Reactions of deprotonated tyrosine and tryptophan with electronically excited singlet molecular oxygen (a1Δ(g)): a guided-ion-beam scattering, statistical modeling, and trajectory study.

The reactions of deprotonated tyrosine (Tyr-H) and tryptophan (Trp-H) with the lowest electronically excited state of molecular oxygen O(2)[a(1)Δ(g)] have been studied in the gas phase, including the measurement of the effects of collision energy (E(col)) on reaction cross sections over a center-of-mass E(col) range from 0.05 to 1.0 eV. Tyr-H and Trp-H were generated using electrospray ionization, and both have a pure carboxylate anion structure in the gas phase. Density functional theory calculations and RRKM modeling were used to examine properties of various complexes, transition states, and products that might be important along the reaction coordinate. It was found that deprotonation of Tyr and Trp results in a large effect on their (1)O(2)-mediated oxidation. For Tyr-H, the reaction corresponds to the formation of a hydroperoxide intermediate, followed by intramolecular H transfer and subsequent dissociation to product ion 4-(2-aminovinyl)phenolate, and neutral H(2)O(2) and CO(2). Despite that the reaction is 1.83 eV exothermic, the reaction cross section shows a threshold-like behavior at low E(col) and increases with increasing E(col), suggesting that the reaction bears an activation barrier above the reactants. Quasi-classical, direct dynamics trajectory simulations were carried out for Tyr-H + (1)O(2) at E(col) = 0.75 eV, using B3LYP/4-31G* level of theory. Trajectories demonstrated the intermediacy of complexes at the early stage of the reaction. A similar product channel was observed in the reaction of Trp-H with (1)O(2), yielding product ion 3-(2-aminovinyl)indol-1-ide, H(2)O(2) and CO(2). However, the reaction cross section of Trp-H is strongly suppressed by E(col) and becoming negligible at E(col) > 1.0 eV, indicating that this reaction proceeds without energy barriers above the reactants.