Rotational Line Strengths and Self-Pressure-Broadening Coefficients for the 1.27-microm, a (1)D(g)-X (3)?(g)(-), v = 0-0 Band of O(2)

Singlet Oxygen Photophysics: From Liquid Solvents to Mammalian Cells

Molecular oxygen, O2, has long provided a cornerstone for studies in chemistry, physics, and biology. Although the triplet ground state, O2(X3Σg-), has garnered much attention, the lowest excited electronic state, O2(a1Δg), commonly called singlet oxygen, has attracted appreciable interest, principally because of its unique chemical reactivity in systems ranging from the Earth's atmosphere to biological cells. Because O2(a1Δg) can be produced and deactivated in processes that involve light, the photophysics of O2(a1Δg) are equally important. Moreover, pathways for O2(a1Δg) deactivation that regenerate O2(X3Σg-), which address fundamental principles unto themselves, kinetically compete with the chemical reactions of O2(a1Δg) and, thus, have practical significance. Due to technological advances (e.g., lasers, optical detectors, microscopes), data acquired in the past ∼20 years have increased our understanding of O2(a1Δg) photophysics appreciably and facilitated both spatial and temporal control over the behavior of O2(a1Δg). One goal of this Review is to summarize recent developments that have broad ramifications, focusing on systems in which oxygen forms a contact complex with an organic molecule M (e.g., a liquid solvent). An important concept is the role played by the M+•O2-• charge-transfer state in both the formation and deactivation of O2(a1Δg).

Effects of Intra-Base Pair Proton Transfer on Dissociation and Singlet Oxygenation of 9-Methyl-8-Oxoguanine-1-Methyl-Cytosine Base-Pair Radical Cations

8-Oxoguanosine is the most common oxidatively generated base damage and pairs with complementary cytidine within duplex DNA. The 8-oxoguanosine-cytidine lesion, if not recognized and removed, not only leads to G-to-T transversion mutations but renders the base pair being more vulnerable to the ionizing radiation and singlet oxygen (1 O2 ) damage. Herein, reaction dynamics of a prototype Watson-Crick base pair [9MOG ⋅ 1MC]⋅+ , consisting of 9-methyl-8-oxoguanine radical cation (9MOG⋅+ ) and 1-methylcystosine (1MC), was examined using mass spectrometry coupled with electrospray ionization. We first detected base-pair dissociation in collisions with the Xe gas, which provided insight into intra-base pair proton transfer of 9MOG⋅+  ⋅ 1MC← → ${{\stackrel{ {\rightarrow} } { {\leftarrow} } } }$ [9MOG - HN1 ]⋅ ⋅ [1MC+HN3' ]+ and subsequent non-statistical base-pair separation. We then measured the reaction of [9MOG ⋅ 1MC]⋅+ with 1 O2 , revealing the two most probable pathways, C5-O2 addition and HN7 -abstraction at 9MOG. Reactions were entangled with the two forms of 9MOG radicals and base-pair structures as well as multi-configurations between open-shell radicals and 1 O2 (that has a mixed singlet/triplet character). These were disentangled by utilizing approximately spin-projected density functional theory, coupled-cluster theory and multi-referential electronic structure modeling. The work delineated base-pair structural context effects and determined relative reactivity toward 1 O2 as [9MOG - H]⋅>9MOG⋅+ >[9MOG - HN1 ]⋅ ⋅ [1MC+HN3' ]+ ≥9MOG⋅+  ⋅ 1MC.

Probing the Transition State-to-Intermediate Continuum: Mechanistic Distinction between a Dry versus Wet Perepoxide in the Singlet Oxygen "Ene" Reaction at the Air-Water Interface

A mechanistic study is reported for the reactions of singlet oxygen (¹O₂) with alkene surfactants of tunable properties. Singlet oxygen was generated either top-down (photochemically) by delivery as a gas to an air-water interface or bottom-up (chemically) by transport to the air-water interface as a solvated species. In both cases, reactions were carried out in the presence of 7-carbon (7C), 9-carbon (9C), or 11-carbon (11C) prenylsurfactants [(CH₃)₂C═CH(CH₂)_nSO₃^- Na⁺ (n = 4, 6, 8)]. Higher "ene" hydroperoxide regioselectivities (secondary ROOH 2 to tertiary ROOH 3) were reached in delivering ¹O₂ top-down through air as compared to bottom-up via aqueous solution. In the photochemical reaction, ratios of 2:3 increased from 2.5:1 for 7C, to 2.8:1 for 9C, and to 3.2:1 for 11C. In contrast, in the bubbling system that generated ¹O₂ chemically, the selectivity was all but lost, ranging only from 1.3:1 to 1:1. The phase-dependent regioselectivities appear to be correlated with the "ene" reaction with photochemically generated, drier ¹O₂ at the air-water interface vs those with wetter ¹O₂ from the bubbling reactor. Density functional theory-calculated reaction potential energy surfaces (PESs) were used to help rationalize the reaction phase dependence. The reactions in the gas phase are mediated by perepoxide transition states with 32-41 kJ/mol binding energy for C═C(π)···¹O₂. The perepoxide species, however, evolve to well-defined stationary structures in the aqueous phase, with covalent C-O bonds and 85-88 kJ/mol binding energy. The combined experimental and computational evidence points to a unique mechanism for ¹O₂ "ene" tunability in a perepoxide continuum from a transition state to an intermediate.

