PHOTOOXYGENATION OF MAGNESIUM meso-TETRAPHENYLPORPHYRIN*

Abstract— On unsensitized photooxygenation magnesium meso -tetraphenylporphyrin underwent oxidative ring cleavage yielding a bilitriene derivative as the sole product. Kinetic studies by quenching technique using singlet-oxygen quenchers, ß-carotene and α-tocopherol, and by substrate direct disappearance technique (Foote and Ching) indicated that only singlet-oxygen process is involved in the photooxygenation, and that the rate of total consumption of singlet oxygen ( k _Q+ k _R) is 1.0 ± 0.4 times 10⁸ M ^-1s^-1.     

LASER FLASH PHOTOKINETIC STUDIES OF ROSE BENGAL SENSITIZED PHOTODYNAMIC INTERACTIONS OF NUCLEOTIDES AND DNA

Abstract— Laser flash photolysis studies of the production of the triplet state of the xanthene dye, rose bengal (RB), have been carried out. The reactions of this state with oxygen to form singlet oxygen and the superoxide anion radical have been observed and yields measured. Quenching of RB(T₁) by oxygen leads to approximately 75% singlet oxygen and 20% superoxide. The reactivity of these species-RB(T₁), O₂(¹Δ_g) and O₂^-—with four nucleotides and DNA have been determined. Only guanine residues showed any noticeable reaction at neutral pH. At higher pH guanine rate constants increased. The consequences to biological photodynamic processes are discussed.     

PHOTOLYSIS OF NITRATE ESTERS—II. SOLUTION PHOTOLYSIS OF ARALKYL NITRATE ESTERS: KINETICS, ESR SPECTRA, AND PHOTOPRODUCTS

Abstract— Irradiation of benzyl nitrate, meso- and dl-hydrobenzoin dinitrates, and cis- and trans -1,2-acenaphthenediol dinitrates in benzene solutions 0.02M in nitrato groups at 265–313 mp decomposed the esters with a quantum yield of 0.4±0.2 in reasonable agreement with earlier work on the gas-phase photolysis of ethyl nitrate. An important primary process was ArRCHONO₂+h v ArRCHO +NO₂, although ArRCHONO₂+h v ArRCHONO + O was not excluded. In secondary reactions solvent benzene was oxidized and nitrated, aldehydes were formed from the nitrate esters, and nitric oxide was evolved. The higher quantum yields of the photodecomposition in ethanol (0.8±0.3) and ether (3±1) solutions were interpreted in terms of a charge-transfer state similar to that found with nitroalkanes in these solvents. Acetaldehyde was produced in a rapid secondary reaction in alcohol solutions at 25° and at 77° K, NO₂ was identified as an intermediate from the ESR spectrum. No stereospecificity was detected in a comparison of rates, photoproducts, and ESR spectra for the stereoisomers. The overall results indicated inter- rather than intramolecular migration of oxy-nitrogen groups in secondary reactions. An apparently new, long-lived, oxynitrogen radical was detected in benzene solutions of nitric oxide and nitrate esters irradiated at 25°.     

USE OF TRIS (2,2'-BIPYRIDINE) RUTHENIUM(II) DICATION IN A NOVEL METHOD FOR THE DETERMINATION OF SINGLET OXYGEN REACTION RATES: APPLICATION TO STUDIES OF GELATIN

Abstract— A novel method for the determination of singlet oxygen reaction rate constants is described and applied to studies of cyclohexadiene in methanol and gelatins in H₂O and D₂O. The technique uses tris (2,2'-bipyridine) ruthenium(II) dication (Ru(bipy)₃₂₊) as both singlet oxygen sensitizer and in situ oxygen concentration monitor during irradiation of sealed samples. Because of the high efficiency with which the luminescence of Ru(bipy)₃2+* can be detected, and the fact that emission lifetimes are used, the method offers some advantages over those previously described. The advantages and disadvantages of the method are discussed. A rate constant of 2.1 (±0.3) x 10⁶ mol^-1 dm³ s^-1 has been determined for the reaction of ¹O₂ with cyclohexadiene in methanol. For two different photographic gelatins the sums of reaction and quenching rate constants are 2.0 (±0.4) x 10⁶ and 3.1 (±2.0) x 10⁵ mol^-1 dm³ s^-1; for swine skin gelatin this value is 3.9 (±2.4) × 10⁵ mol^-1 dm³ s^-1. Chemical reaction, rather than physical quenching, is the dominant reaction route for gelatins and ¹O₂.     

