REACTIVITY OF SINGLET OXYGEN TOWARD AMINO ACIDS AND PEPTIDES

Quenching of singlet oxygen (¹O₂) in D₂O-ethanol by the amino acids tryptophan, tyrosine, histidine, methionine, cysteine and their derivatives was measured by exciting the sensitizers rose bengal or meso-tetra (N-methyl-4-pyridyl)porphyrin tetratosylate in the presence of oxygen and the above quenchers in solution. In our polar solvent, containing 75% D₂O on a molar basis it was found that (1) substitution of the aromatic ring in indole, phenol and imidazole by the electron-donating methyl group increases the total (i.e. nonreactive and reactive) quenching rate constant by a factor of five to eight. Free or blocked amino and carboxyl groups removed by two methylene groups from the ring counteract the above increase in the rate constant. The reactive quenching of singlet oxygen, which leads to oxidative destruction of the aromatic ring, correlates with the above substitution effects. It has been proposed that the quenching process takes place by formation of an exciplex between ¹O₂ and the quencher. Thus our results indicate that the better an electron donor the amino acid residue is the more pronounced is the charge transfer contribution in the exciplex formed with ¹O₂ and the more likely it is to lead to charge separation and hence to a chemical reaction. (2) Oligopeptides in solution or peptide bonds linked to the amino acid residue have only a minor effect on singlet oxygen. It can therefore be expected that the polypeptide chains per se in the protein network will not interact significantly with the single oxygen molecules present. The quenching of the latter should, to a first approximation, depend only on the presence of the above reactive amino acid residues and to their accessibility to ¹O₂ as well as on the effective dielectric constant within the protein structure.     

Flexibility in Proteins: Tuning the Sensitivity to O2 Diffusion by Varying the Lifetime of a Phosphorescent Sensor in Horseradish Peroxidase¶

The heme in horseradish peroxidase (HRP) was replaced by phosphorescent Pt‐mesoporphyrin IX (PtMP), which acted as a phosphorescent marker of oxygen quenching and allowed comparison with another probe, Pd‐mesoporphyrin IX (Khajehpour et al. (2003) Proteins 53, 656–666). Benzohydroxamic acid (BHA), a competitive inhibitor of the enzyme, was also used to monitor its effects on phosphorescence quenching. With the addition of BHA, in the presence of oxygen, the phosphorescence intensity of the protein increased. In contrast, the addition of BHA, in the absence of oxygen, reduced the phosphorescence intensity of the protein. K_d= 18 μM when BHA binds to PtMP‐HRP. The effect of BHA can be explained by two factors: ( 1 ) BHA reduces the accessibility of O₂ to the protein interior and ( 2 ) BHA itself quenches the phosphorescence. Consistent with this, the oxygen quenching of the phosphorescence of PtMP‐HRP gave a quenching constant of k_q= 234 mm Hg^?1 s^?1 in the absence of BHA and k_q= 28.7 mm Hg^?1 s^?1 in the presence of BHA. The quenching rate of BHA is 4000 s^?1. The relative quantum yield of the phosphorescence of the Pt derivative is about six times that of the Pd derivative, whereas the phosphorescence lifetime is approximately eight times shorter. The high quantum yield and suitable lifetime make Pt‐porphyrins appropriate as sensors of O₂ diffusion and flexibility in heme proteins.     

Generation of Reactive Oxygen Species during the Photolysis of 6‐(Hydroxymethyl)pterin in Alkaline Aqueous Solutions

