THE ELECTROPHOTOLUMINESCENCE OF CHLOROPLASTS

Abstract— Delayed light emission emanating from preilluminated chloroplasts can be perturbed with pulsed DC electric fields (200–4000 V cm^-1), The perturbation produces a strong stimulation of chlorophyll luminescence. During the field perturbation the stimulated emission rises to a maximum, typically within 100μs. and then decays. Two kinetic components, R (rapid) and S (slow)†, are distinguished on the basis of their rise and decay times and their field-dependence. The R component increases exponentially at high fields, decays within 100–300μs during the field pulse and collapses with t _1/2= 15 μs at the end of the field pulse. The S component occurs at low fields, exhibits near saturation at 500 V cm^-1, decays with t _1/2 about 3 ms during the field pulse, and collapses with t _1/2= 38μs at the end of the field pulse. Studies using inhibitors, ionophores, electron donors and electron acceptors associate the R component with ion transport processes. The relation to electron transport associated with Photosystem II is discussed.     

DELAYED-LIGHT STUDIES ON PHOTOSYNTHETIC ENERGY CONVERSION-IV. EFFECT OF TRIS POISONING, AND OF ITS REVERSAL BY HYDROGEN DONORS, ON THE MILLISECOND EMISSION FROM CHLOROPLASTS*

Abstract— Destruction of the oxygen-evolving activity of chloroplasts by treatment with 0.8 M Tris-HC1 results in an extremely rapid dark decay of millisecond delayed light. Addition of electron acceptors such K₃Fe(CN)₆ or NADP⁺ does not change the decay characteristics of this msec delayed light. Artificial electron donors such as DPC partially restore the msec delayed emission to the slowly decaying situation which is found in control chloroplasts. Addition of electron acceptors to this photochemically competent system results in more rapid decay and in an increase of emission at 1 msec, as in control chloroplasts. We suggest on the basis of the delayed light data that Tris treatment induces a rapid side reaction which uselessly dissipates the oxidizing and reducing power which is stored by Photoreaction II. Artificial electron donors allow the Tris-poisoned photoreaction to store energy long enough for utilization of the energy by normal photosynthetic reactions, as shown by the flattening of the delayed light curves. In the restored system the normal reactions of electron transport are thus able to compete with the Tris-induced side reaction. This interpretation is supported by the finding that the restored system requires higher exciting intensities for saturation of NADP⁺ reduction than the control system.     

SPECTRAL SENSITIZATION OF SnO2 THIN FILM ELECTRODE BY THE J-AGGREGATE OF CYANINE DYES IN MONOLAYERS

Arachidic acid monolayers containing 5,6,5',6'-dibenzo-I,I-diethyl-2,2'-cyanine chloride were prepared by the spreading method and deposited on the SnO₂ surface using the Langmuir-Blodgett technique. The SnO₂ : dye assembly prepared in this manner was used as a photoelectrode of the conventional electrochemical cell. A distinct J-band was observed in the action spectra of sensitized anodic and cathodic photocurrents. Effects of calcium arachidate barriers inserted between dye layer and either SnO₂ electrode (inside barrier) or electrolyte solution (outside barrier) on the photocurrent were examined. Although the inside barrier was effective in decreasing both anodic and cathodic photocurrents, the outside barrier did not suppress cathodic photocurrent. The following mechanism involving a molecular exciton of the J-aggregate is proposed for the sensitized photocurrent. The anodic photocurrent is caused by hole trapping by some reducing agent and concomitant injection of the electron from molecular exciton to the conduction band of SnO₂. Extraction of conduction-band electron of SnO₂ by molecular exciton and supplying to some oxidizing agent such as dissolved oxygen are responsible for the cathodic photocurrent.     

