REACTION OF RETINOL AND RETINAL WITH SINGLET OXYGEN

Abstract— The rate constants for the reactions of all- trans retinol and retinal with singlet oxygen were measured in a variety of solvents of different polarities. The rate constants increased with increasing solvent dielectric constant, which suggests that a charge transfer mechanism plays a part in the reaction. Further, the rate constant of reaction of singlet oxygen with retinal is greater than that with retinol. Since retinal has a lower ionization potential than retinol, these relative rates also support the hypothesis of charge transfer involvement in the reaction.     

A PRELIMINARY INVESTIGATION OF THE USE OF LIMONENE FOR DETECTING SINGLET OXYGEN AS A COMPONENT OF POLLUTED AIR

Abstract— Exploratory experiments using simulated conditions indicate that the terpene, (+)-limonene can serve to detect O₂(¹Δ_g) in polluted atmospheres. ¹O₂ attack on limonene results in the formation of specific oxidation products in a specific distribution; quantitative identification of these products is a highly specific test for ¹O₂.     

DITHIOPHENES AS SINGLET OXYGEN SENSITIZERS

Abstract— The nematicidal compounds, 5-methyl-2,2'dithienyl, 5-phenyl-2,2'-dithienyl and 5,5'-dichloro-2,2'-dithienyl, generate singlet oxygen upon photoactivation with near-UV light. Two chloro-substituted dithienylethenes have similar properties that are farless efficient. The nematode Aphelenchus avenae was rapidly killed by the photoactivated dithienyls under aerobic but not under anaerobic conditions,indicating that nematicidal activity of these compounds is related to their sensitizing properties. Furthermore,these nematodes did not expire in the presence of these compounds maintained in the dark. Sensitizing properties were studied by following the inhibition ofglucose–6-phosphate dehydrogenase by the photoactivated compounds and protection of enzyme activity by various additions. Only singlet oxygen quenchers were effective. The enzyme was inactivated 3 to 4 times faster in D₂0 compared to H₂0.Bleaching of p-nitrosodimethylaniline in the presence of imidazole and the dithiophenes and a specific reaction with the olefin adamantylideneadamantane were additional evidence of generation of singlet oxygen by the photoactivated compounds.     

SOME PREVALENT BIOMOLECULES AS DEFENSES AGAINST SINGLET OXYGEN DAMAGE

Abstract— We have compared the relative abilities of some putative biological protectors to block oxidation of 2,5-bis(hydroxymethyl)furan (BHMF)† in illuminated solutions containing the photosensitizer rose bengal and in the separated-surface-sensitizer (S-S-S) system involving pure singlet oxygen (¹ΔA_gO₂). While L-histidine is a well-known quencher of singlet oxygen, free L-histidine is not commonly found in high concentrations in nature. L-Carnosine (β-alanyl-L-histidine), however, is present in the striated muscles of many organisms, most notably mammals, in concentrations up to 40 m M . At neutral pH, carnosine quenched singlet oxygen more effectively than did equimolar histidine, both in solubilized sensitizer studies and in the S-S-S system. In the pure singlet oxygen system, 1 m M carnosine reduced the rate of BHMF oxidation as effectively as 3 m M histidine alone, or a mixture of 3 mM histidine and 3 m M β-alanine. The fungal product L-ergothioneine (2-thiol-L.-histidine betaine) and its synthetic analogue, 2-thiolhistidine, at 1 m M blocked photosensitized BHMF oxidation using solubilized rose bengal, as did urate at 0.5 m M . All three compounds failed to reduce the rate of BHMF oxidation by singlet oxygen in the S-S-S system, however. Homocarnosine (-γ-aminobutyryl-L-histidine) gave levels of protection against BHMF oxidation identical to histidine, but is present in the central nervous system only at micromolar concentrations. Neither 1 m M imidazole nor 5 m M urea reduced BHMF oxidation in either system. We conclude that some prevalent biomolecules may afford protection either by preventing singlet oxygen production (urate, L-ergothioneine) or by intercepting singlet oxygen once formed (L-carnosine). Such protective devices may be of importance in natural systems.     

CUTANEOUS PHOTOTOXICITY OF TETRACYCLINE ANTIBIOTICS: GENERATION OF FREE RADICALS and SINGLET OXYGEN DURING PHOTOLYSIS AS MEASURED BY SPIN-TRAPPING and THE PHOSPHORESCENCE OF SINGLET MOLECULAR OXYGEN

Chlortetracycline (CTC) generated an aryl radical in aqueous buffer (pH 7.4) during near UV irradiation, as evidenced by the formation of 2-methyl-2-nitrosopropane spin adducts. The radical was produced via dechlorination, a photoprocess not previously reported for tetracyclines. Demeclocyc-line (DEM), another chlorinated tetracycline, did not produce detectable aryl radicals. Relative ¹O₂ yields obtained by direct luminescence measurements at 1268 nm for five tetracyclines in alkaline ethanol (demeclocycline · tetracycline · chlortetracycline · doxycycline · minocycline) showed that DEM produced approximately three times as much singlet oxygen as CTC. This constitutes direct evidence that tetracyclines sensitize both Type I and Type II photoreactions.     

