Elucidating Potential Energy Surfaces for Singlet O2 Reactions with Protonated, Deprotonated, and Di-Deprotonated Cystine Using a Combination of Approximately Spin-Projected Density Functional Theory and Guided-Ion-Beam Mass Spectrometry

Two-step reaction mechanism reveals new antioxidant capability of cysteine disulfides against hydroxyl radical attack

Cysteine disulfides, which constitute an important component in biological redox buffer systems, are highly reactive toward the hydroxyl radical (^•OH). The mechanistic details of this reaction, however, remain unclear, largely due to the difficulty in characterizing unstable reaction products. Herein, we have developed a combined approach involving mass spectrometry (MS) and theoretical calculations to investigate reactions of ^•OH with cysteine disulfides (Cys-S-S-R) in the gas phase. Four types of first-generation products were identified: protonated ions of the cysteine thiyl radical (⁺Cys-S^•), cysteine (⁺Cys-SH), cysteine sulfinyl radical (⁺Cys-SO^•), and cysteine sulfenic acid (⁺Cys-SOH). The relative reaction rates and product branching ratios responded sensitively to the electronic property of the R group, providing key evidence to deriving a two-step reaction mechanism. The first step involved ^•OH conducting a back-side attack on one of the sulfur atoms, forming sulfenic acid (-SOH) and thiyl radical (-S^•) product pairs. A subsequent H transfer step within the product complex was favored for protonated systems, generating sulfinyl radical (-SO^•) and thiol (-SH) products. Because sulfenic acid is a potent scavenger of peroxyl radicals, our results implied that cysteine disulfide can form two lines of defense against reactive oxygen species, one using the cysteine disulfide itself and the other using the sulfenic acid product of the conversion of cysteine disulfide. This aspect suggested that, in a nonpolar environment, cysteine disulfides might play a more active role in the antioxidant network than previously appreciated.

Elucidating the Reactivity of O2 (a 1Δg): A Study with Amino Acid Anions and Related Sulfur and Oxygen Anionic Species

Rate constants and product ions were determined for a series of anions reacting with singlet molecular oxygen O₂ (a ¹Δ_g) at thermal energy using an electrospray ionization-selected ion flow tube. The 20 naturally occurring amino acids were used to produce corresponding deprotonated anions; only [Cys-H]^- and [Pro-H]^- were found to be reactive with O₂ (a ¹Δ_g), generating OSCH₂CH(NH₂)CO₂^- + HO and C₅H₆NO₂^- + H₂O₂, respectively. The reaction of O₂ (a ¹Δ_g) with [Cys-H]^- has a rate constant more than ten times larger than the reaction of O₂ (a ¹Δ_g) with [Pro-H]^-. Furthermore, reactions of O₂ (a ¹Δ_g) with carboxylic acid and thiol anions were carried out to elucidate the reactivity of the sulfur-containing functional groups. Potential energy surfaces and overall reaction exothermicities were calculated for representative reactions using density functional theory. Reactions in which attack occurs at the sulfur produce HCSO^- as an ionic product. Reactions of several carboxylic acid anions likely proceed through a hydroperoxide intermediate that is analogous to that calculated for reactions with amino acid anions at a higher collision energy. Overall, rate constants for reactions of carboxylic acid anions RC(O)O^- were found to be smaller for larger R groups.