# Assessment of Hydroxyl Radical Reactivity in Sulfur-Containing Amino Acid Models Under Acidic pH

## TL;DR

This study investigates how hydroxyl radicals react with sulfur-containing amino acids under acidic conditions to understand electron transfer processes.

## Contribution

The novel contribution is the analysis of hydroxyl radical reactivity with methionine and cysteine-methylated amino acid models at acidic pH.

## Key findings

- Hydroxyl radical reacts with methionine through two steps: HO• addition to sulfur followed by HO− elimination.
- The presence of nearby amide groups influences the one-electron oxidation mechanism.
- Pulse radiolysis and MS analysis revealed proton flux effects on the reaction at pH 4.

## Abstract

Methionine residues in proteins and peptides are frequently oxidized by losing one electron. The presence of nearby amide groups is crucial for this process, enabling methionine to participate in long-range electron transfer. Hydroxyl radical (HO•) plays an important role being generated in aerobic organisms by cellular metabolisms as well as by exogenous sources such as ionizing radiations. The reaction of HO• with methionine mainly affords the one-electron oxidation of the thioether moiety through two consecutive steps (HO• addition to the sulfur followed by HO− elimination). We recently investigated the reaction of HO• with model peptides mimicking methionine and its cysteine-methylated counterpart, i.e., CH3C(O)NHCHXC(O)NHCH3, where X = CH2CH2SCH3 or CH2SCH3 at pH 7. The reaction mechanism varied depending on the distance between the sulfur atom and the peptide backbone, but, for a better understanding of various suggested equilibria, the analysis of the flux of protons is required. We extended the previous study to the present work at pH 4 using pulse radiolysis techniques with conductivity and optical detection of transient species, as well as analysis of final products by LC-MS and high-resolution MS/MS following γ-radiolysis. Comparing all the data provided a better understanding of how the presence of nearby amide groups influences the one-electron oxidation mechanism.