Influence of blocking groups on photo-oxidation of tyrosine and derivatives.

[Display omitted] • Sensitized photooxidation of Tyr yields formation of three types of products: di-Tyr, Tyr-CBH adducts and Tyr-2 Da. • Blocking of amine or carboxylic group induce formation of more isomers of di-Tyr, while the Tyr-CBH is not visible. • N-Ac-Tyr-OEt derivative with both groups blocked showed the formation of ten different isomers of di-Tyr. One-electron oxidation of tyrosine primarily results in the formation of di-tyrosine, which can induce crosslinks leading to protein damage. In this study, we investigated the 3-carboxybenzophenone-sensitized photo-oxidation of Tyr derivatives through time-resolved and steady-state photolysis under anaerobic conditions to analyze the effects of blocking groups. The mechanism for primary and secondary photoreactions in the sensitized photo-oxidation of Tyr derivatives in aqueous solution was presented based on time-resolved analysis and mass spectrometric characterization of photo-oxidation products. Identified di-Tyr products (in addition to those mentioned more often in the literature, such as 3,3′/3,O') were in some samples presented together with Tyr-CBH adduct (resulting from radical recombination between the tyrosyl radical and CBH•). This publication discusses the possible coulombic effects of interacting ionic species (sensitizer and quencher) on quenching rate constants and the effect of amine groups and steric factors on the distribution of stable products. However, a crucial finding of this work is that the more blocked the Tyr is, the more di-Tyr isomers are formed, suggesting that Tyr residue in proteins may form several forms of di-Tyr cross-links. [ABSTRACT FROM AUTHOR]