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.     

PRIMARY PRODUCTS OF LIQUID WATER PHOTOLYSIS AT 1236, 1470 AND 1849 Å

Abstract— The yield of the primary products of the liquid water photolysis at 1236 and 1470 Å is reported. It was found that besides the dissociation of the excited water molecules into H and OH radicals probably e⁻_aq is also formed. The H and OH radicals were scavenged by means of formate, and the e⁻_aq together with a part of H₂O* by adding carbon dioxide. The quantum yields determined at 1236 Å, are: Φ(H, OH) = 1.03 & 0.02, 0.06 <Φ( e⁻_aq , H₂O*) < 0.12 and at 1470 Å,: Φ(H, OH)=0.72±0.02, 0.037 < Φ( e⁻_aq , H₂O*) <0.075. The quantum yield of high purity water at 1849 Å in the absence of any scavengers is Φ(H, OH)=0.022. Previously published data by us for 1849 Å are also given: Φ(H, OH)=0.33 ± 0.01 and 0.02 < Φ ( e⁻_aq H₂O*) < 0.04. Reaction mechanisms are proposed.     

ACTIVATED OXYGEN: SINGLET MOLECULAR OXYGEN AND SUPEROXIDE ANION

Abstract— Elusive processes associated with molecular oxygen in chemical and biological systems are interpreted in terms of two activated oxygen species, singlet molecular oxygen (¹Σ⁺_g/¹Δ_g) and superoxide anion (X²π_g). The generation and deactivation of singlet oxygen by interaction with organic triplet states are discussed within a comprehensive theoretical framework. Experimental results indicate the anomalous molecular oxygen enhanced luminescence from organic chromophores in polymer matrices results from the deactivation of singlet (¹Δ_g) oxygen by energy transfer to electronically excited states of the chromophore, and three types of oxygen enhanced luminescence have been identified in these systems. Properties of the superoxide anion relevant to its solution chemistry are briefly discussed. Electron transfer theory is used to theoretically examine the generation of singlet oxygen in disproportionation reactions of the superoxide anion, predicting that, depending on the number of water molecules present, the disproportionation reaction is a proficient source of singlet oxygen. A competing quenching process imposes a limit to the steady state concentration of singlet oxygen in most chemical systems. Available experimental results on the quenching of singlet oxygen by superoxide anion are in good agreement with theoretical results obtained via application of electron transfer theory.     

Effect of Magnesium and Calcium Complexation on the Photochemical Properties of Norfloxacin

Abstract— The fluoroquinolone antibiotics can induce skin photosensitivity in some patients and this has been ascribed to the generation of reactive oxygen species, such as singlet oxygen (O₂[¹Δ_g]). We have studied the photochemical properties of the different ionized forms of the fluoroquinolone norfloxacin upon complexation with Mg²+ and Ca²+ ions, as it is proposed that the antibiotic exists mainly as a complex in the blood plasma. We found that the norfloxacin cation (pH < 6) shows no photodegradation after UVA irradiation and has a low quantum yield of O₂(¹Δ_g) generation. The norfloxacin cation does not complex. Ca²+ or Mg²+ ions; when these ions are added to the solution, we observed no changes in the fluorescence quantum yields (φ_flu) and singlet oxygen yields (φ_Δ). In contrast, the neutral (6 ± pH > 8.5) and anionic (pH > 9) forms of norfloxacin are able to complex calcium and magnesium, and their generation of O₂ (¹Δ_g) is decreased by complexation. The neutral zwitterionic form and the anionic form also quench singlet oxygen by both chemical and physical pathways regardless of complex formation, while physical quenching is observed for the cation. At pH > 7.4, norfloxacin photobleaches and complexation to Ca²+ and Mg²+ increases the rate at which photobleaching occurs. Thus, both the pH of the medium and complexation with metal cations may affect the phototoxic potential of this antibiotic.     