Photochemical studies of the reactivity of 6‐(hydroxymethyl)pterin (=2‐amino‐6‐(hydroxymethyl)pteridin‐4(1H)‐one; HPT) in alkaline aqueous solutions (pH 10.2–10.8) at 350 nm and room temperature were performed. The photochemical reactions were followed by UV/VIS spectrophotometry, thin‐layer chromatography (TLC), high‐performance liquid chromatography (HPLC), and an enzymatic method for H₂O₂ determination. In the presence of O₂, 6‐formylpterin (=2‐amino‐3,4‐dihydro‐4‐oxopteridine‐6‐carboxaldehyde; FPT) was the only photoproduct detected. In the absence of O₂, we observed a compound with an absorbance maximum at 480 nm, which was oxidized very rapidly by O₂ in a dark reaction to yield FPT. The quantum yields of substrates disappearance and of photoproducts formation were determined. The formation of H₂O₂ during photooxidation was monitored, and the number of mol of H₂O₂ released per mol of HPT consumed corresponded to a 1 : 1 stoichiometry. HPT was also investigated for efficiency of singlet‐oxygen (¹O₂) production and quenching in aqueous solution. The quantum yield of ¹O₂ production (Φ_Δ=0.21±0.01) was determined by measurements of the ¹O₂ luminescence in the near‐IR (1270 nm) upon continuous excitation of the sensitizer. The rate constant of ¹O₂ total quenching by HPT was determined (k_t=3.1?10⁶ M ^?1 s^?1), indicating that this compound was able to quench ¹O₂. However, ¹O₂ did not participate in the photooxidation of HPT to FPT.     

REACTIVITY OF SINGLET OXYGEN TOWARD LARGE PEPTIDES

Abstract— The reactions of singlet oxygen, ¹O₂, with amino acids and their derivatives have been studied previously. It was found that only five amino acid residues interact readily with ¹O₂. Here we describe its reactions with the large peptides melittin, neuropeptide Y (NPY) and insulin in their native and in their denatured forms. The singlet oxygen quenching by a polypeptide was compared with that of a solution at the same concentration as those of its constituent amino acids, which are known to react efficiently with ¹O₂. It was found that the quenching rate by such a mixture exceeded that of the polypeptides in their native form. The ratio of the rate constants for NPY to that of the corresponding amino acid mixture in solution was 0.75. For melittin in its monomeric form it was 0.83 and for a tetramer of melittin (at high ionic strength) it was 0.70. For native insulin the ratio of the rate constants was 0.55. For oxidized insulin with its -S-S- bridges opened the figure became 0.80. However, the quenching by all the polypeptides in their fully denatured form (in the presence of 6 M urea) equalled that of the corresponding amino acid mixtures. Although polypeptides are generally supposed not to possess a stable secondary structure in solution the effects are explained by shielding of some of the reactive amino acid residues in the chain by temporary folding or incipient secondary structures of the native polypeptide.
It is shown that the kinetics for a homogeneous solution of quenchers applies also to measurements in a polypeptide solution where the quenchers are localized along the polypeptide backbone and thus form clusters in solution.     

THE GENERATION OF HYDROGEN PEROXIDE BY THE UVA IRRADIATION OF HUMAN LENS PROTEINS

Abstract— The water-insoluble proteins from aged human lens are known to contain protein-bound chromophores that act as UVA sensitizers. The irradiation of a sonication-solubilized, water-insoluble fraction from human lenses (55–75 years) with UVA light (1.5 kj/cm², λ > 338 nm) caused an oxygen-dependent photolysis of tryptophan, not seen when either α-crystallin or lysozyme were irradiated. The suggested requirement for active oxygen species was consistent with a linear increase in hydrogen peroxide formation, which was also observed. A final concentration of 55 µM H₂O₂ was attained, with no H₂0₂ being detected in either dark-incubated controls or in irradiated samples of native proteins. The UVA-dependent H₂O₂ formation was increased 50% by superoxide dismutase (SOD) and abolished by catalase, arguing for the initial generation of superoxide anion. A linear photolysis of histidine and tryptophan was also seen; however, the addition of SOD or SOD and catalase had no effect on the photolytic destruction of either amino acid. Superoxide dismutase increased the oxidation of protein SH groups implicating H₂O₂, but SOD and catalase caused a decrease in SH oxidation only at later time periods. The direct addition of H₂O₂ to a water-insoluble sonicate supernatant fraction caused only a slight oxidation of SH groups, but this was increased four- to eight-fold when the protein was denatured in 4.0 M guanidine hydrochloride. Overall, the data suggest a UVA-dependent oxidation of protein SH groups via H₂O₂ generated within the large protein aggregates of the water-insoluble fraction. These data also provide a mechanism for oxidation of the sulfur-containing amino acids in vivo—a process that is known to accompany the formation of age-onset cataracts.     

