Deactivation Of Singlet Oxygen Research Articles

Nonadiabatic Tunneling in Chemical Reactions.

Quantum tunneling can have a dramatic effect on chemical reaction rates. In nonadiabatic reactions such as electron transfers or spin crossovers, nuclear tunneling effects can be even stronger than for adiabatic proton transfers. Ring-polymer instanton theory enables molecular simulations of tunneling in full dimensionality and has been shown to be far more reliable than commonly used separable approximations. First-principles instanton calculations predict significant nonadiabatic tunneling of heavy atoms even at room temperature and give excellent agreement with experimental measurements for the intersystem crossing of two nitrenes in cryogenic matrix isolation, the spin-forbidden relaxation of photoexcited thiophosgene in the gas phase, and singlet oxygen deactivation in water at ambient conditions. Finally, an outlook of further theoretical developments is discussed.

Heavy-atom tunnelling in singlet oxygen deactivation predicted by instanton theory with branch-point singularities

The reactive singlet state of oxygen (O2) can decay to the triplet ground state nonradiatively in the presence of a solvent. There is a controversy about whether tunnelling is involved in this nonadiabatic spin-crossover process. Semiclassical instanton theory provides a reliable and practical computational method for elucidating the reaction mechanism and can account for nuclear quantum effects such as zero-point energy and multidimensional tunnelling. However, the previously developed instanton theory is not directly applicable to this system because of a branch-point singularity which appears in the flux correlation function. Here we derive a new instanton theory for cases dominated by the singularity, leading to a new picture of tunnelling in nonadiabatic processes. Together with multireference electronic-structure theory, this provides a rigorous framework based on first principles that we apply to calculate the decay rate of singlet oxygen in water. The results indicate a new reaction mechanism that is 27 orders of magnitude faster at room temperature than the classical process through the minimum-energy crossing point. We find significant heavy-atom tunnelling contributions as well as a large temperature-dependent H2O/D2O kinetic isotope effect of approximately 20, in excellent agreement with experiment.

Phosphonated mesoporous silica nanoparticles bearing ruthenium complexes used as molecular probes for tracking oxygen levels in cells and tissues.

Molecular oxygen plays an important role in living organisms. Its concentration and fluctuation in cells or tissues are related to many diseases. Therefore, there is a need for molecular systems that can be used to detect and quantify oxygen levels in vitro and in vivo. In this study, we synthesized phosphonated mesoporous silica nanoparticles bearing ruthenium complexes in their pores (pM-Rus) and evaluated their photophysical and biological properties. The pM-Rus were highly soluble in water and showed robust phosphorescence under hypoxic conditions, while the addition of oxygen suppressed this emission. Cellular experiments revealed that pM-Rus with a size of 100 nm showed efficient cellular uptake to emit phosphorescence in hypoxic cells. In addition, pM-Rus have negligible toxicity to cells due to the blockage of direct contact between ruthenium complexes and intracellular biomolecules and the deactivation of singlet oxygen (1O2) generated by photoexcitation of ruthenium complexes before leaking out of the pores. Animal experiments confirmed that pM-Rus showed robust emission at hypoxic regions in mice. Thus, pM-Rus are promising oxygen probes for living systems.

Heme is responsible for enhanced singlet oxygen deactivation in cytochrome c.

The deactivation of singlet oxygen, the lowest electronic excited state of molecular oxygen, by proteins is usually described through the interaction of singlet oxygen with certain amino acids. Changes in accessibility of these amino acids influence the quenching rate and the phosphorescence kinetics of singlet oxygen. In the cellular environment, however, numerous proteins with covalently bound or encapsulated cofactors are present. These cofactors could also influence the deactivation of singlet oxygen, and these have received little attention. To confront this issue, we used cytochrome c (cyt c) and apocytochrome c (apocyt c) to illustrate how the heme prosthetic group influences the rate constant of singlet oxygen deactivation upon acidic pH-induced conformational change of cyt c. Photo-excited flavin mononucleotide (FMN) was used to produce singlet oxygen. Our data show that the heme group has a significant and measurable effect on singlet oxygen quenching when the heme is exposed to solvents and is therefore more accessible to singlet oxygen. The effect of amino acids and heme accessibility on the FMN triplet state deactivation was also investigated.

Unraveling the Effect of Singlet Oxygen on Metal-O2 Batteries: Strategies Toward Deactivation.