Photophysical properties of the lowest excited singlet and triplet states of thio- and seleno-psoralens

The decay processes of the lowest excited singlet and triplet states of five heteropsoralens (HPS) were investigated by steady-state and shift-phase fluorometry and by laser-flash photolysis in different solvents. The emission spectra of HPS are detectable only in trifluoroethanol (TFE), where fluorescence lifetimes (τ_F) and quantum yields (φ_F) were measured. The triplet lifetimes (τ_T), triplet (φ_T) and singlet-oxygen production (φ_Δ) quantum yields were determined in benzene, ethanol and TFE by laser-flash photolysis. Semiempirical (INDO/1-CI) calculations allowed the nature of the lowest excited singlet and triplet states and transition probabilities to be obtained. Theoretical and experimental results indicate that the two lowest excited singlet states S₁ and S₂ of HPS are close-lying and different in nature (π,π* and n,π*). The "proximity effect" between these two states controls the photophysical properties of HPS as it does for the other furocoumarins. However, HPS have a peculiar behavior with respect to the related compounds because they are fluorescent and have, in three cases, detectable intersystem crossing only in TFE. This behavior can be tentatively explained by a different energy gap and/or order between the S₁ and S₂ states.     

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.     

TIME-RESOLVED STUDIES OF SINGLET-OXYGEN EMISSION FROM L1210 LEUKEMIA CELLS LABELED WITH 5-(N-HEXADECANOYL)AMINO EOSIN. A COMPARISON WITH A ONE-DIMENSIONAL MODEL OF SINGLET-OXYGEN DIFFUSION AND QUENCHING

Abstract— Time-resolved measurements were made of near-infrared emission from 5-( N -hexadecanoyl)amino-eosinlabeled L1210 leukemia cells following pulsed-laser excitation. The cells were suspended in phosphate-buffered saline made with deuterium oxide solvent. A significant fraction of the emission occuring10–80 μs after the laser pulse was due to singlet oxygen. This singlet-oxygen emission is believed to result from singlet oxygen generated near the cell-membrane surface, where 5-( N -hexadecanoyl)amino eosin is known to concentrate, and then diffusing out into the buffer. The intensity and the kinetics of the experimentally observed singlet-oxygen emission were in excellent agreement with the predictions of a theoretical one-dimensional model of singlet-oxygen diffusion and quenching.
During the10–80 μs time period studied, most of the singlet oxygen was located in the buffer. Thus, the use of water-soluble singlet-oxygen quenchers, such as histidine, provide one means of separating the singlet-oxygen emission quenchers, such as histidine, provide one means of separating the singlet-oxygen emission from other sources of light during this time interval.     

ON THE PRIMARY PHOTOPROCESS OF 124-kdalton PHYTOCHROME

The photoreaction between P_τ and the first detectable intermediate, lumi-R, of 124-kdalton oat phytochrome has been investigated at low temperatures. The temperature dependence of the quantum yields of the photoreactions, P_τ to lumi-R and lumi-R to P_τ, has been determined. From measurements over a temperature range from 119 to 155 K, an activation barrier of 3.6 ± 0.5 kJ mol ₁ is found for the photoreaction of P_τ with 661-nm actinic light. A higher value (5.7 ± 0.7 kJ mol ^-1) is found for the photoreaction of lumi-R to P^τ. with 698-nm actinic light. Increased quantum yields are found in deuterated buffer solutions at low temperatures. The activation energies for deuterated phytochrome (3.2 ± 0.7 kJ mol^–1 for P_τ with 661-nm irradiation and 6.2 ± 1.2 kJ mol^-1 for lumi-R at 698-nm irradiation) are identical within the limits of error with those of protonated phytochrome. The lack of a deuterium effect for the activation energies favors the Z,E-isomerization rather than proton transfer or tautomerization for the chromophore photochemistry during P_τ⇄lumi-R conversion.     