THE SINGLET OXYGEN REACTIVITY OF BILIVERDIN

Abstract— In 1, 1, 2-trichlorotrifluoroethane solution biliverdin physically quenches singlet oxygen at a rate of 8 × l0^sM-¹s^-1 and reacts chemically at 6 × 10 ⁵M-¹s^-1 to give a red product. In D, O solution the rate constants are PD dependent and range from 1.5–6 times 10¹⁰M-¹s^-1 for quenching and the chemical rate varies from 3–5 × 10⁸ M^-1 s^-1 to give colorless products.     

pH DEPENDENCE OF SINGLET OXYGEN PRODUCTION IN AQUEOUS SOLUTIONS USING THIAZINE DYES AS PHOTOSENSITIZERS

Abstract— The production of singlet oxygen by thiazine dye photosensitization, as measured by the rate of photooxidation of tryptophan, was found to be very sensitive to changes of pH in the range 5–9. For methylene blue in aerated solutions, the production of ¹O₂* is approximately five times more efficient in basic than in acidic medium. This was shown to be related to the p K 's of the triplet dyes, by evaluating the yields of ¹O₂* from the lifetimes and the quenching rate constants for the two ionic species of sensitizer triplets measured by laser flash photolysis. Changes in the quenching rate constants of the thiazine triplet states can be correlated with the triplet energies.     

QUENCHING OF SINGLET OXYGEN BY HUMAN PLASMA

Direct measurements of the decay of singlet oxygen phosphorescence at 1270 nm were made in human plasma diluted with various amounts of deuterium oxide. The Stern-Volmer plot of the singlet oxygen lifetimes was linear up to 15% plasma concentration (vol/vol). Extrapolation of these measurements to 100% plasma content gave a singlet oxygen lifetime of 1.04 +/- 0.03 microseconds in human plasma. Biological molecules accounted for 77% of the total singlet oxygen quenching while water accounted for 23% of the quenching. The contributions of various types of biological molecules to the total singlet oxygen quenching were calculated from their plasma concentrations and their quenching constants. Plasma proteins quenched most of the singlet oxygen. Uric acid also quenched a significant amount of singlet oxygen (12%). Tocopherols, carotenoids, ascorbic acid and bilirubin made only small contributions to the total singlet oxygen quenching (less than or equal to 4%).     

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.     

RATES OF REACTION OF SINGLET OXYGEN WITH SULFIDES

Abstract—Reaction rate constants for the reaction of singlet oxygen with a series of 24 sulfides in chloroform have been measured by inhibition of the self-sensitized photooxidation of rubrene. The reaction rate constant is sensitive to steric effects, decreasing as the carbons α- to sulfur become more highly substituted. Addition of a methyl group to each of the carbons α- to sulfur decreases the rate constant by about a factor of 10. From a series of p - and m -substituted thioanisoles, a ρ of -1.67 ± 0.09 was found. A much better correlation was found with σ than with σ⁺ indicating there is no resonance interaction with the reaction center. Typical rate constants are: di- n -butyl sulfide, 2.3 × 10⁷ M ^-1 s^-1; CBZ-L-methionine methyl ester, 1.4 × 10⁷; di-s-butyl sulfide, 1.8 × 10⁶; di- t -butyl sulfide, 1.3 × 10⁵; and thioanisole, 2.3 × 10⁶.     

ON THE PRODUCTION OF NITROXIDE RADICALS BY SINGLET OXYGEN REACTION: AN EPR STUDY

Abstract— The production of free radicals by reaction of 2,2,6,6-tetramethyl-4-piperidinol with singlet oxygen was studied by EPR spectroscopy. The rate constant of the amine was found to be equal to 8 ×10⁵ M ^-1s^-1 in ethanol and to 4 × 10⁷M^-1s^-1 in phosphate buffer (pH 8). Competition experiments were performed with singlet oxygen quenchers such as NaN₃, DABCO and the quenching rate constants were found to be consistent with the literature values. The EPR method proved to be a valuable technique to study the reaction of singlet O₂ with the sterically hindered amine without any interfering effect.     