Aprotic metal-O2 batteries have attracted the interest of the research community due to their high theoretical energy density that target them as potential energy storage systems for automotive applications. At present, these devices show various practical problems, which hinder the attainment of the high theoretical energy densities. Among the main limitations, we can highlight the irreversible parasitic reactions that lead to premature death of the battery. The degradation processes, mainly related to the electrolyte, lead to the formation of secondary products that accumulate throughout the cycling in the air electrode. This accumulation of predominantly insulating products results in the blocking of active sites, promoting less efficiency in system performance. Recently, it has been discovered that the superoxide intermediate radical anion is involved in the generation of the reactive oxygen singlet species (1O2) in metal-O2 batteries. The presence of singlet oxygen is intimately linked with electrolyte degradation processes and with carbon-electrode corrosion reactions. This review analyzes the nature of singlet oxygen, while clarifying its toxic role in metal-O2 batteries. Besides, the main mechanisms of deactivation of singlet oxygen are presented, trying to inspire the research community in the development of new molecules capable of mitigating the harmful effects related to this highly reactive species.

Modeling the Effect of Solvents on Nonradiative Singlet Oxygen Deactivation: Going beyond Weak Coupling in Intermolecular Electronic-to-Vibrational Energy Transfer.

For almost 50 years, attempts have been made to account for the pronounced solvent effect on the lifetime of singlet molecular oxygen, O2(a1Δg). This process is dominated by the O2(a1Δg) → O2(X3Σg-) nonradiative transition. Given the comparatively low O2(a1Δg) excitation energy of ∼7880 cm-1, existing models have been built upon a foundation of electronic-to-vibrational (e-to-v) energy transfer in which C-H and O-H stretching modes in the solvent act as the dominant energy sink. The latter accounts for large H/D solvent isotope effects on the O2(a1Δg) lifetime. However, recent experiments showing a pronounced temperature effect on the O2(a1Δg) lifetime in some solvents reveal limitations in these models. We have developed a general and computationally tenable model that accounts for both temperature and H/D solvent isotope effects on the O2(a1Δg) lifetime. A key feature of our approach is the need to strike a balance in the oxygen-solvent interaction between weak and strong coupling. In the weak coupling limit, the O2(a1Δg) → O2(X3Σg-) transition probability is determined by the overlap of vibrational wave functions, and this is the main component defining the H/D isotope effects. In the strong coupling limit, the transition probability is determined by an activated process and thus accounts for the observed temperature dependence. In addition to resolving a long-standing oxygen-dependent problem, our model may provide useful insights into a wide range of bimolecular interactions that involve e-to-v energy transfer.

Deactivation of singlet oxygen by cerium oxide nanoparticles

The influence of cerium oxide nanoparticles on singlet oxygen generation upon photosensitizer irradiation was studied. Solutions of tetra-15-crown-5-phthalocyanines (aluminium(III), gallium(III) and indium(III) hydroxide, 1Al, 1Ga, 1In) in DMSO were used as photosensitizers.Ceria nanoparticles (CNPs) were shown to inhibit the photobleaching process of the crown-phthalocyanines induced by UV and visible irradiation of these photosensitizers. The photoprotective properties of CNPs were shown to depend on their size and lattice parameter, with these in turn being determined by conditions of CNPs’ synthesis. In addition, the deactivation of the reactive oxygen species and the UV-shielding effect of ceria nanoparticles were demonstrated.

Morin Flavonoid Adsorbed on Mesoporous Silica, a Novel Antioxidant Nanomaterial.

Morin (2´,3, 4´,5,7-pentahydroxyflavone) is a flavonoid with several beneficial health effects. However, its poor water solubility and it sensitivity to several environmental factors avoid its use in applications like pharmaceutical and cosmetic. In this work, we synthetized morin-modified mesoporous silica nanoparticles (AMSNPs-MOR) as useful material to be used as potential nanoantioxidant. To achieve this, we characterized its adsorption kinetics, isotherm and the antioxidant capacity as hydroxyl radical (HO•) scavenger and singlet oxygen (1O2) quencher. The experimental data could be well fitted with Langmuir, Freundlich and Temkin isotherm models, besides the pseudo-second order kinetics model. The total quenching rate constant obtained for singlet oxygen deactivation by AMSNPs-MOR was one order of magnitude lower than the morin rate constant reported previously in neat solvents and lipid membranes. The AMSNPs-MOR have good antioxidant properties by itself and exhibit a synergic effect with morin on the antioxidant property against hydroxyl radical. This effect, in the range of concentrations studied, was increased when the amount of morin adsorbed increased.

Holographic monitoring of spatial distributions of singlet oxygen in water

A method for monitoring spatial distributions of singlet oxygen in biological media has been developed. Singlet oxygen was generated using Radachlorin® photosensitizer, while thermal disturbances caused by nonradiative deactivation of singlet oxygen were detected by the holographic interferometry technique. Processing of interferograms yields temperature maps that characterize the deactivation process and show the distribution of singlet oxygen species.

