Experimental and Trajectory Study on the Reaction of Protonated Methionine with Electronically Excited Singlet Molecular Oxygen (a¹Δ_g): 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).

Mass Spectrometry Study of Multiply Negatively Charged, Gas-Phase NaAOT Micelles: How Does Charge State Affect Micellar Structure and Encapsulation?

“…Mass spectra were acquired on a home-made guided-ionbeam tandem mass spectrometer described in detail previ- ously [8,9,[31][32][33][34]. The apparatus consists of an ESI [35,36] ion source, radio frequency (rf) hexapole ion guide [37], quadrupole mass filter, rf octopole ion guide surrounded by a scattering cell, second quadrupole mass filter, and a detector.…”

“…Sample solutions were sprayed into ambient atmosphere through an electrospray needle at a flow rate (0.03 or 0.04 ml h -1 ) optimized for each solution concentration. The electrospray needle was prepared in house [8,9,[31][32][33][34], and biased at −1,900 to −2,400 V relative to ground. Negatively charged droplets formed from ESI were fed into a heated desolvation capillary assembly.…”

Mechanistic and Kinetic Study of Singlet O2 Oxidation of Methionine by On-Line Electrospray Ionization Mass Spectrometry

“…We first investigated the oxidation of protonated and deprotonated Met in the absence of water [35,36]. The reaction systems were then augmented by adding explicit number of water ligands [35,36], which accounts for microsolvation. The parallel gas-and solution-phase study helps us evaluate evolution of Met oxidation dynamics and kinetics from the gas phase, through microsolvation, to the aqueous solution.…”

Alternatives to the ‘water oxidation pathway’ of biological ozone formation

“…2. Tomono et al [36] found that decomposition of the pure 1 O 2 generator, 1-methylnaphthalene-4-endoperoxide (MNPE) at 37°C and pH 7.4 in the presence of cholesterol resulted in the formation of secosterol aldehyde B and a small amount of secosterol aldehyde A, and that addition of IgG resulted in a decrease in the former aldehyde and an increase in the latter. Increased formation of secosterol A in the presence of IgG is easily explained by the antibodycatalyzed ozone formation from 1 O 2 .…”

“…Between pH 6 and 10, the reaction of methionine 4 with 1 O 2 in water proceeds via persulfoxide 5 and the cyclic dehydromethionine 6 to form methionine sulfoxide 7 [36] (Fig. 3) [39], the reaction of 4 with O 3 to form 7 may also proceed through a trioxysulfoxide adduct, 8 (Fig.…”