Guided-Ion-Beam Scattering and Direct Dynamics Trajectory Study on the Reaction of Deprotonated Cysteine with Singlet Molecular Oxygen

We present a study on the gas-phase reaction of deprotonated cysteine with the lowest electronically excited state of molecular oxygen O-2[a(1)Delta(g)], 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 O-1(2) (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 O-1(2), and the formation of OSCH2CH(NH2)CO2- radical via elimination of center dot 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.