Mechanism of singlet oxygen deactivation in an electric discharge oxygen – iodine laser

We have determined the influence of the reaction of molecular singlet oxygen with a vibrationally excited ozone molecule on the removal rate of in an electric-discharge-driven oxygen – iodine laser. This reaction has been shown to be a major channel of loss at the output of an electric-discharge singlet oxygen generator. In addition, it can also contribute significantly to the loss of in the discharge region of the generator.

Functions of tocopherols in the cells of plants and other photosynthetic organisms.

Tocopherol synthesis has only been observed in photosynthetic organisms (plants, algae and some cyanobacteria). Tocopherol is synthesized in the inner membrane of chloroplasts and distributed between chloroplast membranes, thylakoids and plastoglobules. Physiological significance of tocopherols for human and animal is well-studied, but relatively little is known about their function in plant organisms. Among the best characterized functions oftocopherols in cells is their ability to scavenge and quench reactive oxygen species and fat-soluble by-products of oxidative stress. There are the data on the participation of different mechanisms of α-tocopherol action in protecting photosystem II (PS II) from photoinhibition both by deactivation of singlet oxygen produced by PSII and by reduction of proton permeability of thylakoid membranes, leading to acidification of lumen under high light conditions and activation of violaxanthin de-epoxidase. Additional biological activity of tocopherols, independent of its antioxidant functions have been demonstrated. Basic mechanisms for these effects are connected with the modulation of signal transduction pathways by specific tocopherols and, in some instances, by transcriptional activation of gene expression.

Deactivation of Singlet Oxygen by Titanium Dioxide in Aqueous Solution Studied by Phosphorescence Quenching with Porphyrin Photosensitizers

Deactivation of singlet oxygen (1O2) by TiO2 nanoparticles was investigated for the first time. 1O2 was produced by pulse laser-induced photosensitization with either cationic or anionic porphyrin dye. Generation and decay of 1O2 were measured by its phosphorescence at 1270 nm in the presence of seven commercially available TiO2 powders of various sizes and crystallites. To clarify the quenching mechanisms, the deactivation of the triplet excited state of porphyrin dyes (3D*) with TiO2 powders was also measured by the transient T–T absorption. The Stern–Volmer plot suggests that the quenching should take place dynamically by second-order reaction. The obtained rate constants for the deactivation of 1O2 and 3D* with TiO2 were increased with the size of the particles, which is attributable to the increase of the local concentration of 1O2 and 3D* around the TiO2 particles. The intrinsic rate constant for the deactivation of 1O2 by TiO2 was elucidated to be 1.0 × 108 M–1 s–1.

Free radical scavenging properties of phytofluene and phytoene isomers as compared to lycopene: a combined experimental and theoretical study.

Free radical scavenging properties of Lycopene (LYC), Phytofluene (PF), and Phytoene (PE) are studied experimentally, considering the electron transfer mechanism and using the ABTS(•+) decolouration assay. Density Functional Theory is used to obtain the theoretical UV-visible spectra of ABTS (neutral and cation) and to determine the free radical scavenger capacity in terms of the electron donor mechanism and the deactivation of singlet oxygen. The experimental data agreed very well with the theoretical results. Considering both mechanisms, LYC, PF, and PE can be considered good free radical scavengers, with LYC turning out to be the best and PE the worst antiradical. However, the triplets that formed following the deactivation of singlet oxygen presented similar capacity for donating electrons. In the case of triplets, the antiradical capacity of LYC, PF, and PE is similar in terms of the electron donor mechanism. Although the results indicated that PF and PE are not as effective antiradicals as LYC, which is to be expected due to the number of conjugated double bonds, they do present a higher antioxidant capacity than expected considering the small number of conjugated double bonds.

Efficiency of porous silicon photosensitizer in the singlet oxygen-mediated oxidation of organic compounds

The efficiency of singlet oxygen photosensitization by porous silicon is compared with that of a conventional dye photosensitizer tetraphenylporphine by means of singlet oxygen-mediated photooxidation of α-terpinene. Based on photoluminescence measurements it was concluded that the efficiency of porous silicon as a sensitizer is much lower than that of the conventional organic dye. The main reasons for this are the low quantum yield of long living photoinduced electronic excitations (excitones) confined in silicon nanocrystals and deactivation of singlet oxygen by H-terminated internal surface of porous silicon powder. Efficient quenching of excitons in porous silicon by 1,3-diphenylisobenzofuran via the direct energy transfer was demonstrated. This process is believed to be responsible for photobleaching of 1,3-diphenylisobenzofuran at its millimolar concentrations.