Robust Benzo[g, h, i ]perylenetriimide Dye‐Sensitized Electrodes in Air‐Saturated Aqueous Buffer Solution

Highly electron deficient benzo[ghi]perylenetriimide (BPTI) chromophores were persistently anchored to a metal oxide electrode surface and reversible formation of their radical anions was shown in air‐saturated aqueous buffer solution. Our results show a very low reaction‐rate constant of BPTI^.? with O₂ (k=1.92±0.05×10^?2 s^?1). BPTI is a robust chromophore that can be used as the electron acceptor in molecule‐based artificial photosynthetic devices for direct water splitting in aqueous phase.     

Theoretical Study of the Reaction Formalhydrazone with Singlet Oxygen. Fragmentation of the C=N Bond,Ene Reaction and Other Processes

Photobiologic and synthetic versatility of hydrazones has not yet been established with ¹O₂ as a route to commonly encountered nitrosamines. Thus, to determine whether the “parent” reaction of formalhydrazone and ¹O₂ leads to facile C=N bond cleavage and resulting nitrosamine formation, we have carried out CCSD(T)//DFT calculations and analyzed the energetics of the oxidation pathways. A [2 + 2] pathway occurs via diradicals and formation of 3‐amino‐1,2,3‐dioxazetidine in a 16 kcal/mol^?1 process. Reversible addition or physical quenching of ¹O₂ occurs either on the formalhydrazone carbon for triplet diradicals at 2–3 kcal mol^?1, or on the nitrogen (N(3)) atom forming zwitterions at ~15 kcal/mol^?1, although the quenching channel by charge‐transfer interaction was not computed. The computations also predict a facile conversion of formalhydrazone and ¹O₂ to hydroperoxymethyl diazene in a low‐barrier ‘ene’ process, but no 2‐amino‐oxaziridine‐O‐oxide (perepoxide‐like) intermediate was found. A Benson‐like analysis (group increment calculations) on the closed‐shell species are in accord with the quantum chemical results.     

DETERMINATION OF THE ACRYLAMIDE QUENCHING CONSTANT FOR PROTEIN AND MODEL INDOLE TRIPLETS

Abstract— The decay of the indole triplet of single tryptophan-containing proteins and model compounds can be readily measured at room temperature in aqueous solution by monitoring the triplet-triplet absorption or phosphorescence emission following a 265 nm exciting laser pulse. The quenching action of acrylamide on the triplet excited state of indole side chains was studied in an analogous fashion to that previously done at the singlet level (Eftink and Ghiron, 1977). The acrylamide triplet quenching constant (^tk_q) ranged from a high of 7.8 times 10⁸M^-1 s^-1 for the exterior indole of corticotropin (ACTH) to a low of 2 times 10⁵ Af^-1 s^-1 for the interior indole of ribonuclease T, (RNase T,). The ratio (7) of these values with their respective acrylamide singlet quenching constants (^tk_q),(γ=^tk_q⁸K_q) ranged from a high of 0.22 for ACTH to a low of 0.001 for RNase T₁,. Acrylamide is also an inefficient quencher of model indoles in various solvents (i.e. it has a γ less than 1). The magnitude of γ varied from a high of 0.3 in H₂0 to a low of 0.02 in acetonitrile, but did not correlate with viscosity, dielectric constant or polarity. The lower efficiency observed for internal indole groups can not be explained by that class of models which predict the presence of static quenching at the triplet level, since none was observed. The present results confirm the observation of Calhoun et al. of a large discrepancy between acrylamide's singlet and triplet quenching constants for buried indole side chains, but suggest that it may be largely explained by the fact that acrylamide is an inefficient quencher of the indole triplet state (1983). The magnitude of this inefficiency is probably determined by specific microenvironmental factors. Thus, unlike ⁸K_q, the environmentally sensitive ^lk_H cannot be easily used to characterize the dynamics of proteins.     

