pH-DEPENDENCE OF RHODAMINE B SEMIQUINONE DISMUTATION RATE IN AQUEOUS ALCOHOLIC SOLUTION

Abstract— A single transient species is observed in absorption following the flash irradiation of dilute deoxygenated aqueous alcoholic solutions of rhodamine B in hte pH range 5.5–7.0 The second-order decay constant for this semiquinone radical is an approximate linear function of the prevailing hydrogen ion concentration, the transient persisting for a period of from several seconds in neutral solution to several hours at pH = 12.
The slow rate of decay and its pH-dependence are explained in terms of an electron or H-atom transfer between the (unobserved) protonated and (observed) unprotonated semiquinone radical with simultaneous regeneration of hte D^+> and production of the leuco-dye DH:—
D·+ DH·⁺→ D⁺+ DH
Insofar as the reactant concentrations are controlled by the acid dissociation constant K^a of DH^.+ the data are consistent with a value of 5.5±1.0 for pK_a and a pH-independent rate constant K_o of 1.3±0.5×10⁶ 1. Mole^-1 sec^-1 at 20C.     

THE PRODUCTION OF SINGLET MOLECULAR OXYGEN BY ZINC(II) PHTHALOCYANINE IN ETHANOL AND IN UNILAMELLAR VESICLES. CHEMICAL QUENCHING AND PHOSPHORESCENCE STUDIES

Abstract— Zn(II)phthalocyanine (ZnPc) generates O₂(¹Δ_g) with a quantum yield of ca. 0.4 upon photocxcitation at 354 or 600 nm in ethanolic solution as determined by time-resolved phosphorescence studies at 1270 nm and photooxidation experiments using 1,3-diphenylisobenzofuran (DPBF) as substrate. The quantum yield of photooxidation slightly increases upon incorporation of ZnPc into unilamellar liposomes of dipalmitoylphosphatidylcholine. Under our irradiation conditions (600 nm, 18^°C, and short light exposure times), DPBF(5–50 μM) undergoes photooxidation by a pure Type II mechanism; the rate constant for the O₂(¹Δ_g) + DPBF reaction is (1.1 ±0.1) x 10⁹ M^-1 s^_1 in ethanol solution and determined to be about two orders of magnitude smaller when both ZnPc and DPBF are embedded into liposomes.     

AN INVESTIGATION OF RETINAL AS A SOURCE OF SINGLET OXYGEN

Abstract— All -trans retinal is dissolved in alcohols and illuminated at 365 nm in the presence of a singlet oxygen acceptor, 2,5-dimethylfuran. Illumination produces the photosensitized oxidation of the acceptor which is measured by the disappearance of its 215 nm absorption band. A kinetic study is carried out and β_DMF is 1.6 × 10^--⁴ M . The quantum yield of ¹O₂ production from the light-excited retinal is estimated to 0.096. The retinal sensitized photooxidation of dimethylfuran is inhibited by a ¹O₂ quencher, 1,4-diazabicyclo(2,2,2)-octane, and enhanced by deuteration of the solvent. Deuterated solvents are known to increase ¹O₂ lifetime.
The production of ¹O₂ from retinal is briefly discussed in relation to the damage which may be induced by light in the visual cells.     

PULSE RADIOLYSIS STUDIES IN MODEL LIPID SYSTEMS: FORMATION AND BEHAVIOR OF PEROXY RADICALS IN FATTY ACIDS

Abstract— Radiolytic formation and peroxidation of fatty acid radicals have been investigated by pulse radiolysis techniques in oleate, linoleate, linolenate and arachidonate systems. A strong absorption band at 280 nm associated with conjugated radicals, R_conj, formed in polyunsaturated fatty acid moieties has been used as a probe for kinetic processes occurring at doubly allylic sites in the hydrocarbon chain. Formation of R_conj by O^- has been found to be more efficient than the less selective OH radical. Peroxidation of R_conj is shown to be somewhat slower, ( k _R+ O₂˜ 3 × 10⁸ M ^-1 s^-1), than O₂ reactions with radicals in oleate ( k _R+ O₂= 1 × 10⁹ M ^-1 s^-1). Peroxy radicals generated in these reactions disappear slowly by essentially second order processes (2 k RO₁˜ 10⁷ M ^-1 s^-1). The superoxide radical, O^-₂, shows little if any reactivity towards 0.01 M linolenate or arachidonate over periods of 20 s.     