Photophysics of Squaraine Dyes: Role of Charge-Transfer in Singlet Oxygen Production and Removal

The unique optical properties of squaraines render these molecules useful for applications that range from xerography to photodynamic therapy. In this regard, squaraines derived from the condensation of nitrogen-based heterocycles and squaric acid have many promising attributes. Key solution-phase photophysical properties of six such squaraines have been characterized in this study. One feature of these molecules is a pronounced absorption band in the region approximately 600-720 nm that has significant spectral overlap with the fluorescence band (i.e., the Stokes shift is small). As such, effects of emission/reabsorption yield unique excitation wavelength dependent phenomena that are manifested in quantum yields of both fluorescence and sensitized singlet oxygen production. Comparatively small squaraine-sensitized yields of singlet oxygen production and, independently, large rate constants for squaraine-induced deactivation of singlet oxygen are consistent with a model in which there is appreciable intra- and intermolecular charge-transfer in the squaraine and squaraine-oxygen encounter complex, respectively. The results reported herein should be useful in the further development of these compounds for a range of oxygen-dependent applications.

Linear free energy relationship analysis of solvent effect on singlet oxygen reactions with mono and disubstituted anthracene derivatives

Detection of O 2( 1Δ g) emission, λ max = 1270 nm, following laser excitation and steady-state methods were employed to measure the total reaction rate constant, k T, and the reactive reaction rate constant, k R, for the reaction between singlet oxygen and seven mono and disubstituted anthracenes. In most solvents, the rate constants for the quenching of singlet oxygen by monosubstituted anthracenes are in the order of 10 6 M −1 s −1, whereas for the disubstituted compounds the values are in the order of 10 7 M −1 s −1 indicating that both, mono and disubstituted anthracenes are good quenchers of singlet oxygen. The reactive rate constants were very close to the total rate constants, implying that quenching is essentially a chemical process. The analysis of solvent effect on k T and k R by using LSER equations indicates that singlet oxygen deactivation by the anthracene derivatives is accelerated by solvents with large values of both, dipolarity/polarizability and Hildebrand solvent parameters, being inhibited by hydrogen bond donor solvents in some cases. Solvent effect analysis support the formation of a loose, zwitterionic like exciplex for 9-methylanthracene and 9-methoxyanthracene, a loose exciplex with partial charge separation for di-substitutes anthracene and a compact exciplex with partial charge separation for anthracene and 9-phenylanthracene.

Nonradiative Deactivation of Singlet Oxygen (1O2) by Cubane and Its Derivatives

The bimolecular quenching rate constants of singlet oxygen ((1)O2) by cubane and cubane derivatives were determined and found to be in the order of 10(3)-10(4) M(-1) s(-1). These values represent larger values than expected for aliphatic alkanes as a model for C-H vibrational deactivation. This result is explained by the occurrence of two different deactivation mechanisms: energy transfer to cubane C-H vibrational modes and the formation of a charge-transfer complex between (1)O2 and cubane ((1)O2(*-)...cubane(*+)).

Vibrational deactivation of singlet oxygen: does it play a role in stereoselectivity during photooxygenation?

Oxazolidinone-substituted enecarbamates offer a system to explore vibrational quenching and the strategic placement of CH bonds as a method for manipulating the stereoselectivity of photoreactions.

Deactivation of singlet oxygen by single-wall carbon nanohorns

Abstract The deactivation rate constant of singlet oxygen 1O2 by single-wall carbon nanohorns (SWNHs) was determined. The estimated rate constant by SWNHs, 8 × 104 L g−1 s−1, was significantly larger than the physical quenching rate constant by α-Tocopherol, 0.9 × 104 L g−1 s−1. The origin of this remarkable efficiency of SWNHs for 1O2 deactivation is ascribed to the unique curvature of the graphitic tube in their horn shaped structure.

Subcellular, Time-Resolved Studies of Singlet Oxygen in Single Cells

In time-resolved and spatially resolved experiments, singlet molecular oxygen, O2(a1Deltag), was created in a single nerve cell upon irradiation of a sensitizer incorporated in the cell using a focused laser beam. The singlet oxygen thus produced was detected by its infrared phosphorescence. Data obtained indicate that in both the cytoplasm and the nucleus of the cell, this reactive species is approximately 1-2 orders of magnitude longer-lived than previously believed. The data demonstrate that deactivation of singlet oxygen in the cell is dominated by interactions with the solvent not cellular constituents such as proteins. These results provide a new perspective for mechanistic studies of the role of O2(a1Deltag) in photoinduced cell death and intracellular signaling.