Formation of a new quinone methide intermediate during the oxidative transformation of 3,4-dihydroxyphenylacetic acids: Implication for eumelanin biosynthesis

Oxidation of dopa and dopamine requires a net removal six electrons to prod uce indolequinones, the monomeric precursors of eumelanin pigment. On the o ther hand, their 6-fluoroderivatives suffer only four-electron oxidation to yield the same products (M. E, Rice, B, Mogaddam, C, R, Creveling, and R. L. Kirk, Anal. Chem, 59, 1534-1536, 1987). Taking advantage of this novel f luorochemistry, we reexamined the oxidative mechanism of 3,4-dihydroxypheny lacetic acid and 6-fluoro-3,4-dihydroxyphenylacetic acid to throw more ligh t on the nature of reactive intermediates formed during the reaction, Enzym atic or chemical oxidation of 3,4-dihydroxyphenylacetic acid generated the transient o-quinone which exhibited rapid intramolecular cyclization and si de chain modification to produce 2,5,6-trihydrobenzofuran and 3,4-dihydroxy mandelic acid, respectively. However, when 6-fluoro-3,4-dihydroxyphenylacet ic acid was oxidized either by tyrosinase or by sodium periodate, the resul tant quinone uniquely exhibited only cyclization coupled with loss of fluor ide ion. This clean reaction allowed us to establish the structures of the transient reactive intermediates, Two interconvertable isomeric forms of th e product were isolated and characterized from the reaction mixture. If the oxidation was carried out in water, a yellow quinolactone accumulated in t he reaction mixture. This compound was instantaneously converted to a purpl e quinone methide upon addition of a trace amount of sodium phosphate. Pass age through a C,, HPLC column caused the reverse transformation, The struct ures of these products were established by semiempirical molecular orbital calculations and NMR spectrometry, comparison of the oxidation mechanisms o f melanin precursors, dopa and dopamine, with that of 3,4-dihydroxyphenylac etic acids reveals that a similar quinone methide intermediate is likely to be formed during eumelanin biosynthesis. (C) 1999 Academic Press.