title: Experimental and trajectory study on the reaction of protonated methionine with electronically excited singlet molecular oxygen (a1Δg): reaction dynamics and collision energy effects.
abstract: The reaction of protonated methionine with the lowest electronically excited state of molecular oxygen O(2)(a(1)Δ(g)) was studied in a guided ion beam apparatus, including the measurement of reaction cross sections over a center-of-mass collision energy (E(col)) range of 0.1-2.0 eV. A series of electronic structure and RRKM calculations were used to examine the properties of various complexes and transition states that might be important along the reaction coordinate. Only one product channel is observed, corresponding to generation of hydrogen peroxide via transfer of two hydrogen atoms (H2T) from protonated methionine to singlet oxygen. At low collision energies, the reaction approaches the collision limit and may be mediated by intermediate complexes. The reaction shows strong inhibition by collision energy, and becomes negligible at E(col) > 1.25 eV. A large set of quasi-classical direct dynamics trajectory simulations were calculated at the B3LYP/6-21G level of theory. Trajectories reproduced experimental results and provided insight into the mechanistic origin of the H2T reaction, how the reaction probability varies with impact parameter, and the suppressing effect of collision energy. Analysis of the trajectories shows that at E(col) = 1.0 eV the reaction is mediated by a precursor and/or hydroperoxide complex, and is sharply orientation-dependent. Only 20% of collisions have favorable reactant orientations at the collision point, and of those, less than half form precursor and hydroperoxide complexes which eventually lead to reaction. The narrow range of reactive collision orientations, together with physical quenching of (1)O(2) via intersystem crossing between singlet and triplet electronic states, may account for the low reaction efficiency observed at high E(col).
authors.affiliation: Department of Chemistry and Biochemistry, Queens College and the Graduate Center of the City University of New York, 65-30 Kissena Boulevard, Flushing, New York 11367, USA.
authors.forename: Yigang
authors.initials: Y
authors.lastname: Fang
authors.forename: Fangwei
authors.initials: F
authors.lastname: Liu
authors.forename: Andrew
authors.initials: A
authors.lastname: Bennett
authors.forename: Shamim
authors.initials: S
authors.lastname: Ara
authors.forename: Jianbo
authors.initials: J
authors.lastname: Liu
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D006861:Hydrogen Peroxide / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D008715:Methionine / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D055668:Photochemical Processes; D011522:Protons; D011789:Quantum Theory; D026082:Singlet Oxygen / Q000737:chemistry*; D053719:Tandem Mass Spectrometry
chemical_list: D011522:Protons; D026082:Singlet Oxygen; D008715:Methionine; D006861:Hydrogen Peroxide