LUMIFLAVIN-SENSITIZED PHOTOOXYGENATION OF INDOLE

Abstract— The lumiflavin-sensitized photooxygenation of indole in aqueous solutions has been investigated by means of steady light photolysis and flash photolysis. The semiquinone of lumiflavin and the half-oxidized radical of indole were formed by the reaction between triplet lumiflavin and indole (3.7 times 10⁹ M ^-1 s^-1). The semiquinone anion radical of lumiflavin reacted with oxygen to form superoxide radical. The triplet state of lumiflavin also reacted with oxygen forming singlet oxygen, ¹O₂. But the reaction between ¹O₂ and indole (7 times 10⁷ M^_l s^_1; estimated from steady light photolysis using Rose Bengal as a sensitizer) was far less efficient than the reaction between indole and triplet lumiflavin. The quantum yield of the lumiflavin-sensitized photooxygenation of dilute indole via radical processes was much higher than that via ¹O₂ processes, though appreciable ¹O₂ was formed.     

OPTICAL ABSORPTION AND ELECTRON SPIN RESONANCE SPECTRA OF CATION RADICALS OF DIMERIC CHLOROPHYLL a IN LOW-TEMPERATURE MATRICES

Abstract— A chlorophyll (Chl) a solution in 3-methylpentane at 77 K exhibits an absorption spectrum with a distinct peak at 706 nm in the red-band region. The formation of the 706 nm absorbing species (S706) was reversible with respect to temperature change; no chemical change was observed. γ-Irradiation of the rigid 3-methylpentane solution at 77 K yields an absorption spectrum which can be ascribed to S706⁺ and S706⁻. When carbon tetrachloride, an electron scavenger, was added to the solution, the absorption of S706⁺ survived, which has peaks at 850 and 956 nm. It is assumed that the S706 is hydrogen-bonded dimeric Chi a , which may be regarded as a model of P700 in photosynthesis. Cation radicals of monomeric Chi a were formed in a γ-irradiated sec -butyl chloride solution at 77 K, and an absorption spectrum with peaks at 730 and 813 nm was recorded. ESR spectra of the cation radicals of S706 and monomeric Chi a are of a similar shape but their linewidths are 7.5 and 11.0 Gauss, respectively. The linewidth narrowing observed for S706⁺ is clear evidence for the assumption that S706 is dimeric Chi a. Comparison was made of the absorption spectrum of S706⁺ with the light-induced spectrum of P700 reported earlier.     

Fast Redox Perturbation of Aqueous Solution by Photoexcitation of Pyranine

Abstract— Intense illumination (60-120 MW/cm²) of an oxygen-free aqueous solution of pyranine (8-hydroxypyrene-l,3,6-tri-sulfonate) by the third harmonic frequency of an Nd-Yag laser (355 nm) drives a two successive-photon oxidative process of the dye. The first photon excites the dye to its first electronic singlet state. The second photon interacts with the excited molecule, ejects an electron to the solution and deactivates the molecule to a ground state of the oxidized dye (φ⁺). The oxidized product, φ⁺, is an intensely colored compound (Λ_max= 445 nm, ε= 43 000 ± 1000 M ⁻¹ cm⁻¹) that reacts with a variety of electron donors like quinols, ascorbate and ferrous compounds. In the absence of added reductant, φ⁺ is stable, having a lifetime of -10 min. In acidic solutions the solvated electrons generated by the photochemical reaction react preferentially with H⁺. In alkaline solution the favored electron acceptor is the ground-state pyranine anion and a radical, φ, of the reduced dye is formed. The reduced product is well distinguished from the oxidized one, having its maximal absorption at 510 nm with e = 25 000 ± 2000 M^-l cm⁻¹. The oxidized radical can be reduced either by φ^- or by other electron donors. The apparent second-order rate constants of these reactions, which vary from 10⁶ up to 10⁹M⁻¹ s⁻¹, are slower than the rates of diffusion-controlled reactions. Thus the redox reactions are limited by an energy barrier for electron transfer within the encounter complex between the reactants.     

PRIMARY PROCESSES IN THE LASER FLASH PHOTOLYSIS AND PULSE RADIOLYSIS OF l-EPHEDRINE

Abstract— Transient absorption spectra produced by laser flash-photolysis of an aqueous solution of ephedrine have been measured under a variety of conditions. Ephedrine was found to photoionise via a biphotonic process. The apparent yield of photoionisation increases with lowering of pH, a value of 8.8 being found for the p K _a associated with this change. The cation radical absorption spectrum has been determined using the techniques of both pulse radiolysis and laser flash photolysis. The extinction coefficient of the cation at 295 nm was determined to be 1.37 × 10⁴ dm³ mol^-1 cm^-1 and 1.2 × 10⁴ dm³ mol-¹cm-¹ by the two techniques, respectively, at pH 11. It is also shown that the rate constant for electron abstraction by the azide radical to form the ephedrine cation is controlled by protonation of the amine group in the side chain. The ephedrine anion radical spectrum and its extinction coefficient at 305 nm were also determined. The excited states responsible for photoionisation and photodegradation are discussed.     