PHOTOOXIDATION AND ELECTRON-TRANSFER PROCESSES IN FREE-BASE TETRAPHENYLPORPHIN PROBED BY RESONANCE RAMAN SPECTROSCOPY

Abstract
We report here the resonance Raman studies of photooxidation of free base tetraphenylporphin (H₂TPP) in the presence of external electron acceptors such as CCl₄ and chloranil under selective laser irradiation. From the dependence of photooxidation on the concentration of electron acceptors, polarity of solvents, excitation lines and temperatures, we have inferred that a weak triplet exciplex formed between the excited H₂TPP and electron acceptor in non-polar solvents serves as transient species and the light-induced intermolecular charge transfer from H₂TPP to the electron acceptor is the primary process involved in photooxidation. Observation of partial photooxidation in the rigid matrix at low temperatures has been interpreted to be due to long-range quantum mechanical electron tunneling process. Almost complete photooxidation is observed in a soft matrix as the donor and acceptor molecules can attain favorable relative orientation and separation for electron transfer during the excited state lifetime of the exciplex.     

CHARGE TRANSFER COMPLEXES OF PHEOPHYTIN a WITH NITROAROMATICS. ELECTRON TRANSFER FROM EXCITED SINGLET OF PHEOPHYTIN a TO NITROAROMATICS

Abstract— The interaction of pheophytin a (Pheo) with seven nitroaromatic acceptors of varying ring sizes and electron acceptor abilities has been studied both in the ground and excited states. The ground state association constants ( K ) of the 1: 1 complexes of donor (Pheo) and acceptors were found to increase with increasing electron affinities of the different acceptors. All the nitroaromatic compounds efficiently quench the singlet emission of Pheo and the quenching follows the Stern-Volmer (SV) relationship. The SV constants ( K _sv) for different quenchers follow the same order as that of the K values. The reduction potential of Pheo₊/Pheo_* obtained from the quenching data agrees well with the theoretically predicted value. A charge transfer interaction between the singlet excited state of Pheo and the nitroaromatics is suggested from the dependence of quenching rate constants on the electron affinities of the acceptors.     

PHOTOOXIDATION AND ELECTRON-TRANSFER PROCESSES IN FREE-BASE TETRAPHENYLPORPHIN PROBED BY RESONANCE RAMAN SPECTROSCOPY

Abstract —We report here the resonance Raman studies of photooxidation of free base tetraphenylporphin (H₂TPP) in the presence of external electron acceptors such as CCl₄ and chloranil under selective laser irradiation. From the dependence of photooxidation on the concentration of electron acceptors, polarity of solvents, excitation lines and temperatures, we have inferred that a weak triplet exciplex formed between the excited H₂TPP and electron acceptor in non-polar solvents serves as transient species and the light-induced intermolecular charge transfer from H₂TPP to the electron acceptor is the primary process involved in photooxidation. Observation of partial photooxidation in the rigid matrix at low temperatures has been interpreted to be due to long-range quantum mechanical electron tunneling process. Almost complete photooxidation is observed in a soft matrix as the donor and acceptor molecules can attain favorable relative orientation and separation for electron transfer during the excited state lifetime of the exciplex.     

THE USE OF LIPID HEADGROUP CHARGE TO ENHANCE PHOTOINDUCED ELECTRON TRANSFER IN VESICLES: REACTIONS OF FUNCTIONALIZED PYRENE WITH A TWO COMPONENT VIOLOGEN SYSTEM

Abstract—
Kinetics of photoinduced electron transfer from a lipid functionalized pyrene, 1-(10-(6(8)-octadecylpyrenyl)decanoyl)-2-hexanoyl-.sn-glycero-3-phosphorylcholine (OPyPC), to a two component viologen acceptor system have been measured by laser flash photolysis. N, N'-tetramethylene-2,2'-bi-pyridinium ion (DQ²⁺) and N, N' -dipropyl-4,4'-bipyridinium sulfonate (PVS), have been utilized as the primary and secondary acceptors. It has been shown that utilization of a lipid with a net negatively charged phosphatidylglycerol headgroup provides a driving force for localizing high concentrations of primary acceptor (DQ²⁺) in the region of donor. Subsequently, the charged interface can act to maintain long-term separation between the oxidized pyrene donor (OPyPC⁺) and the reduced secondary acceptor, PVS^-. When a dioleoyl lipid is used, reaction of (OPyPC⁺) with the double bond competes significantly with back reaction. However, substitution of diphytylphosphatidylglycerol for the dioleoyl analog results in a cation lifetime of about 0.5 ms and a continued very long-lived reduced species (˜4 h). Quantum yields of ˜0.15 may be obtained in this system.     

