Reaction of Protonated Tyrosine with Electronically Excited Singlet Molecular Oxygen (a¹Δ_g): An Experimental and Trajectory Study

Abstract

Reaction of protonated tyrosine with the lowest electronically excited singlet state of molecular oxygen, (1)O(2) (a(1)Delta(g)), is reported over the center-of-mass collision energy (E(col)) range from 0.1 to 3.0 eV, using an electrospray-ionization, guided-ion-beam scattering instrument, in conjunction with ab initio electronic structure calculations and direct dynamics trajectory simulations. Only one product channel is observed, corresponding to generation of hydrogen peroxide via transfer of two hydrogen atoms from protonated tyrosine. Despite being exoergic, the reaction is in competition with physical quenching of (1)O(2) and is very inefficient. At low E(col), the reaction may be mediated by intermediate complexes and shows strong inhibition by collision energy. At high E(col), the reaction efficiency drops to approximately 1% and starts to have contribution from a direct mechanism. Quasi-classical trajectory simulations were performed to probe the mechanism at high collision energies. Analysis of trajectories shows that, at E(col) of 3.0 eV, a small fraction of hydrogen peroxide (25%) is produced via a direct, concerted mechanism where two hydrogen atoms are transferred simultaneously, but most hydrogen peroxide (75%) is formed by dissociation of hydroperoxide intermediates. According to ab initio calculations and trajectory simulations, collisions also lead to formation of various endoperoxides, and dissociation of endoperoxides may play a role in physical quenching of (1)O(2). The apparatus and experimental techniques are described in detail.

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

“…The operation, calibration, and data analysis procedures for the mass spectrometer were described previously [57]. In brief, the apparatus consists of an ion source, radio frequency (rf) hexapole ion guide, quadrupole mass filter, rf octopole ion guide surrounded by a scattering cell, second quadrupole mass filter, and a pulse-counting detector.…”

Oxidation of gas-phase hydrated protonated/deprotonated cysteine: how many water ligands are sufficient to approach solution-phase photooxidation chemistry?

“…To avoid the complexities/interferences arising from solutionphase experiments and simplify the interpretation of the oxidation mechanisms of amino acids, we have looked at the reactions of 1 O 2 with protonated and deprotonated amino acids in the gas phase [7][8][9][10][11] using electrospray-ionization (ESI) 12,13 mass spectrometry and guided-ion-beam scattering methods. 14 One advantage of investigating biomolecules in the gas phase is that it allows one to observe single molecules separated from bulk solution environments.…”