Production and loss of O2(1Δ g ) at atmospheric pressure using microwave-driven microplasmas

We have used arrays of microwave-generated microplasmas operating at atmospheric pressure to generate high concentrations of singlet molecular oxygen, O₂(¹Δ _g ), which is of interest for biomedical applications. The discharge is sustained by a pair of microstrip-based microwave resonator arrays which force helium/oxygen gas mixtures through a narrow plasma channel. We have demonstrated the efficacy of both NO and less-hazardous N₂O additives for suppression of ozone and associated enhancement of the O₂(¹Δ _g ) yield. Quenching of O₂(¹Δ _g ) by ozone is sufficiently suppressed such that quenching by ground state molecular oxygen becomes the dominant loss mechanism in the post-discharge outflow. We verified the absence of other significant gas-phase quenching mechanisms by measuring the O₂(¹Δ _g ) decay along a quartz flow tube. These measurements indicated a first-order rate constant of (1.2 ± 0.3) × 10^-24 m³ s^-1, slightly slower than but consistent with prior measurements of singlet oxygen quenching on ground state oxygen. The discharge-initiated reaction mechanisms and data analysis are discussed in terms of a chemical kinetics model of the system.

Singlet Oxygen Oxidation of the Radical Cations of 8-Oxo-2'-deoxyguanosine and Its 9-Methyl Analogue: Dynamics, Potential Energy Surface, and Products Mediated by C5-O2 -Addition

8-Oxo-2'-deoxyguanosine (OG) is the most common DNA lesion. Notably, OG becomes more susceptible to oxidative damage than the undamaged nucleoside, forming mutagenic products in vivo. Herein the reactions of singlet O₂ with the radical cations of 8-oxo-2'-deoxyguanosine (OG^.+ ) and 9-methyl-8-oxoguanine (9MOG^.+ ) were investigated using ion-molecule scattering mass spectrometry, from which barrierless, exothermic O₂ -addition products were detected for both reaction systems. Corroborated by static reaction potential energy surface constructed using multi-reference CASPT2 theory and molecular dynamics simulated in the presence of the reactants' kinetic and internal energies, the C5-terminal O₂ -addition was pinpointed as the most probable reaction pathway. By elucidating the reaction mechanism, kinetics and dynamics, and reaction products and energetics, this work constitutes the first report unraveling the synergetic damage of OG by ionizing radiation and singlet O₂ .

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

We report a kinetics and mechanistic study on the ¹O₂ oxidation of 9-methylguanine (9MG) and the cross-linking of the oxidized intermediate 2-amino-9-methyl-9H-purine-6,8-dione (9MOG^OX) with N^α-acetyl-lysine-methyl ester (abbreviated as LysNH₂) in aqueous solutions of different pH. Experimental measurements include the determination of product branching ratios and reaction kinetics using mass spectrometry and absorption spectroscopy, and the characterization of product structures by employing collision-induced dissociation. Strong pH dependence was revealed for both 9MG oxidation and the addition of nucleophiles (water and LysNH₂) at the C5 position of 9MOG^OX. The ¹O₂ oxidation rate constant of 9MG was determined to be 3.6 × 10⁷ M^-1·s^-1 at pH 10.0 and 0.3 × 10⁷ M^-1·s^-1 at pH 7.0, both of which were measured in the presence of 15 mM LysNH₂. The ωB97XD density functional theory coupled with various basis sets and the SMD implicit solvation model was used to explore the reaction potential energy surfaces for the ¹O₂ oxidation of 9MG and the formation of C5-water and C5-LysNH₂ adducts of 9MOG^OX. Computational results have shed light on reaction pathways and product structures for the different ionization states of the reactants. The present work has confirmed that the initial ¹O₂ addition represents the rate-limiting step for the oxidative transformations of 9MG. All of the downstream steps are exothermic with respect to the starting reactants. The C5-cross-linking of 9MOG^OX with LysNH₂ significantly suppressed the formation of spiroiminodihydantoin (9MSp) resulting from the C5-water addition. The latter became dominant only at the low concentration (∼1 mM) of LysNH₂.