Oxidation of diazo compounds by triphenyl phosphite ozonide. Quenching of singlet oxygen by diazo compounds

The reaction of triphenyl phosphite ozonide with various types of diazo compounds results in their oxidation, which is accomplished by singlet oxygen (¹O₂) evolved during thermal decomposition of the ozonide. A decrease in the ionization potential of the substrate results in an increase in the overall rate constant of quenching of¹O₂. In the case of 9-diazofluorene, the main channel of¹O₂ quenching is physical quenching.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1567–1571, September, 1994.The work was carried out with the financial support of the Russian Foundation for Basic Research (Project No. 93-03-5231).     

Singlet-Oxygen Quenching by Carotenoids: Steady-state luminescence experiments

The near-infrared luminescence of singlet oxygen (¹O₂) has been measured in order to determine the efficiency of ¹O₂ quenching by two carotenoid compounds, β-carotene and canthaxanthin. 1H-Phenalen-1-one and rose bengal have been used as photosensitizers in those steady-state luminescence experiments. Stern-Volmer analysis of the ¹O₂ luminescence in solutions of CCl₄ and CD₃OD, containing different concentrations of the carotenoids, has shown a very efficient quenching by canthaxanthin. The rate constants are about a factor of 2 below the diffusion limited values for the given solvents, confirming earlier results in benzene. In comparison, the efficiency of ¹O₂ quenching by β-carotene is slightly lower than that by canthaxanthin in non-polar solvents and is reduced by an order of magnitude in CD₃OD, due to the aggregation of this quencher.     

Synthesis,Antiproliferative Activity and DNA-Binding Properties of Nitrogen and Sulfur Heterocyclic Norcantharidin Acylamide Acid

Three novel norcantharidin acylamide acids (L¹?N‐thiadiazole norcantharidin acylamide acid, C₁₀H₁₁N₃O₄S; L²?N‐thiazole norcantharidin acylamide acid, C₁₁H₁₂N₂O₄S and L³?N‐benzothiazole norcantharidin acylamide acid, C₁₅H₁₄N₂O₄S) were synthesized by the reactions of norcantharidin (NCTD?7‐oxabicyclo[2,2,1]heptane‐2,3‐dicarboxylic acid anhydride, C₈H₈O₄) with 2‐amino‐1,3,4‐thiadiazole (C₂H₃N₃S), 2‐aminothiazole (C₃H₄N₂S) and 2‐aminobenzothiazole (C₇H₆N₂S), respectively. Their structures were characterized by elemental analysis, IR, and NMR. The inhibition rates of L³ was much higher than those of L¹ and L² against human hepatoma cells SMMC7721 cell lines in vitro. The interaction between the compounds and DNA was studied by means of fluorescence quenching studies and viscosity measurements. The emission intensities decreased obviously with increasing concentration of the compounds in the fluorescence quenching experiments. The linear Stern‐Volmer quenching constant K_sq values were 0.62 (L¹), 0.55 (L²) and 1.08 (L³), respectively. The binding abilities followed the trend from high to low were L³, L¹ and L², respectively. The results of viscosity measurements showed that L¹ and L² might bind to DNA via partial intercalation, while L³ bound mainly in intercalation.     