氮氧自由基的研究(ⅩⅥ)——哌啶氮氧自由基电子转移反应的循环伏安特性

应用循环伏安法研究了哌啶氮氧自由基(1-3)在酸性甲醇中的电化学可逆氧化和还原特性。实验表明这些氮氧自由基可视为电子受体及给体的良好模型化合物,且1-3在酸性的非水溶液中的还原反应进行得很快,而减少酸性将有利于其氧化反应。     

FLASH PHOTOLYSIS-ELECTRON SPIN RESONANCE STUDIES OF THE DYNAMICS OF PHOTOSYSTEM I IN GREEN-PLANT PHOTOSYNTHESIS—II. INTACT AND BROKEN SPINACH CHLOROPLASTS*

Abstract —The transient oxidation and subsequent reduction of P700⁺ in spinach chloroplasts has been monitored by flash photolysis-electron spin resonance spectroscopy in the presence of various donors and acceptors. In general, the results obtained correlate well with results on Photosystem I subchloroplast particles, with two major differences. For Type A and B intact chloroplasts in the presence of 3-(3, 4-dichlorophenyl)-1, 1-dimethylurea (DCMU), the electron acceptor methyl viologen has no effect on the decay kinetics. This phenomenon is interpreted in terms of a functioning cyclic electron flow path around Photosystem I. Also, the photoresponse of Signal I depends on the length of the photolyzing flash. This is interpreted in terms of the existence of a primary electron donor to P700⁺ with a transfer time of ? 10 μs.     

AN FMN-PHOTOSYSTEM I PHOTOVOLTAIC CELL

Abstract— We have made a photovoltaic cell using Photosystem I subchloroplast particles isolated according to the method of Shiozawa et al. (1974). The particles were placed on a filter between two compartments one of which contained the electron donor, K₄Fe(CN)₆ and the other the electron acceptor, FMN. Upon illumination with white light ( I = 80 W/m²) a potential of 300 mV was observed across a 3000 Ω load resistance. Both Photosystem I photochemistry and direct photoreactions of FMN contribute to the process. A power output of 20 μW was observed for a 2 cm² filter containing 60 μg chlorophyll. This corresponds to 0.1 W/m². The power efficiency was 0.13%. The short circuit current was 108 μA.     

SPECTROSCOPY AND PHOTOCHEMISTRY OF 8-SUBSTITUTED 5-DEAZAFLAVINS

Abstract— Absorption, fluorescence and phosphorescence spectra as well as fluorescence and phosphorescence quantum yields of 8-X-5-deazaflavins (X = C1, NO₂, p -NO₂-C₆H₄, N(CH₃)₂, NH₂, p -NH₂-C₆H₄, p -N(CH₃)₂-C₆H₄-N=N) were determined. It was found that all these data are highly influenced by the substituent at position 8 of the 5-deazaisoalloxazine skeleton. Also the photoreduction of 8-X-5-deazaflavins in the presence of electron donors was studied. It was established that the photoreduction leads to the formation of a 5,5'-dimer and/or a 6,7-dihydro compound. Reduction of the C(6)-C(7) bond is promoted by strong electron-donating substituents and bulky electron donors. 5-Deazaftavins with a reducible substituent at position 8 exhibit reduction of the substituent prior to the reduction of the 5-deazaisoalloxazine skeleton.     

