Guided-Ion-Beam Scattering and Direct Dynamics Trajectory Study on the Reaction of Deprotonated Cysteine with Singlet Molecular Oxygen
2013
Fang, Yigang | Liu, Fangwei | Emre, Rifat | Liu, Jianbo
We present a study on the gas-phase reaction of deprotonated cysteine with the lowest electronically excited state of molecular oxygen O₂[a¹Δg], including the measurement of the effects of collision energy (Ecₒₗ) on reaction cross sections over a center-of-mass Ecₒₗ 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 Ecₒₗ range. This product channel is driven by the electronic excitation energy of ¹O₂ (the so-called dissociative excitation transfer), and is strongly suppressed by Ecₒₗ. Two minor channels correspond to the formation of HSCH₂C(NH)CO₂– + H₂O₂ via abstraction of two hydrogen atoms from HSCH₂CH(NH₂)CO₂– by ¹O₂, 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 Ecₒₗ = 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.
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