Oxidation of peptidyl 3,4-dihydroxyphenylalanine analogues: implications for the biosynthesis of tunichromes and related oligopeptides.

The o-diphenolic amino acid l-3,4-dihydroxyphenylalanine (dopa), the enamine α, β-dehydro-3,4-dihydroxyphenylalanine (Δ-dopa), and/or hydroxylated derivatives thereof, are integrated into the primary sequence of many scleroproteins and polyphenolic oligopeptides such as the celenamides, tunichromes, and halocyamines. After oxidation of N-acetyldopa ethyl ester, a low mol wt analogue of peptidyl dopa, the resultant o-quinone tautomerized to (Z)-α, β-dehydro-3,4-dihydroxyphenylalanine ethyl ester. We have characterized this Δ-dopa derivative and an acetate 1,4-addition product formed during the synthesis. Tautomerization of peptidyl dopa quinone to A-dopa may be involved in the biosynthesis of A-dopa-containing oligopeptides.