Collision Dynamics of Protonated N-Acetylmethionine with Singlet Molecular Oxygen (a¹Δ_g): The Influence of the Amide Bond and Ruling Out the Complex-Mediated Mechanism at Low Energies

Abstract

It has been proposed (J. Phys. Chem. B 2011, 115, 2671) that the ammonium group is involved in the gas-phase reaction of protonated methionine (MetH(+)) with singlet oxygen (1)O2, yielding hydrogen peroxide and a dehydro compound of MetH(+) where the -NH3(+) transforms into cyclic -NH2-. For the work reported, the gas-phase reaction of protonated N-acetylmethionine (Ac-MetH(+)) with (1)O2 was examined, including the measurements of reaction products and cross sections over a center-of-mass collision energy (Ecol) range from 0.05 to 1.0 eV using a guided-ion-beam apparatus. The aim is to probe how the acetylation of the ammonium group affects the oxidation chemistry of the ensuing Ac-MetH(+). Properties of intermediates, transition states, and products along the reaction coordinate were explored using density functional theory calculations and Rice-Ramsperger-Kassel-Marcus (RRKM) modeling. Direct dynamics trajectory simulations were carried out at Ecol of 0.05 and 0.1 eV using the B3LYP/4-31G(d) level of theory. In contrast to the highly efficient reaction of MetH(+) + (1)O2, the reaction of Ac-MetH(+) + (1)O2 is extremely inefficient, despite there being exoergic pathways. Two product channels were observed, corresponding to transfer of two H atoms from Ac-MetH(+) to (1)O2 (H2T), and methyl elimination (ME) from a sulfone intermediate complex. Both channels are inhibited by collision energies, becoming negligible at Ecol > 0.2 eV. Analysis of RRKM and trajectory results suggests that a complex-mediated mechanism might be involved at very low Ecol, but direct, nonreactive collisions prevail over the entire Ecol range and physical quenching of (1)O2 occurs during the early stage of collisions.

Evolution of oxidation dynamics of histidine: non-reactivity in the gas phase, peroxides in hydrated clusters, and pH dependence in solution

“…However, gas-phase experiments are no less important considering their capability in providing detailed pictures of reactions and their environmental and atmospheric implications. We have investigated the reactions of 1 O 2 with AAs and model dipeptides in the gas phase [26][27][28][29][30][31] using ionbeam-scattering methods 32 and electrospray-ionization mass spectrometry (ESI MS). 33 Gas-phase experiments can distinguish the intrinsic reactivities of AAs from solvent and counter-ion effects, providing a basis for understanding their photooxidation mechanisms.…”

Reaction Kinetics, Product Branching, and Potential Energy Surfaces of 1O2-Induced 9-Methylguanine–Lysine Cross-Linking: A Combined Mass Spectrometry, Spectroscopy, and Computational Study

“…According to the calibration results of effective [ 1 O 2 ] in the reaction solutions (see details in the Supporting Information), 58% of solution-phase 1 O 2 was physically quenched by collisions with 15 mM LysNH 2 (most likely due to its amide group). Consequently, the 1 O 2 oxidation of 9MG in the presence of LysNH 2 appeared to be much slower compared to that in the pure 9MG solution.…”

“…On the basis of its p K a values, LysNH 2 is completely protonated at pH 7.0 and consists of 78% protonated LysNH 3 + and 22% neutral LysNH 2 at pH 10.0. LysNH 2 has three basic sites, which can be protonated: the -N ε H 2 group (p K a = 10.54), the -N α H group (p K a = 1.25 estimated from that of protonated N -methylacetamide), and the carbonyl oxygen of the N -acetyl group (as the charge in the ensuing HNCOH + is stabilized by amide resonance with a protonated imine structure HN + C-OH). , To identify the global minimum LysNH 3 + , we calculated all three protonated structures for each of the first 10 low-energy LysNH 2 conformers in Figure S3. The resulting 32 protonated structures, their relative enthalpies at 298 K, and Cartesian coordinates are provided in Figure S4 and in the Supporting Information.…”

“…These could presumably facilitate crossing reaction barriers and forming the 2,5-dione products from [LysNH− 9MSp − H] − and 5-LysNH 2 + −9MOG. 81 ). Consequently, the 1 O 2 oxidation of 9MG in the presence of LysNH 2 appeared to be much slower compared to that in the pure 9MG solution.…”