THE KINETICS OF REDUCTION OF CYTOCHROME c BY SEMI-REDUCED FLAVIN

Abstract— Flavin mononucleotide radicals, FMNH', generated by laser flash photolysis of FMN in the presence of the electron donors, histidine, guanosine monophosphate or EDTA, were found to reduce cytochrome c with an apparent rate constant of 6 ± 10⁷ M ⁻¹ s⁻¹. These flash photolysis results were, however, complicated by the electron donor radicals formed simultaneously which, particularly with EDTA, also lead to reduction of cytochrome c. Pulse radiolysis of a nitrous oxide saturated aqueous solution of FMN containing a high concentration of HCOONa, leads to the exclusive formation of FMNH'. By adding small concentrations of cytochrome c to this solution, a rate constant of 4.0 ± 10⁻¹ M ⁻¹ s⁻¹ was obtained for the reduction of cytochrome c by FMNH'. Replacement of the HCOONa by EDTA in such solutions leads to further routes for reduction of cytochrome c on radiolysis. as in the photolytic situation. The relevance of these results to flavin-photosensitised reduction of cytochrome c and other components of the mitochondrial electron transport chain is discussed.     

电沉积Cu2xIn2-2xSe2薄膜的光电化学研究

用电沉积法制得Cu_2xIn_2-2xSe₂(铜铟硒)(0[1]并用EDAX对其组成进行分析。对薄膜电极的光电化学性能、光谱响应、能隙与x的依赖关系进行了研究。借助于现场微区扫描光电流谱观察了热处理、薄膜厚度、光极化对薄膜电极的光电性能影响。研究了Pb(NO₃)₂有效的浸渍对薄膜光电性能的影响。     

PHOTOREDUCTION OF NAD TO NADH IN SEMICONDUCTOR DISPERSIONS

Abstract— Band gap illumination of TiO₂ (anatase) dispersions in weakly alkaline solutions of nicotinamide-adenine-dinucleotide (NAD⁺) leads in the presence of rhodium trisbipyridyl complex [Rh(bipy)³⁺₃], to continuous generation of biologically active cofactor NADH. Effects of pH, NAD⁺ and Rh(bipy)³⁺₃ concentration on the efficiency of this photoconversion process are investigated. The reaction proceeds already in aqueous solution in the absence of external electron donors but it is enhanced significantly in the presence of 10% methanol.     

INTERFACIAL ELECTRON TRANSFER INVOLVING RADICAL IONS OF CAROTENE AND DIPHENYLHEXATRIENE IN MICELLES AND VESICLES

Abstract— The radical cations and anions of diphenylhexatriene have been produced and characterized in homogenous and micellar solutions by pulse radiolysis and laser flash photolysis techniques. Both types of radical ions were formed in cyclohexane on pulse radiolysis. The radical cation was formed in dichloroethane on pulse radiolysis, and by two photon photoionization in ethanol, dichloroethane, and various micelles. Both radical ions have intense ( 10⁵ M ^-1 cm^-1) absorption peaks at600–650nm. The cation peak occurs at slightly shorter wavelengths than that of the anion.
In micelles and vesicles the radical anion of carotene was formed by electron transfer from e_a– on pulse radiolysis. The radical cation was formed on pulse radiolysis of micellar solutions containing Br^-₂ as counterion, presumably by electron transfer to Br₂^-. The spectra agree with those of the radical cation and anion of carotene that have previously been obtained in homogenous solutions (Dawe and Land, 1975).
Electron transfer in micelles and vesicles from the radical anion of biphenyl to carotene and diphenylhexatriene, and from the radical anions of these to inorganic acceptors has been studied.     

LIGHT-INDUCED ELECTRON TRANSPORT ACROSS SEMICONDUCTOR ELECTRODE/REACTION-CENTER FILM/ELECTROLYTE INTERFACES

Abstract— Reaction center (RC) complexes isolated from the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26 were dried as a film onto platinum and semiconductor (SnO₂) electrodes. The light-induced primary charge separation which occurs across the biological complex couples electrically with the SnO₂ but not with the metal electrode on the time scale of observation. As the working electrode in a two-electrode photoelectrochemical cell, RC-coated SnO₂ generated photovoltages as high as 80 mV and photocurrents as high as 0.5µA·cm² when exposed to light of λ >600nm. The number of quinone molecules per RC strongly influences the photovoltage and photocurrent observed. Photo-effects generated by RC electrodes persist after several days of storage; however, the kinetics and polarity of the effects are subject to change. The potential use of RC electrodes lies more as a new probe of photosynthetic electron transport rather than as a solar energy conversion device because modification to the RCs and their environment affect the electrical properties of the cell. An energy-level model is proposed to explain how the photoelectrochemical cell functions.     

