Efficient decarboxylation of oxidized Cysteine unveils novel Free-Radical reaction pathways.

[Display omitted] • Methionine and S-Methyl-Cysteine oxidation leads to efficient decarboxylation. • Oxidation of S-Methyl-Cysteine yields alpha-aminoalkyl radicals. • Alpha-aminoalkyl radicals undergo beta-scission and hydrolysis. • LC-MS analysis reveals the formation of multiple isomeric photoproducts through free-radical reactions. The present study explores the photosensitized oxidation process of Acetyl-Methyl-Cysteine (Ac-MeCys) within a neutral pH aqueous solution. A comprehensive mechanism elucidating this phenomenon is put forth, leveraging outcomes from nanosecond laser flash photolysis (nsLFP) investigations and analysis of resultant stable compounds. The nsLFP findings furnish insights into the transient entities arising from the interaction between the photosensitizer, 3-carboxybenzophenone (3CB), and Ac-MeCys. These insights encompass the quantum yields of these transients. Additionally, the evaluation of enduring products entails the utilization of liquid chromatography and high-resolution mass spectrometry techniques. The principal reaction observed involves the formation of α-amidoalkyl radicals (αN) through decarboxylation, which supersedes the more prevalent α-thioalkyl radicals (αS) reported previously for analogous compounds such as Ac-Methionine. The αN radicals experience two distinct pathways for decay: they either combine with ketyl radicals to yield the αN-3CBH radical coupling product, or undergo beta-scission and hydrolysis, relinquishing CH 3 SH and giving rise to a corresponding alcohol. This investigation highlights how the distance relationship between the thioether group and the peptide backbone influences the oxidation trajectory of Ac-MeCys in comparison to Ac-Met. [ABSTRACT FROM AUTHOR]