Singlet O2 Oxidation of a Deprotonated Guanine-Cytosine Base Pair and Its Entangling with Intra-Base-Pair Proton Transfer

We report an experimental and computational study on the ¹ O₂ oxidation of gas-phase deprotonated guanine-cytosine base pair [G ⋅ C-H]^- that is composed of 9HG ⋅ [C-H]^- and 7HG ⋅ [C-H]^- (pairing 9H- or 7H-guanine with N1-deprotonated cytosine), and 9HG ⋅ [C-H]^- _PT and 7HG ⋅ [C-H]^- _PT (formed by intra-base-pair proton transfer from the N1 of guanine to the N3 of [C-H]^- ). The conformer-averaged reaction product ions and cross section were measured over a center-of-mass collision energy range from 0.1 to 0.5 eV using a guided-ion-beam tandem mass spectrometer. To explore conformation-specific reactivity, collision dynamics of ¹ O₂ with each of the four [G ⋅ C-H]^- conformers was simulated at B3LYP/6-31G(d). Trajectories showed that the ¹ O₂ oxidation of the base pair entangles with intra-base-pair proton transfer, and prefers to occur in a collision when the base pair adopts a proton-transferred structure; trajectories also indicate that the 9HG-containing base pair favors stepwise formation of 4,8-endoperoxide of guanine, whereas the 7HG-containing base pair prefers concerted formation of guanine 5,8-endoperoxide. Using trajectory results as a guide, potential energy surfaces (PESs) along all possible reaction pathways were established using the approximately spin-projected ωB97XD/6-311++G(d,p)//B3LYP/6-311++G(d,p) method. PESs have not only rationalized trajectory findings but provided more accurate energetics and indicated that the proton-transferred base-pair conformers have lower activation barriers for oxidation than their non-proton-transferred counterparts.

Capturing Transient Endoperoxide in the Singlet Oxygen Oxidation of Guanine

The chemistry of singlet O2 toward the guanine base of DNA is highly relevant to DNA lesion, mutation, cell death, and pathological conditions. This oxidative damage is initiated by the formation of a transient endoperoxide through the Diels-Alder cycloaddition of singlet O2 to the guanine imidazole ring. However, no endoperoxide formation was directly detected in native guanine or guanosine, even at -100 °C. Herein, gas-phase ion-molecule scattering mass spectrometry was utilized to capture unstable endoperoxides in the collisions of hydrated guanine ions (protonated or deprotonated) with singlet O2 at ambient temperature. Corroborated by results from potential energy surface exploration, kinetic modeling, and dynamics simulations, various aspects of endoperoxide formation and transformation (including its dependence on guanine ionization and hydration states, as well as on collision energy) were determined. This work has pieced together reaction mechanisms, kinetics, and dynamics data concerning the early stage of singlet O2 induced guanine oxidation, which is missing from conventional condensed-phase studies.

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

We report a reaction apparatus developed to monitor singlet oxygen ((1)O2) reactions in solution using on-line ESI mass spectrometry and spectroscopy measurements. (1)O2 was generated in the gas phase by the reaction of H2O2 with Cl2, detected by its emission at 1270 nm, and bubbled into aqueous solution continuously. (1)O2 concentrations in solution were linearly related to the emission intensities of airborne (1)O2, and their absolute scales were established based on a calibration using 9,10-anthracene dipropionate dianion as an (1)O2 trapping agent. Products from (1)O2 oxidation were monitored by UV-Vis absorption and positive/negative ESI mass spectra, and product structures were elucidated using collision-induced dissociation-tandem mass spectrometry. To suppress electrical discharge in negative ESI of aqueous solution, methanol was added to electrospray via in-spray solution mixing using theta-glass ESI emitters. Capitalizing on this apparatus, the reaction of (1)O2 with methionine was investigated. We have identified methionine oxidation intermediates and products at different pH, and measured reaction rate constants. (1)O2 oxidation of methionine is mediated by persulfoxide in both acidic and basic solutions. Persulfoxide continues to react with another methionine, yielding methionine sulfoxide as end-product albeit with a much lower reaction rate in basic solution. Density functional theory was used to explore reaction potential energy surfaces and establish kinetic models, with solvation effects simulated using the polarized continuum model. Combined with our previous study of gas-phase methionine ions with (1)O2, evolution of methionine oxidation pathways at different ionization states and in different media is described.

How protonation and deprotonation of 9-methylguanine alter its singlet O2 addition path: about the initial stage of guanine nucleoside oxidation

Protonated and deprotonated 9-methylguanine follow completely different oxidation routes.

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.

Kinetics for the Reactions of O- and O2- with O2(aΔg) Measured in a Selected Ion Flow Tube at 300 K

The kinetics of the reactions of O- and O2- with O2(a1Deltag) have been studied at 300 K in a selected ion flow tube (SIFT). The O2(a1Deltag) concentrations have been determined using emission at 1270 nm from the O2(a1Deltag, v=0-->X3Sigmag-, v=0) transition measured with an InGaAs detector calibrated against absolute spectrally dispersed emission measurements. The rate constants measured for O- and O2- are 1.1x10(-10) and 6.6x10(-10) cm3 s-1, respectively, with uncertainties of +/-35%. The O2- reaction only produces electrons and can be described as Penning detachment, while the O- reaction has been found to produce both O2- and e-. The O2- branching fraction has a lower limit of approximately 0.30. Comparison of the present results to previous measurements found in the literature provides a resolution to a previous discrepancy in the rate constant values.