Mechanism of photochromic transformations and photodegradation of an asymmetrical 2,3-diarylcyclopentenone.

A mechanistic study of the photochromic properties and photodegradation processes of an asymmetrical diarylcyclopentenone bearing thiophene and benzothiophene units using stationary photolysis, nanosecond laser flash photolysis and time-resolved luminescence was performed. It was found that the light-induced reversible isomerization of (3-(2,5-dimethyltiophen-3-il)-2-(2-methyl-1-benzylthiophen-3-il)cyclopent-2-en-1-one, compound 1) from open to closed form is a common photochromic transformation inherent to diarylethenes, while the photodegradation process proceeds in two ways. The first is a formal 1,2-dyotropic rearrangement, proceeding without the participation of oxygen. The second is the oxygen-dependent mechanism involving the excitation of the open form 1A into the triplet state, quenching of the latter by dissolved oxygen, and oxidation of the initial compound by singlet oxygen.

Molecular oxygen migration through the xenon docking sites of human hemoglobin in the R-state.

A nanosecond laser flash-photolysis technique was used to study bimolecular and geminate molecular oxygen (O2) rebinding to tetrameric human hemoglobin and its isolated α and ß chains in buffer solutions equilibrated with 1atm of air and up to 25atm of xenon. Xenon binding to the isolated α chains and to the α subunits within tetrameric hemoglobin was found to cause a decrease in the efficiency of O2 escape by a factor of ~1.30 and 3.3, respectively. A kinetic model for O2 dissociation, rebinding, and migration through two alternative pathways in the hemoglobin subunits was introduced and discussed. It was shown that, in the isolated α chains and α subunits within tetrameric hemoglobin, nearly one- and two-third escaping molecules of O2 leave the protein via xenon docking sites, respectively. The present experimental data support the idea that O2 molecule escapes from the ß subunits mainly through the His(E7) gate, and show unambiguously that, in the α subunits, in addition to the direct E7 channel, there is at least one alternative escape route leading to the exterior via the xenon docking sites.

Photosensitized singlet oxygen luminescence from the protein matrix of Zn-substituted myoglobin.

A nanosecond laser near-infrared spectrometer was used to study singlet oxygen ((1)O2) emission in a protein matrix. Myoglobin in which the intact heme is substituted by Zn-protoporphyrin IX (ZnPP) was employed. Every collision of ground state molecular oxygen with ZnPP in the excited triplet state results in (1)O2 generation within the protein matrix. The quantum yield of (1)O2 generation was found to be equal to 0.9 ± 0.1. On the average, six from every 10 (1)O2 molecules succeed in escaping from the protein matrix into the solvent. A kinetic model for (1)O2 generation within the protein matrix and for a subsequent (1)O2 deactivation was introduced and discussed. Rate constants for radiative and nonradiative (1)O2 deactivation within the protein were determined. The first-order radiative rate constant for (1)O2 deactivation within the protein was found to be 8.1 ± 1.3 times larger than the one in aqueous solutions, indicating the strong influence of the protein matrix on the radiative (1)O2 deactivation. Collisions of singlet oxygen with each protein amino acid and ZnPP were assumed to contribute independently to the observed radiative as well as nonradiative rate constants.

Towards understanding non-equivalence of α and ß subunits within human hemoglobin in conformational relaxation and molecular oxygen rebinding.