Kinetic studies of the deactivation of O2(1Σg+) and O(1D)

The kinetics of the deactivation of O₂(¹Σ_g⁺) is studied in real time. O₂(¹Σ_g⁺) is generated in this system by the O(¹D) + O₂ reaction following O₃laser flash photolysis in the presence of excess O₂, and it is monitored by its characteristic emission band at 762 nm. Quenching rate constants were obtained for O₂, O₃, N₂, CO₂, H₂O, CF₄and the rare gases. Since O(¹D) is the precursor for the formation of O₂(¹Σ_g⁺), the addition of an O(¹D) quencher effectively lowers the initial concentration of O₂(¹Σ_g⁺). By measuring the initial intensity of the 762 nm fluorescence signal, the relative quenching efficiencies were determined for O(¹D) quenching by N₂, CO₂, Xe, and Kr with respect to O₂; the results are in good agreement with literature values.     

Synthesis of glass ceramics containing finely dispersed particles of aluminum-doped M-type strontium hexaferrite

Glasses with nominal compositions SrFe₁₀Al₂O₁₉+4(SrB₂O₄+Sr₂B₂O₅) (1) and SrFe₉Al₃O₁₉+4(SrB₂O₄+Sr₂B₂O₅) (2) were prepared by rapid quenching of melts. Thermal treatment of glass samples at 600–900 °C resulted in crystallization of the magnetic phase SrFe_12−x Al_xO₁₉ (x = 1.1±0.1) and strontium borates. Platelet hexaferrite particles with average sizes from (250×60) nm² to (450×140) nm² were prepared. The coercive force of glass ceramics is 580 and 475 kA m⁻¹ for glasses 1 and 2, respectively. The coercive force of 580 kA m⁻¹ is the highest known value compared to hexaferrite particles prepared earlier by glass crystallization.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 74–77, January, 2005.     

INFLUENCE OF THE PEPTIDE BOND ON THE SINGLET MOLECULAR OXYGEN-MEDIATED (O2[g]) PHOTOOXIDATION OF HISTIDINE and METHIONINE DIPEPTIDES. A KINETIC STUDY

The dye-sensitized photooxidation of l -histidine (His) and l -methionine (Met) and their simplest dipeptides with glycine (Gly) (His-Gly, Gly-His, Gly-Met) and Met-methyl ester (Met-ME) mediated by singlet molecular oxygen (O₂[_g]) was studied. The overall rate constants in acetonitrile-H₂O (K_t) for O₂(¹_g) quenching were measured by time-resolved phosphorescence detection. In H₂O a competitive kinetic method was employed. In both solvents the reactive rate constants (K_t,) were determined to discriminate between the overall and physical contributions to the quenching. The kinetic and mechanistic aspects of the interaction are discussed. For His-Gly, the peptide bond has practically no effect on the kinetics of photooxidation. For Gly-His the overall rate constant is much higher than that for His and His-Gly, in both H₂O and acetonitrile-H₂O. The main contribution to k₁ (for Gly-His) is the physical quenching of O₂(¹_g)- In water the k_t/k_r ratio for free His and His-Gly is 1.0, reaching a value of 2.0 in the organic solvent-H₂O mixture. The rates of-NH₂ loss upon sensitized photooxidation in all cases parallel the trend of k_r values. The main results for the His series indicate that: (1) a polar environment favors autoprotection (i.e. an increase in the contribution of physical quenching) against photodynamic effects; (2) only the rate constant for reactive interaction with O₂[_g] does not depend on the location of the peptide bond involving His. For Met derivatives the k_t, values are higher in both solvents than that for free Met. Only for the free amino acid in H₂O is the interaction with O₂(¹_g) totally reactive. For Gly-Met and Met-ME the physical quenching prevails: k_t, is, in both solvents, about one order of magnitude higher than k_r. According to our results on -NH₂ loss and on the basis of previous investigations by others, the photooxidative products distribution in the Met series indicates that Gly-Met yields only dehydroMet, whereas Met and Met-ME produce a mixture of Met-sulfoxide and the Met-dehydro compound.     