FLUORESCENCE AND PHOTOOXIDATION OF 1—ANILINONAPHTHALENE—8—SULFONIC ACID IN REVERSED MICELLAR SOLUTIONS AND THE EFFECT OF CCl4

Abstract— The 1-anilinonaphthalene-8-sulfonic acid solubilized in dodecylammonium propionate reversed micellar cyclohexanic solutions, emitted a strong fluorescence, and was photooxidized under aerobic conditions. Carbon tetrachloride (CCl₄) highly quenched the fluorescence and remarkably enhanced the oxidation reaction. The fluorescence quenching obeyed the Stern-Volmer relation, and the photooxidation was caused by the singlet oxygen generated by the photosensitization of the dye. From the kinetic analysis, it was known that the intersystem crossing rate from the dye excited singlet to triplet was enhanced by CCl₄. Carbon tetrachloride did not quench the triplet state. The ratio of quantum yields for the oxidation in the presence and absence of CCl₄ was independent of the oxygen concentration in the reaction mixture. The fluorescence quenching constant and the intersystem crossing rate were obtained at various solubilized water contents.     

POTENTIAL PHOTOSENSITIZERS FOR PHOTODYNAMIC THERAPY—II. PHOTOPHYSICAL PROPERTIES OF [26] PORPHYRIN

Abstract— –Photophysical properties of [26] porphyrin (26 P) were investigated in chloroform. The quantum yields of fluorescence, of S₁→ T₁ intersystem crossing and singlet oxygen formation were measured. The purity, stability, the strong absorption in the red (δ_max= 783 nm; ε_max= 28 000 M ¹ cm^-1) and the ability of singlet oxygen formation recommend 26 P as potential photosensitizer for tumor therapy.     

RADIATION PROTECTION BY SUPEROXIDE DISMUTASE

Abstract— Protection of X-irradiated mice by bovine superoxide dismutase is enhanced when the enzyme is given intravenously both before and after the exposure. With the combined treatment, the LD₅₀₍₃₀₎ dose is increased from 734 ± 8 to 1144 ± 15rad for a dose reduction factor of 1.56 ± 0.04. This protection occurs in a dose range where hematological damage is an important contributor to animal lethality. The proliferative capacity of bone marrow stem cells, X-irradiated in air, is protected by exogenous superoxide dismutase. The enzyme increased the D₀ from 105 ± 6 to 290 ± 34rad, an increase that represents 83% of the oxygen enhancement ratio of 3.3. In N₂ and N₂O, the D₀ of the stem cells is 348 ± 50 and 327 ± 55 rad, respectively, and the enzyme does not significantly change these values.     

SINGLET OXYGEN YIELDS AND RADICAL CONTRIBUTIONS IN THE DYE-SENSITISED PHOTO-OXIDATION IN METHANOL OF ESTERS OF POLYUNSATURATED FATTY ACIDS (OLEIC, LINOLEIC, LINOLENIC AND ARACHIDONIC)

Abstract— The triplet state characteristics (spectrum, lifetime and quantum yield) for four dye sensi tisers [methylene blue (MB), erythrosin (ER), haematoporphyrin (HP) and riboflavin (RF)] were determined in methanol by laser flash photolysis and singlet oxygen yields (0.60 to 0.48) from time-resolved measurements of the 1270 nm near infrared emission. The reaction of singlet oxygen with four long chain unsaturated phenyl esters [oleate (18: 1), linoleate (18: 2), linolenate (18: 3) and arachidonate (20: 4)] was followed quantitatively using the singlet oxygen luminescence technique and also, after continuous420–700 nm irradiation, by HPLC and other analysis of the isomeric product monohydroperoxides. The overall quantum yield of photooxidation (∼10^-2) was shown to be consistent with the observed singlet oxygen quenching constants(2–12 times 10⁴ dm³ mol^-1 s^-1) for the four esters studied and the singlet oxygen lifetime in methanol (τ∼ 9 μs). The isomer product distribution was interpreted in terms of a dual singlet oxygen and radical mechanism, the radical contribution increasing with sensitiser in the order ER = MB < HP ≪ RF, but also showing some dependence on substrate unsaturation. Evidence is presented for singlet oxygen quenching by MB and RF ( k_O = 1.6 and 6.0 times 10⁷ dm³ mol^-1 s^-1) and for the accelerated photobleaching of the dye sensitisers in the presence of the unsaturated esters.     