Picosecond to millisecond laser time-resolved transient absorption spectroscopy was used to study molecular oxygen (O2) rebinding and conformational relaxation following O2 photodissociation in the α and ß subunits within human hemoglobin in the quaternary R-like structure. Oxy-cyanomet valency hybrids, α2(Fe2+-O2)ß2(Fe3+-CN) and α2(Fe3+-CN)ß2(Fe2+-O2), were used as models for oxygenated R-state hemoglobin. An extended kinetic model for geminate O2 rebinding in the ferrous hemoglobin subunits, ligand migration between the primary and secondary docking site(s), and nonexponential tertiary relaxation within the R quaternary structure, was introduced and discussed. Significant functional non-equivalence of the α and ß subunits in both the geminate O2 rebinding and concomitant structural relaxation was revealed. For the ß subunits, the rate constant for the geminate O2 rebinding to the unrelaxed tertiary structure and the tertiary transition rate were found to be greater than the corresponding values for the α subunits. The conformational relaxation following the O2 photodissociation in the α and ß subunits was found to decrease the rate constant for the geminate O2 rebinding, this effect being more than one order of magnitude greater for the ß subunits than for the α subunits. Evidence was provided for the modulation of the O2 rebinding to the individual α and ß subunits within human hemoglobin in the R-state structure by the intrinsic heme reactivity through a change in proximal constraints upon the relaxation of the tertiary structure on a picosecond to microsecond time scale. Our results demonstrate that, for native R-state oxyhemoglobin, O2 rebinding properties and spectral changes following the O2 photodissociation can be adequately described as the sum of those for the α and ß subunits within the valency hybrids. The isolated ß chains (hemoglobin H) show similar behavior to the ß subunits within the valency hybrids and can be used as a model for the ß subunits within the R-state oxyhemoglobin. At the same time, the isolated α chains behave differently to the α subunits within the valency hybrids.

Molecular oxygen binding with alpha and beta subunits within the R quaternary state of human hemoglobin in solutions and porous sol-gel matrices.

Nanosecond laser flash-photolysis technique was used to study bimolecular and geminate molecular oxygen (O2) rebinding to alpha and beta subunits within oxygenated human adult hemoglobin in solutions and porous wet sol-gel matrices. Plasticity associated with the tertiary structure within R-state hemoglobin is explored through measurements that focus on the functional properties of hemoglobin under conditions designed to tune the tertiary structure without inducing the R to T transition. Inequivalence in the O2 binding to the alpha and beta hemes within the R quaternary structure is studied. The individual kinetic properties of the alpha and beta subunits within the hemoglobin encapsulated in sol-gels and aged as the oxy derivative are shown to be independent of proton concentration over the pH range from 6.3 to 8.5. However, buffer effects on the subunits' properties are revealed in sol-gel-free mediums. Interestingly, the alpha and beta subunits within the encapsulated hemoglobin possess the O2 rebinding properties which fall within the range of the ones for oxygenated hemoglobin in the buffer solutions. The combined results show a pattern in which there is a progression of functional properties that are ascribed to a family of conformational substates of R-state hemoglobin. O2 rebinding to the alpha and beta subunits within the oxygenated R-state hemoglobin in both solutions and wet sol-gels is revealed to be modulated by tertiary structural changes in two quite different ways. The possible structural changes, which modify the O2 rebinding properties, are discussed.

Direct measurements of the reactivity of singlet oxygen to some persistent herbicides in aqueous solutions.

Reactivity of the singlet oxygen (SO), which is assumed to be one of the important oxidizers in natural waters, towards to a set of persistent herbicides, was measured for the first time using time resolved luminescence technique. It was observed that rate constants of SO reactions with the majority of studied herbicides are less than 106 M-1s-1 allowing to conclude about negligible participation of SO in oxidation of the compounds in natural waters.

Mechanistic investigation of humic substances assisted photodegradation of imipramine under simulated sunlight.

Imipramine (IMI) is a frequently prescribed tricyclic antidepressant and widely detected in the natural waters, while the environmental fate of IMI is yet poorly understood. Here, we investigated the photodegradation of IMI under simulated sunlight in the presence of humic substances (HS), typically including humic acid (HA) and fulvic acid (FA). The direct and indirect IMI photodegradation was found to increase both with increasing pH and with deoxygenation of the reaction solutions. The excited triplet state of HS (3HS⁎) was mainly responsible for the photosensitized degradation of IMI according to the steady-state quenching and direct time-resolved experiments. The electron transfer interaction between 3HS⁎ and IMI was observed by laser flash photolysis (LFP) with bimolecular reaction rate constants of (4.9 ± 0.4) × 109 M-1 s-1. Evidence of electron transfer from IMI to 3HS⁎ was further demonstrated by the photoproduct analysis. The indirect photodegradation was triggered off in the side chain of IMI with the nonbonding nitrogen electron transferring to 3HS⁎, followed by hydroxylation, demethylation and cleavage of the side chain. Very important that HS photosystem does not lose its efficiency with decreasing of IMI concentration, meaning that the studied photosystem still be used at environmentally relevant concentrations of IMI. These results suggest that photodegradation could be an important attenuation pathway for IMI in HS-rich and anaerobic natural waters.