Tryptophan exposure and accessibility in the chitooligosaccharide-specific phloem exudate lectin from pumpkin (Cucurbita maxima). A fluorescence study

The exposure and accessibility of the tryptophan residues in the chitooligosaccharide-specific pumpkin (Cucurbita maxima) phloem exudate lectin (PPL) have been investigated by fluorescence spectroscopy. The emission λ_max of native PPL, seen at 338 nm was red-shifted to 348 nm upon denaturation by 6 M Gdn.HCl in the presence of 10 mM β-mercaptoethanol, indicating near complete exposure of the tryptophan residues to the aqueous medium, whereas a blue-shift to 335 nm was observed in the presence of saturating concentrations of chitotriose, suggesting that ligand binding leads to a decrease in the solvent exposure of the tryptophan residues. The extent of quenching was maximum with the neutral molecule, acrylamide whereas the ionic species, iodide and Cs⁺ led to significantly lower quenching, which could be attributed to the presence of charged amino acid residues in close proximity to some of the tryptophan residues. The Stern–Volmer plot for acrylamide was linear for native PPL and upon ligand binding, but became upward curving upon denaturation, indicating that the quenching occurs via a combination of static and dynamic mechanisms. In time-resolved fluorescence experiments, the decay curves could be best fit to biexponential patterns, for native protein, in the presence of ligand and upon denaturation. In each case both lifetimes systematically decreased with increasing acrylamide concentrations, indicating that quenching occurs predominantly via a dynamic process.     

Photokinetics in tetraphenylporphyrin – molecular oxygen system at gas/solid interfaces: effect of singlet oxygen quenchers on oxygen-induced delayed fluorescence

The oxygen quenching of excited states and concomitant delayed fluorescence (DF) of meso-tetraphenylporphyrin (TPP) adsorbed on alumina (Al₂O₃) were studied at different TPP and O₂ concentrations and temperatures by the diffuse-reflectance laser flash technique. The formation of ¹O₂ in the course of ³TPP quenching by ³O₂ is followed by the energy transfer from ¹O₂ to ³TPP (¹O₂ feedback) with the generation of TPP fluorescent state. The global kinetic analysis of DF revealed variations on kinetic parameters with surface loading which match the aggregation of TPP on the surface. In concentrated samples the energy exchange between ¹O₂ and ³O₂ accelerates the ¹O₂ feedback more than 10 times. The key role of ¹O₂ in oxygen-induced DF is confirmed by the DF quenching by coadsorbed ¹O₂ quenchers (NaN₃, 3-methylindole, 1,4-diazabicyclo[2.2.2]octane). This process is in part controlled by the surface which enhances the efficiencies of amine quenchers but reduces that of NaN₃ when compared with the corresponding efficiencies in solution.     

Rhodotorula aurantiaca penicillin V acylase: Active site characterization and fluorometric studies

Penicillin V acylase (PVA), a member of newly evolved Ntn-hydrolase superfamily, is a pharmaceutically important enzyme to produce 6-aminopenicillanic acid. Active site characterization of recently purified monomeric PVA from Rhodotorula aurantiaca (Ra-PVA), the yeast source, showed the involvement of serine and tryptophan in the enzyme activity. Modification of the protein with serine and tryptophan specific reagents such as PMSF and NBS showed partial loss of PVA activity and substrate protection. Ra-PVA was found to be a multi-tryptophan protein exhibiting one tryptophan, in native and, four in its denatured condition. Various solute quenchers and substrate were used to probe the microenvironment of the putative reactive tryptophan through fluorescence quenching. The results obtained indicate that the tryptophan residues of Ra-PVA were largely buried in hydrophobic core of the protein matrix. Quenching of the fluorescence by acrylamide was collisional. Acrylamide was the most effective quencher amongst all the used quenchers, which quenched 71.6% of the total intrinsic fluorescence of the protein, at a very less final concentration of 0.1 M. Surface tryptophan residues were found to have predominantly more electropositively charged amino acids around them, however differentially accessible for ionic quenchers. Denaturation led to shift in λ_max from 336, in native state, to 357 nm and more exposed to the solvent, consequently increase in fluorescence quenching with all quenchers. This is an attempt towards the conformational studies of Ra-PVA.     

