Peptide Cysteine Thiyl Radicals Abstract Hydrogen Atoms from Surrounding Amino Acids: The Photolysis of a Cystine Containing Model Peptide

Abstract

Peptide cysteine thiyl radicals were generated through UV-photolysis of disulfide precursors, in order to follow intramolecular reactions of those radicals with neighboring amino acids. When reactions were carried out in D(2)O, there was a significant incorporation of deuterium specifically into the C(alpha)-H bonds of glycine residues in positions i+1 and i-1 to the Cys residue, indicating a fast reversible H-atom transfer. This H-atom transfer occurred prior to the formation of final, nonradical products including free thiol, thioaldehyde, and aldehyde. Such fast H-atom transfer is relevant to biologic conditions of oxidative stress and to the stabilization of proteins against oxidation, where the formation of carbon-centered radicals in proteins may lead to fragmentation, intramolecular cross-linking, aggregation and/or epimerization.

Stability of Protein Pharmaceuticals: An Update

“…The photoionization of Trp can reduce disulfide bonds by electron transfer, resulting in chemical and physical degradation of the protein (193,(223)(224)(225). Studies with different proteins have demonstrated the ability of photoexcited Trp to reduce disulfide bonds in both the liquid (223,224,226,227) and solid-state matrixes (225).…”

Structure and Reactivity of theN-Acetyl-Cysteine Radical Cation and Anion: Does Radical Migration Occur?

“…T here are numerous examples where radicals play key roles in the active site of enzymes [1][2][3][4][5][6][7][8][9][10][11][12]. These radicals are normally located on a reactive amino acid side chain (cysteine [2], methionine, tyrosine, or tryptophan [10]) or on the peptide backbone, namely at the α-carbon positon of glycine [13].…”

“…Radicals in amino acids, peptides, and their derivatives have been examined using various experimental [7,8,21] and computational techniques [13][14][15].…”

Mobile protons versus mobile radicals: Gas-phase unimolecular chemistry of radical cations of cysteine-containing peptides

“…The side-chain thiolate radicals of cysteine residues represent an important class of radicals in peptide and protein chemistry that are involved in a number of enzyme reactions [24] and are implicated in protein damage [25]. Recent work by Schoeneich's group has demonstrated that the side-chain thiolate radicals of cysteine residues can abstract hydrogen atoms from amino acids intermolecularly and also intramolecularly from adjacent amino acid residues [67][68][69][70][71][72]. Given the potential importance of these reactions in protein damage, as well as these being excellent model systems to examine side-chain-to-backbone HAT in gas-phase peptides, here we use a combination of multistage mass spectrometry experiments, collision-induced dissociation, deuterium labeling, and density functional theory (DFT) calculations to study the fragmentation reactions of regiospecifically generated radical cations of simple cysteine-containing di-and tripeptides.…”