliest forms of cancer as they have a high recurrence but as yet no effective chemotherapy.(7, 2) The drug resistance of melanoma has been attributed to the presence of melanin, a redox-active polymeric pigment formed from the oxidation of tyrosine within cells.(5) The formation of melanin itself depends on fine control of oxidative chemistry a peroxide-dependent enzyme, tyrosinase, catalyzes two successive reactions, the hydroxylation of tyrosine and die oxidation of the product L-dopa, Scheme 1.(4) The product of dopa oxidation cyclizes to a 5,6-dihydroxyindole (DHI) intermediate, which is highly reactive and gives rise to black eumelanin polymers by a pathway dependent on further oxidation by oxygen.(5)... 

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