New insights into mechanism of direct UV photolysis of p-arsanilic acid.

The mechanism of direct UV photolysis of p-arsanilic acid (p-ASA), a widely used veterinary drug, was revised by means of laser flash photolysis coupled with high resolution liquid chromatography - mass spectrometry (LC-MS). None of p-ASA triplet state or singlet oxygen was found to directly participate in the photodegradation of p-ASA as it was assumed in previous works. Here we demonstrate that the main primary photoprocess is a monophotonic ionization (ϕion266nm = 0.032) leading to the formation of hydrated electron and corresponding anilinyl cation radical. These primary species react with dissolved oxygen yielding secondary reactive oxygen species. The final organic photoproducts, such as aminophenol and different dimeric products, originate from various reactions between these secondary species. The generation of inorganic arsenic, both As(V) and As(III), was also observed in agreement with previous works. For the first time we report the quantum yield of p-ASA photodegradation, which decreases from 0.058 to 0.035 with the excitation wavelength from 222 to 308 nm.

Photooxidation of herbicide amitrole in the presence of fulvic acid.

Fulvic acid (Henan ChangSheng Corporation) photoinduced degradation of non-UVA-absorbing herbicide amitrole (3-amino-1,2,4-triazole, AMT) as a way for its removal from polluted water was investigated in details. It was shown that the main primary species generated by fulvic acid under UVA radiation, triplet state and hydrated electron, are not directly involved in the herbicide degradation. AMT decays in reactions with secondary intermediates, reactive oxygen species, formed in reactions of the primary ones with dissolved oxygen. Singlet oxygen is responsible for 80% of herbicide oxidation, and •OH and O2-• radicals-for the remaining 20% of AMT. It was found that quantum yield of AMT photodegradation (ϕ 365nm) decreases linearly from 2.2 × 10-3 to 1.2 × 10-3 with the increase of fulvic acid concentration from 1.1 to 30 mg L-1. On the contrary, the increase of AMT concentration from 0.8 to 25 mg L-1 leads to practically linear growth of ϕ 365nm value from 1.8 × 10-4 to 4 × 10-3. Thus, the fulvic acid exhibits a good potential as UVA photooxidizer of organic pollutants sensitive to the singlet oxygen (ϕ 532nm(1O2) = 0.025 at pH 6.5).

Does photodissociation of molecular oxygen from myoglobin and hemoglobin yield singlet oxygen?

Time-resolved luminescence measurements in the near-infrared region indicate that photodissociation of molecular oxygen from myoglobin and hemoglobin does not produce detectable quantities of singlet oxygen. A simple and highly sensitive method of luminescence quantification is developed and used to determine the upper limit for the quantum yield of singlet oxygen production. The proposed method was preliminarily evaluated using model data sets and confirmed with experimental data for aqueous solutions of 5,10,15,20-tetrakis(4-N-methylpyridyl) porphyrin. A general procedure for error estimation is suggested. The method is shown to provide a determination of the integral luminescence intensity in a wide range of values even for kinetics with extremely low signal-to-noise ratio. The present experimental data do not deny the possibility of singlet oxygen generation during the photodissociation of molecular oxygen from myoglobin and hemoglobin. However, the photodissociation is not efficient to yield singlet oxygen escaped from the proteins into the surrounding medium. The upper limits for the quantum yields of singlet oxygen production in the surrounding medium after the photodissociation for oxyhemoglobin and oxymyoglobin do not exceed 3.4×10(-3) and 2.3×10(-3), respectively. On the average, no more than one molecule of singlet oxygen from every hundred photodissociated oxygen molecules can succeed in escaping from the protein matrix.