We present a study on the gas-phase reaction of deprotonated cysteine with the lowest electronically excited state of molecular oxygen O2[a(1)Δg], including the measurement of the effects of collision energy (E(col)) on reaction cross sections over a center-of-mass E(col) range from 0.1 to 1.0 eV. Deprotonated cysteine was generated using electrospray ionization, and has a carboxylate anionic structure (HSCH2CH(NH2)CO2(-)) in the gas phase. Three product ion channels were observed. The dissociation of HSCH2CH(NH2)CO2(-) to NH2CH2CO2(-) and neutral CH2S has the largest cross section over the entire E(col) range. This product channel is driven by the electronic excitation energy of (1)O2 (the so-called dissociative excitation transfer), and is strongly suppressed by E(col). Two minor channels correspond to the formation of HSCH2C(NH)CO2(-) + H2O2 via abstraction of two hydrogen atoms from HSCH2CH(NH2)CO2(-) by (1)O2, and the formation of OSCH2CH(NH2)CO2(-) radical via elimination of ·OH from an intermediate complex, respectively. Density functional theory calculations were used to locate various complexes, transition states, and products. Quasi-classical direct dynamics trajectory simulations were carried out at E(col) = 0.2 eV using the B3LYP/4-31G(d) level of theory. Trajectory results were used to guide the construction of a reaction coordinate, discriminate between different mechanisms, and provide additional mechanistic insights. Analysis of trajectories highlights the importance of complex mediation at the early stages of all reactions, and suggests a partially concerted mechanism for H2O2 elimination.
We present a study on the reactions of singlet oxygen O2[a(1)Δg] 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 (1)O2 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) + (1)O2 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.
It has been proposed (J. Phys. Chem. B 2011, 115, 2671) that the ammonium group is involved in the gas-phase reaction of protonated methionine (MetH(+)) with singlet oxygen (1)O2, yielding hydrogen peroxide and a dehydro compound of MetH(+) where the -NH3(+) transforms into cyclic -NH2-. For the work reported, the gas-phase reaction of protonated N-acetylmethionine (Ac-MetH(+)) with (1)O2 was examined, including the measurements of reaction products and cross sections over a center-of-mass collision energy (Ecol) range from 0.05 to 1.0 eV using a guided-ion-beam apparatus. The aim is to probe how the acetylation of the ammonium group affects the oxidation chemistry of the ensuing Ac-MetH(+). Properties of intermediates, transition states, and products along the reaction coordinate were explored using density functional theory calculations and Rice-Ramsperger-Kassel-Marcus (RRKM) modeling. Direct dynamics trajectory simulations were carried out at Ecol of 0.05 and 0.1 eV using the B3LYP/4-31G(d) level of theory. In contrast to the highly efficient reaction of MetH(+) + (1)O2, the reaction of Ac-MetH(+) + (1)O2 is extremely inefficient, despite there being exoergic pathways. Two product channels were observed, corresponding to transfer of two H atoms from Ac-MetH(+) to (1)O2 (H2T), and methyl elimination (ME) from a sulfone intermediate complex. Both channels are inhibited by collision energies, becoming negligible at Ecol > 0.2 eV. Analysis of RRKM and trajectory results suggests that a complex-mediated mechanism might be involved at very low Ecol, but direct, nonreactive collisions prevail over the entire Ecol range and physical quenching of (1)O2 occurs during the early stage of collisions.
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