Molecular Requirement of Chlorosomal Chlorophylls. Self-Organization of a Chlorophyll Derivative Possessing a Hydroxyl Group at Ring II

Aggregation of zinc 7¹-hydroxyl-13²-demethoxycarbonyl-pheophytin a (Zn-7¹-OH-Chl) was examined in relation to the structure and function of the self-aggregates of 3¹-OH-type chlorophylls (Chi) in chlorosomes of green photosynthetic bacteria. The Zn-7¹-OH-Chl aggregates yielded a Q_y absorption band at 712 nm with a 1.2-fold larger width (full width at half maximum, 500 cm⁻¹) than the monomer's (420 cm⁻¹). Infrared and NMR spectroscopies revealed that each molecule in the aggregate links together with simultaneous coordination (C7¹-OH…Zn) and hydrogen bonding (C7¹-OH … O=C13¹). A nonlinear alignment of the constituent molecules in the oligomeric structure was assumed. Despite the similar molecular linkages, linearly aligned Q_y, moments in the Zn-3¹-OH-Chl aggregate gave a chlorosome-like broader, more redshifted Q_y band (740 nm; 670 cm⁻¹, 2.1-fold larger than the monomer's). Because it is advantageous for efficient light harvesting and energy transfer to have several Q_y, spectral components, spread over a wide spectral range, that can act as the energy gradient, it is concluded that not only the intermolecular linkages but the linear locations of OH, C=0 and Mg in the molecule are crucial for photosynthetic antenna of the self-assembled chiorosomal Chl.     

STUDIES ON SOME INTERMEDIATES AND PRODUCTS OF THE PHOTOREDUCTION OF 9,10-ANTHRAQUINONE *,†

Abstract— The mechanism of the photoreduction of 9,10-anthraquinone (AQ) in alcohol and hexane has been studied by flash photolysis. The fluorescence spectrum of the photoproduct, 9,10-dihydroxy anthracene shows a large shift between hexane and ethanol. The quantum yields of photoreduction for AQ are solvent-dependent, the reaction between the solvent radical and AQ determining the quantum yield.
The absorption spectrum of the 9,10-anthrasemiquinone (AQH.) has a long-wavelength absorption band with peaks at 631 and 678 nm. The second-order decay constants for AQH. were estimated to be 1.3 × 10⁹, 6.7 × 10⁸ and 2.0 × 10⁸ M ^-1 sec^-1 in ethanol, 2-propanol and ethylene glycol, respectively.
A long-wavelength absorption band was observed for 9,10-anthrasemiquinone radical anion, having peaks at 776 and 860 nm; epsi;_max= 1900 at 776 nm. This spectrum is compared with the spectra of 9,10-dihydroxy anthracene mono- and di-anions. The 9,10-anthrasemiquinone radical anion was found to photoreduce quantitatively to 9,10-dihydroxy anthracene mono-anion with a quantum yield of 0.1.     

SPECTRAL AND TIME DEPENDENCE STUDIES OF THE ULTRA WEAK BIOLUMINESCENCE EMITTED BY THE BACTERIUM Escherichia coli

Abstract— Weak luminescence was detected using photon counting equipment, from oxygenated, liquid cultures of Escherichia coli during two stages of its growth cycle. The first period of emission occurred during the exponential phase of growth and comprised a UV(210–330 nm) band and a visible region(450–620 nm) band, the total intensity being (1.65 ± 0.12) x 10³ counts s^-1. The second period of emission occurred during the stationary phase of growth and comprised only a visible region(450–620 nm) band of intensity (8.72 ± 0.15) x 10³ counts s^-1. When the growth temperature was raised from 306.15 to 310.15 K, the above emission intensities were approximately halved, but the spectra were not changed significantly. No luminescence was observed at either temperature when the E. coli was grown anaerobically. The visible region luminescence was attributed to excited carbonyl groups and excited singlet O₂ dimers formed during the decomposition of lipid peroxides. The UV component was tentatively assigned to oxidative side reactions accompanying the synthesis of proteins.     