Photochemical and photophysical properties of lumazine in aqueous solutions

Lumazine (pteridine-2,4(1H,3H)-dione, LU) was investigated for its efficiency of singlet oxygen (¹O₂) production and quenching in aqueous solution. The quantum yield of ¹O₂ production (Φ_Δ) was determined by measurements of the ¹O₂ luminescence in the near-infrared upon continuous excitation of the sensitizer. Values of Φ_Δ are sensitive to the pH and were found to be 0.44 ± 0.01 and 0.080 ± 0.004 in acidic and alkaline media, respectively. The photochemical stability of LU was investigated under different pH conditions, in the presence and in the absence of O₂. The photochemical consumption of LU in aqueous solution at room temperature under irradiation at 350 nm was followed by UV–vis spectrophotometry and HPLC. Values of the quantum yields of LU disappearance are low, indicating that LU is rather photostable under physiological conditions.     

Rate constants for reactions of iodine atoms in solution

Laser flash photolysis (at 248 or 308 nm) or aryl iodides in water or water/methanol solutions produces iodine atoms and phenyl radicals. Iodine atoms react rapidly with added I^? to form I₂^? but do not react rapidly with O₂ (k ? 10⁷ L mol^?1 s^?1). Iodine atoms oxidize phenols to phenoxyl radicals, with rate constants that vary from 1.6 × 10⁷ L mol^?1 s^?1 for phenol to about 6 × 10⁹ L mol^?1 s^?1 for 4-methoxyphenol and hydroquinone. Ascorbate and a Vitamin E analogue are also oxidized very rapidly. N-Methylindole is oxidized by I atoms to its radical cation with a diffusion-controlled rate constant, 1.9 × 10¹⁰ L mol^?1 s^?1. Iodine atoms also oxidize sulfite and ferrocyanide ions rapidly but do not add to double bonds. The phenyl radicals, produced along with the I atoms, react with O₂ to give phenylperoxyl radicals, which react with phenols much more slowly than I atoms. © 1995 John Wiley & Sons, Inc.     

PHOTOLUMINESCENCE OF SINGLET OXYGEN IN PIGMENT SOLUTIONS

Luminescence of ¹O₂ (1270 nm) accompanying energy transfer to oxygen from photoexcited (triplet) molecules of sensitizers in air saturated solutions has been investigated. The luminescence was observed in CC1₄, CS₂ and freon with the use of porphyrins, chlorophylls, pheophytins and aromatic hydrocarbons as sensitizers. The lifetime and quantum yield of the luminescence depended on the nature of the solvents. pigments and their concentrations. The maximum values of these parameters were equal to 28 ± 5 ms and 5 ± 4 times 10^--⁵, respectively. The quantum yield of ¹O₂ generation by pigments has been measured and the results used for determining the quantum yields of intersystem crossing in the pigment molecules. The rate constants of ¹O₂ reaction with different substances have been determined with the aid of luminescence quenching. It has been shown that along with β-carotene. Chls, pheophytins, and some porphyrins are also very active quenchers of ¹O₂, The quenching effect depends on their molecular structure and on the presence and nature of the central metal atom. Quenching ¹O₂ by the pigments is due mainly to a “physical” mechanism (without destruction of the pigments). The destructive “chemical” quenching is by 1–4 orders of magnitude less effective and is accompanied with photochemiluminescence of the pigments. The experiments on ¹O₂ generation and quenching indicate that energy of triplet states of bacteriochlorophyll and bacteriopheophytin is somewhat higher than that of ¹Δ_g oxygen. The data demonstrate wide possibilities of the luminescence studied as a method for investigating ¹O₂ reactivity and photophysical properties of sensitizers.