alpha,beta-Dehydro-3,4-dihydroxyphenylalanine derivatives: potential schlerotization intermediates in natural composite materials.

Proteins containing the post-translationally modified amino acid L-3,4-dihydroxyphenylalanine (DOPA) undergo autosclerotization as a means of assuring cohesive resilience in many structural matrices found in nature. To explore the chemical mechanism of sclerotization, we examined the oxidation products of relatively simple analogs of a peptidyl DOPA residue, such as N-acetylDOPA ethyl ester and N-acetyldopamide, together with those of several oligopeptides. Oxidation, induced by either of two catecholoxidases or by sodium periodate, resulted in the Lewis base catalyzed formation of derivatives of the unusual amino acid 3,4-dihydroxy-alpha,beta-dehydroDOPA (delta DOPA). The N-acetyl delta DOPA ethyl ester representative of this group of derivatives was characterized by NMR and uv spectroscopy. A variety of peptides developed analogous uv spectra upon oxidation. A similar reaction was observed upon oxidation of 3,4-dihydroxyphenylpropanoic (dihydrocaffeic) acid, but not after oxidation of N-acetyldopamine. Evidence is presented that this conversion is the result of a rearrangement of the DOPA quinone moiety to its delta DOPA tautomer, and that this tautomerization can be a dominant fate for peptidyl DOPA quinone, provided a Lewis base catalyst is available and competing reactions are minimized. Formation of delta DOPA in natural or synthetic polymers would increase the variety of crosslinks available to sclerotizing matrices. delta DOPA has been found in naturally occurring oligopeptides isolated by other workers from several marine species.