MESODIPHENYLHELIANTHRENE—THE MOST REACTIVE SINGLET OXYGEN ACCEPTOR

Abstract— The self-sensitized photooxidation of mesodiphenylhelianthrene in various solvents has been investigated. The involvement of ¹O₂ as the reactive intermediate in the formation of the endoperoxide has been demonstrated by the quenching of the photooxidation by the efficient ¹O₂-quencher β-carotene. The rate constant for the addition of ¹O₂ to mesodiphenylhelianthrene has been determined to be k _R≅ 10¹⁰ M ^-1 s^-1, which is the highest value hitherto known in the literature. The probability factor p , which describes the concentration independent part of the overall quantum yield, has been measured to be p =0.17.     

EFFECT OF TEMPERATURE AND pH ON THE QUENCHING OF TRIPLET LUMIFLAVIN IN THE PRESENCE AND ABSENCE OF FERRI- AND FERROCYANIDE

Abstract— –Flash photolysis at 450 nm over the temperature range 0.8–60°C was used to determine Arrhenius parameters for the first and second order disappearance of triplet lumiflavin (1.66 µ .M ) at a flash energy of 2 kj in deaerated phosphate buffer at varying pH:
³Lf → Lf₀
³Lf +³Lf → Lf₀+ Lf₀
Arrhenius parameters were also determined for the pseudo first-order quenching of triplet lumiflavin by 10 µ M ferri- and ferrocyanide ions,
³Lf + Fe³⁺→ Fe³⁺→ Lf₀+ Fe³⁺ (energy transfer)
³Lf + Fe²⁺→ Lf^-+ Fe³⁺ (electron transfer)
and for disappearance of the semireduced lumiflavin in the presence of ferrocyanide at pH 6.8, by the second-order reaction
Lf^-+Lf ^-→ Lf₀+ Lf⁼.     

POLARIZED UV-ABSORPTION SPECTRA OF RETINAL ISOMERS-I. MEASUREMENTS IN EXTREMELY THIN MONOCRYSTAL PLATELETS

Abstract Crystals of all- trans retinal and both different forms of 11- cis , 12-s- cis retinal were grown on quartz slides with faces (101), (001) and (101), respectively, forming thin platelets of less than 0.2 μm thickness. Polarized UV absorption spectra at room temperature were measured in the range from 20 to 43 × 10³ cm⁻¹ with a microscope-spectrophotometer. In this spectral range three diffuse absorption bands were observed for all crystal types at similar wave numbers. A main absorption band was found at 25–28 × 10³ cm⁻¹, and two further bands at 32–34 and 38–40 × 10³ cm⁻¹. In case of all- trans retinal the latter band is by far the weakest in this spectral range. Additionally, the crystal spectrum of all- trans retinal shows a shoulder at the low wavenumber side of the main band which cannot be resolved in the corresponding solution spectrum. In the crystal spectra of 11- cis , 12-s- cis retinal, however, only a strong dissymmetry is observed at this side of the main band.     

THE EFFECT OF CHEMICAL STRUCTURE ON THE PHOTOSENSITIZING EFFICIENCIES OF PORPHYRINS

Abstract— A kinetic investigation was performed on the photooxidation of methionine sensitized by various porphyrins at different oxygen concentrations. The rate of photooxidation was found to be strongly dependent on the nature of the sensitizer. In the case of hematoporphyrin, chelation of Mg²⁺ and Zn²⁺ and especially of Cu²⁺ and Fe²⁺ caused a significant decrease of the photosensitizing efficiency. Fluorescence and/or flash photolysis studies showed that such a decrease is ascribed to an enhancement of the non-radiative decay of the first excited singlet state as well as to a reduction of the triplet lifetime. The sensitizing efficiency is also dependent on the nature of the porphyrin side chains. A reaction mechanism involving ¹O₂ as the oxidizing agent is proposed.     

COOPERATION OF CHARGES IN PHOTOSYNTHETIC O2 EVOLUTION–I. A LINEAR FOUR STEP MECHANISM

Abstract— Using isolated chloroplasts and techniques as described by Joliot and Joliot[6] we studied the evolution of O₂ in weak light and light flashes to analyze the interactions between light induced O₂ precursors and their decay in darkness. The following observations and conclusions are reported: 1. Light flashes always produce the same number of oxidizing equivalents either as precursor or as O₂. 2. The number of unstable precursor equivalents present during steady state photosynthesis is ∼ 1.2 per photochemical trapping center. 3. The cooperation of the four photochemically formed oxidizing equivalents occurs essentially in the individual reaction centers and the final O₂ evolution step is a one quantum process. 4. The data are compatible with a linear four step mechanism in which a trapping center, or an associated catalyst, ( S ) successively accumulates four + charges. The S ⁴⁺ state produces O₂ and returns to the ground state S ₀. 5. Besides S ₀ also the first oxidized state S ⁺ is stable in the dark, the two higher states, S²⁺ and S³⁺ are not. 6. The relaxation times of some of the photooxidation steps were estimated. The fastest reaction, presumably S *₁← S ₂, has a (first) half time ≤ 200 μsec. The S *₂ state and probably also the S *₀ state are processed somewhat more slowly (˜ 300–400 μsec).     