CHEMILUMINESCENCE OF SOME REACTIONS WITH MOLECULAR OXYGEN

Abstract— Many inorganic oxidation reactions involving a variety of oxidizing agents show chemiluminescence in the spectral region of 400–600 mp. Excited O₂O₂-associates are assumed to be the emitting molecules formed by bimolecular recombination of HO _x O₂-radicals. Oxidation of sodium sulfite with molecular oxygen in the presence of Cu- and Fe-basic oxides shows chemiluminescence which originates from the reactions of the heavy metal complexes of OH- and O, H-radicals with O₂. The simplest way of producing a radical from O₂ is by the uptake of one electron. Suitable electron donors, such as hydro-quinones and semiquinones, emit light if treated with oxygen. In certain organic solvents OH- can also act as electron donor. Its presence causes the formation of semiquinones from quinones in the absence of oxygen. The chemiluminescence which is observable upon treatment of alkaline dimethylsulfoxide, dioxane, pyridine-water and alcohol-water mixtures with oxygen is also attributable to electron transfer from OH- to O₂.     

A NEW METHOD FOR THE DETECTION OF SINGLET OXYGEN IN AQUEOUS SOLUTIONS

Abstract— In experiments on the interception of reactive intermediates of strongly oxidizing character in dye (S) sensitized photooxidations using p -nitrosodimethylaniline (RNO) as a selective scavenger, it has been observed that some substrates (A) or ¹O₂ acceptors (like imidazole derivatives) induce the bleaching of RNO as followed spectrophotometrically at 440 nm. Since singlet oxygen (¹O₂) does not react chemically with RNO, this bleaching is a consequence of ¹O₂ capture by the imidazole ring which results in the formation of a trans-annular peroxide intermediate [¹O₂] capable of inducing the bleaching of RNO (-RNO). In the absence of RNO, [¹O₂] decomposes or rearranges into the final oxygenation product ¹O₂: ¹Δ_g Thus, the system imidazole plus RNO can be used as a sensitive and selective test for the presence of ¹O₂ in aqueous solutions. The method can also be applied in the presence of sensitizing dyes which, under visible irradiation, can partially bleach RNO even in the absence of imidazole derivatives. In such a case, the bleaching of RNO is strongly increased by the presence of imidazoles with a characteristic dependence on their concentration. The separation of the product of RNO bleaching by thin layer chromatography can serve as additional proof of the presence of ¹O₂ in the system. The imidazole plus RNO method has been applied to a number of sensitizing and non-sensitizing dyes.     

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.     

THE EFFECT OF 1,4-DIAZABICYCLO[2.2.2]OCTANE ON THE RADIOSENSITIVITY OF BACTERIA

Abstract— Hydroxyl radicals ('OH) are scavenged by 1,4-diazabicyclo[2.2.2]octane (DABCO) at a diffusion-controlled rate of 1.25 ± 0.1 × 10⁹ M ^-1s^-1. Unlike other efficient 'OH scavengers which exhibit protection of bacteria against irradiation both in oxic and hypoxic conditions, DABCO has been shown to protect Serratia marcescens and various strains of Escherichia coli only in oxic conditions.
DABCO appears to eliminate a component of the sensitization afforded by oxygen in all strains of E. coli tested. The level of this protection increases from ∼15% in the wild type AB 1157 to ∼100% in the recA uvrA mutant AB 2480. It is suggested that DABCO protects against lethal events that can occur on macromolecules other than DNA such as the cell membrane.
Results with added glycerol, as well as work in D₂O solution, indicate that DABCO is more likely to be acting by scavenging radicals rather than by quenching ¹O₂. If ¹O₂ is a component of the sensitization afforded by oxygen, then it is unlikely to be formed in a hydrophilic environment in the cell.