Oxidation of gas-phase hydrated protonated/deprotonated cysteine: how many water ligands are sufficient to approach solution-phase photooxidation chemistry?

Abstract

We present a study on the reactions of singlet oxygen O₂[a¹Δ_g] with hydrated protonated and deprotonated cysteine (Cys) in the gas phase, including measurements of the effects of collision energy (E_col) and hydration number on reaction cross sections over a center-of-mass E_col range from 0.05 to 1.0 eV. The aim is to probe how successive addition of water molecules changes the oxidation chemistry of Cys in the gas phase. Hydrated clusters, generated by electrospray ionization, have structures of HSCH₂CH(NH₃⁺)CO₂H(H₂O)_1,2 and HSCH₂CH(NH₂)CO₂⁻(H₂O)_1,2 for protonated and deprotonated forms, respectively. In contrast to ¹O₂ reactions with dehydrated protonated/deprotonated Cys of which hydroperoxide products all decomposed, reactions with hydrated protonated/deprotonated Cys yielded stable hydroperoxide products, analogous to photooxidation reaction of Cys in solution. We investigated the number of water ligands necessary to produce a stable hydroperoxide, and found that a single water molecule suffices—that is, to relax nascent, energized hydroperoxide in the hydrated cluster by elimination of water. Hydrated protonated Cys shows higher reaction efficiency than the hydrated deprotonated one, particularly with the addition of the second water ligand. Reactions of hydrated protonated/deprotonated Cys are suppressed by E_col, becoming negligible at E_col ≥ 0.5 eV. Density functional theory calculations were used to locate reaction coordinates for these systems. Quasi-classical, direct dynamics trajectory simulations were performed for HSCH₂CH(NH₃⁺)CO₂H(H₂O) + ¹O₂ at the B3LYP/4-31G(d) level of theory. Analysis of trajectories highlights the importance of complex mediation in the early stages of the reaction, and illustrates that water can catalyze proton transfer within the hydrated complex.