Singlet O2 Reactions with Radical Cations of 8-Bromoguanine and 8-Bromoguanosine: Guided-Ion Beam Mass Spectrometric Measurements and Theoretical Treatments

8-Bromoguanosine is generated in vivo as a biomarker for early inflammation. Its formation and secondary reactions lead to a variety of biological sequelae at inflammation sites, most of which are mutagenic and linked to cancer. Herein, we report the formation of radical cations of 8-bromoguanine (8BrG^•+) and 8-bromoguanosine (8BrGuo^•+) and their reactions toward the lowest excited singlet molecular oxygen (¹O₂)─a common reactive oxygen species generated in biological systems. This work aims to investigate synergistic, oxidatively generated damage of 8-brominated guanine and guanosine that may occur upon ionizing radiation, one-electron oxidation, and ¹O₂ oxidation. Capitalizing on measurements of reaction product ions and cross sections of 8BrG^•+ and 8BrGuo^•+ with ¹O₂ using guided-ion beam tandem mass spectrometry and augmented by computational modeling of the prototype reaction system, 8BrG^•+ + ¹O₂, using the approximately spin-projected ωB97XD/6-31+G(d,p) density functional theory, the coupled cluster DLPNO-CCSD(T)/aug-cc-pVTZ and the multireference CASPT2(21,15)/6-31G**, probable reaction products, and potential energy surfaces (PESs) were mapped out. 8BrG^•+ and 8BrGuo^•+ present similar exothermic oxidation products, and their reaction efficiencies with ¹O₂ increase with decreasing collision energy. Both single- and multireference theories predicted that the two most energetically favorable reaction pathways correspond to ¹O₂-addition to the C8 and C5-positions of 8BrG^•+, respectively. The CASPT2-calculated PES represents the best quantitative agreement with the experimental benchmark, in that the oxidation exothermicity is close to the water hydration energy of product ions and, thus, is able to eliminate a water ligand in the product ions.

Oxidation Dynamics of Methionine with Singlet Oxygen: Effects of Methionine Ionization and Microsolvation

We report an in-depth study on the gas-phase reactions of singlet O2[a(1)Δg] with methionine (Met) at different ionization and hydration states (including deprotonated [Met - H](-), hydrated deprotonated [Met - H](-)(H2O)1,2, and hydrated protonated MetH(+)(H2O)1,2), using guided-ion-beam scattering mass spectrometry. The measurements include the effects of collision energy (Ecol) on reaction cross sections over a center-of-mass Ecol range from 0.05 to 1.0 eV. The aim of this study is to probe the influences of Met ionization and hydration on its oxidation mechanism and dynamics. Density functional theory calculations, Rice-Ramsperger-Kassel-Marcus modeling, and quasi-classical, direct dynamics trajectory simulations were performed to examine the properties of various complexes and transition states that might be important along reaction coordinates, probe reaction potential energy surfaces, and to establish the atomic-level mechanism for the Met oxidation process. No oxidation products were observed for the reaction of [Met - H](-) with (1)O2 due to the high-energy barriers located in the product channels for this system. However, this nonreactive property was altered by the microsolvation of [Met - H](-); as a result, hydroperoxides were captured as the oxidation products for [Met - H](-)(H2O)1,2 + (1)O2. For the reaction of MetH(+)(H2O)1,2 + (1)O2, besides formation of hydroperoxides, an H2O2 elimination channel was observed. The latter channel is similar to what was found in the reaction of dehydrated MetH(+) with (1)O2 (J. Phys. Chem. B 2011, 115, 2671). The reactions of hydrated protonated and deprotonated Met are all inhibited by Ecol, becoming negligible at Ecol ≥ 0.5 eV. The kinetic and dynamical consequences of microsolvation on Met oxidation and their biological implications are discussed.

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

Oxidation of histidine by (1)O2 is an important process associated with oxidative damage to proteins during aging, diseases and photodynamic therapy of tumors and jaundice, and photochemical transformations of biological species in the troposphere. However, the oxidation mechanisms and products of histidine differ dramatically in these related environments which range from the gas phase through aerosols to aqueous solution. Herein we report a parallel gas- and solution-phase study on the (1)O2 oxidation of histidine, aimed at evaluating the evolution of histidine oxidation pathways in different media and at different ionization states. We first investigated the oxidation of protonated and deprotonated histidine ions and the same systems hydrated with explicit water molecules in the gas phase, using guided-ion-beam-scattering mass spectrometry. Reaction coordinates and potential energy surfaces for these systems were established on the basis of density functional theory calculations, Rice-Ramsperger-Kassel-Marcus modeling and direct dynamics simulations. Subsequently we tracked the oxidation process of histidine in aqueous solution under different pH conditions, using on-line UV-Vis spectroscopy and electrospray mass spectrometry monitoring systems. The results show that two different routes contribute to the oxidation of histidine depending on its ionization states. In each mechanism hydration is essential to suppressing the otherwise predominant dissociation of reaction intermediates back to reactants. The oxidation of deprotonated histidine in the gas phase involves the formation of 2,4-endoperoxide and 2-hydroperoxide of imidazole. These intermediates evolve to hydrated imidazolone in solution, and the latter either undergoes ring-closure to 6α-hydoxy-2-oxo-octahydro-pyrrolo[2,3-d]imidazole-5-carboxylate or cross-links with another histidine to form a dimeric product. In contrast, the oxidation of protonated histidine is mediated by 2,5-endoperoxide and 5-hydroperoxide, which convert to stable hydrated imidazolone end-product in solution. The contrasting mechanisms and reaction efficiencies of protonated vs. deprotonated histidine, which lead to pH dependence in the photooxidation of histidine, are interpreted in terms of the chemistry of imidazole with (1)O2. The biological implications of the results are also discussed.

Collision dynamics of protonated N-acetylmethionine with singlet molecular oxygen (a(1)Δg): the influence of the amide bond and ruling out the complex-mediated mechanism at low energies

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.