PHOTOOXIDATION OF EPINEPHRINE SENSITIZED BY METHYLENE BLUE—EVIDENCE FOR THE INVOLVEMENT OF SINGLET OXYGEN AND OF SUPEROXIDE

Abstract— The photooxidation of epinephrine, sensitized by methylene blue or by chlorophylls, excited with red light, involves the reduction of two molecules of oxygen to hydrogen peroxide per molecule of epinephrine oxidized to adrenochrome. The initial rates of these reactions are not affected by low concentrations of catalase. In 99 mol % D₂O, rates of methylene blue sensitized photooxidations are accelerated as much as 5.2 times over rates in ordinary water. Azide anion is a more effective inhibitor of this reaction in D₂O than in H₂O. Half maximal inhibitions are obtained by 1.3 mM azide in H₂O and by 0.1 mAf azide in D₂O. Isotope effects and azide sensitivities point to photooxidation of epinephrine in D₂O primarily by a singlet oxygen pathway; in H₂O, non-singlet oxygen pathways become more predominant. Superoxide dismutase (SOD) markedly inhibits rates of the photooxidations in H₂O and in D₂O; about 25% at 10^-9 M SOD, and 50% at 10^-6 M SOD in H₂O. In the photooxidation in H₂O, both by non-singlet and singlet oxygen mechanisms, the amount of superoxide produced is equivalent to the amount of O₂ consumed in the photooxidation of epinephrine; the superoxide thus formed participates in the oxidation of epinephrine.     

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.     

CHEMICAL REACTION RATES OF AMINO ACIDS WITH SINGLET OXYGEN

Abstract— The chemical reactions of amino acids with singlet oxygen have been measured in D₂O solution where the singlet oxygen was generated directly by irradiation of the oxygen ³∑_g^-→¹δ_g+ lv electronic transaction with the 1.06 μm output of an Nd-Yag laser. Chemical reaction was measured as amino acid loss by an amino acid analyzer or by fluorescence in the cases of tryptophan and tyrosine.
The chemical rate constants, in units of 10⁷ M ^-1s^-1, are histidine 10, tryptophan 3, methionine 1.7, tyrosine 0.8 and alanine 0.2, In the cases of histidine, methionine and alanine the interaction appears to be entirely chemical, i.e. there is no evidence for physical quenching in addition to the chemical reaction. The histidine chemical reaction rate constant shows an increase with pD with a p K of 6.9.     

Determination of Fluorescence Yields, Singlet Lifetimes and Singlet Oxygen Yields of Water-Insoluble Porphyrins and Metalloporphyrins in Organic Solvents and in Aqueous Media

Abstract— Fluorescence quantum yields and singlet lifetimes for a wide range of hydrophilic to hydrophobic porphyrins and metalloporphyrins have been determined in toluene, methanol or acetone. Photosensitized singlet oxygen yields have been determined in the same solvents. For some porphyrins, the same quantities were determined in an aqueous medium, through use of an amphiphilic polymer to solubilize the porphyrin sensitizer and target molecule, 1, 3-diphenylisobenzofuran. Because rate constants for the deactivation of singlet oxygen ( k _d) and for its reaction with a target molecule (k _a ) are unknown in such aqueous polymer systems, a new method was developed for evaluating yields of singlet oxygen formation that also provides a value for the ratio k_d/k_a. A variation observed in quantum yield of singlet oxygen production for the aqueous polymer system with variation in initial concentration of the target molecule is discussed.