PHOTOSENSITIZATION OF PYRIMIDINES BY 2-METHYLNAPHTHOQUINONE IN WATER: A LASER FLASH PHOTOLYSIS STUDY

2-Methylnaphthoquinone (MQ) has been excited in water with a 20 ns laser flash at 353 nm and the resultant transient species have been observed optically. Triplet-state MQ (³MQ) decays on a sub-microsecond time scale. It has been characterized in terms of its absorption spectrum and quantum yield. Rate constants have been measured for the decay of ³MQ in infinitely dilute solution, for self-quenching by ground-state MQ, and for reactions of ³MQ with oxygen, thymine, uracil, 6-methyluracil, and orotic acid. The interaction of ³MQ with pyrimidines involves charge transfer to give the pyrimidine cation radical and the MQ : anion radical. These reactions are discussed in relation to the mechanism of pyrimidine photooxidation sensitized by MQ.     

ÉTUDE EN SPECTROSCOPIE PAR ÉCLAIR DES COLORANTS THIAZINIQUES EN SOLUTION AQUEUSE

Abstract— p K values of ionisation equilibrium of thiazines dyes in their triplet state have been measured in aqueous solutions by spectroscopy. The triplet-triplet absorption bands, in the red part of the spectrum, are given for thionine, azurs and methylene blue. It is shown that, in the pH range 4–9, the equilibria investigated correspond to a second protonation of thiazines dyes that occurs in the triplet state:
³TH⁺+ H⁺→³TH₂²⁺
designating by ³TH⁺ the thiazine cation similar to the species stable in neutral aqueous solutions.     

LEAKAGE OF 86Rb+ AFTER ULTRAVIOLET IRRADIATION OF Escherichia coli K-12

Abstract— Stationary phase cultures of a DNA repair proficient Escherichia coli K-12 strain showed a release of intracellular material as assessed by three different methods (260 nm absorption; [methyl-³H]thymidine leakage and ⁸⁶Rb⁺ leakage) after broad-band (Black-Light Blue) near-UV radiation but not after far-UV (254 nm) radiation. As a control response for membrane damage to cells, this leakage of intracellular material was also determined by each method after mild-heat (52°C) treatment of E. coli K-12. An action spectrum for the release of ⁸⁶Rb⁺ from E. coli K-12 after irradiation with monochromatic wavelengths, from 254 to 405 nm, is also presented. The action spectrum for lethality (F₃₇ values) obtained for this strain, shows that leakage of ⁸⁶Rb⁺ occurs at fluences equivalent to or slightly less than fluences causing inactivation at wavelengths above 305 nm. In contrast, at wavelengths below 305 nm, leakage of ⁸⁶Rb⁺ from irradiated cells can be induced but only at fluences significantly greater than was required to cause cell inactivation. These results indicate, therefore, that near-UV radiation can induce a damaging effect on the cell's permeability barrier which may be significant in causing the death of the cell, whereas the effect is not significant in causing the death of cells by far-UV radiation where DNA damage is known to be the main cause of lethality.     

CHLORIDE ENHANCEMENT OF QUENCHING OF TRIPLET METHYLENE BLUE BY Fe(II)

Abstract— The influence of chloride ion on the rate of decay of triplet methylene blue in 0.01 M acid in the absence and presence of ferrous ions was investigated by means of laser flash-photolysis monitored by kinetic spectrophotometry. Chloride weakly accelerates decay of ³MBH^2± in aqueous solution in the absence of Fe(II). Quenching of ³MBH²⁺ by Fe(II) is more strongly catalyzed by Cl^- in both water and 50 v/v% aq. CH₃CN. The uncatalyzed quenching constant, k ₅, is of the order of 1 × 10⁶ M ^-1 s^-1 while in 4.8 M aqueous chloride ( μ – 7.2 M ) k ₅= (37.2 ± 1.8) × 10⁶ M ^-1 s^-1. A possible role of chloride is as a bridging species in quenching via electron transfer between ³MBH²⁺ and Fe(II).