THE CHROMOPHORES AS ENDOGENOUS SENSITIZERS INVOLVED IN THE PHOTOGENERATION OF SINGLET OXYGEN IN SPINACH THYLAKOIDS

Abstract— The photogeneration of singlet oxygen (¹O₂) from thylakoids and the chromophores involved as endogenous sensitizers were investigated using chloroplasts and thylakoids isolated from spinach. The blue light-induced inhibition kinetics of photosynthetic electron transport and that of CTvCF, ATPase were also studied. The spectral dependence of the generation of ¹O₂ from thylakoid membranes, measured by the imidazole plus RNO method, clearly demonstrated that the Fe-S centers play an important role in ¹O₂ generation, acting as sensitizers in thylakoids. The photoinhibition of the electron transport in isolated chloroplasts was strikingly depressed by a lipid-soluble '0₂ quencher and enhanced by deuterium oxide substitution, indicating that the inhibition processes are mainly mediated by ¹O₂ which is produced via photodynamic activation. The involvement of chloroplast cytochromes in the production of ¹O₂ was deduced from the action spectrum for the photodynamic inhibition of the electron carrier chain. The results obtained from the kinetic studies appear consistent with the involvement of some components such as the Fe-S centers and cytochrome chromophores of the carrier chain in the generation of ¹O₂.     

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.     

PHOTOOXIDATION OF 8-METHOXYPSORALEN WITH SINGLET OXYGEN*

Abstract— The in vitro photooxidation of 8-methoxypsoralen (8-MOP) with singlet oxygen is studied. Irradiation of 8-MOP(295–400 or320–400 nm) in the presence of oxygen for 72 h results in the formation of a product (1.4%) which is identified as 6-formyl-7-hydroxy-8-methoxycoumarin by aid of IR, NMR, MS and co-chromatography with an authentic sample. A study of this reaction in the presence of l,4-diazobicyclo(2,2,2)octane, a singlet oxygen scavenger, indicates the involvement of ¹O₂ in the formation of this compound. In addition to this, formation of a novel dimer of 8-MOP is reported.     

OXODIPYRROMETHENES AS REACTIVE SINGLET OXYGEN ACCEPTORS. MEASUREMENT OF THEIR CHEMICAL REACTION RATES BY A LASER FLASH PHOTOLYSIS TECHNIQUE

Abstract— Singlet oxygen has been generated directly in 1,1,2-trichloro, 1,2,2-trifluoroethane solution by irradiation of the oxygen dimol 2³∑-_g→2¹δ_g transition with a pulsed dye laser and the ³∑_g^-→¹δ_g+ lv transition with a continuous Nd-YAG laser. The rates of chemical reaction and physical quenching of singlet oxygen so generated has been measured for a series of substituted oxodipyrro-methenes. The results show that the oxodipyrromethenes react with singlet oxygen at rates comparable to that for 1,3-diphenylisobenzofuran. The rate of quenching of singlet oxygen by ground state oxygen has been measured to be 2.5±0.3 × 10³ M^-l s^-1.     

不同硫醚化合物和单线态氧反应能力的研究

本工作采用Young提出的方法,利用二苯基苯并呋喃及硫醚类化合物对单线态氧的竞争反应,通过荧光强度的比较,测定了一系列烷基和芳基硫醚化合物和单线态氧反应的速度常数。对所得结果,联系立体效应和电子诱导效应进行了讨论。     

单线态氧的检测及分析应用研究进展

综述了单线态氧（^1O2）的测定方法。介绍了单线态氧的ESR、磷光光度、分光光度、化学发光和荧光法的基本原理、方法研究进展和分析应用情况,并重点介绍了化学发光法和荧光法,同时比较了这几种方法的特点。     

PHOTOCHEMISTRY OF cis-POLYISOPRENE AND ITS SINGLET OXYGEN ADDUCT

Abstract— Reaction of singlet oxygen (¹Δ_g, ¹O₂) with cis -polyisoprene yields an allylic hydroperoxide with an olefinic double bond shifted in the polymer chain. The photochemical decomposition of the resultant hydro-peroxide and the subsequent polymer chain scission kinetics have been studied in the absence of oxygen. Quantum yields of hydroperoxide decomposition range from 3.1 to 8.4 in cyclohexane, depending on the initial amount of hydroperoxide in the polymer. On the other hand, the quantum yields for polymer chain scission are low, and vary with the frequency of the incident light. The ratio for number of polymer scissions per number of hydroperoxy groups decomposed is of the order of 10^-2. The polymer chain degradation is sensitized by the addition of ketones. Based on these data, a reaction mechanism for the overall photodegradation of the cis -polyisoprene initiated by singlet oxygen is proposed.