PHOTORESPONSES OF SEMICONDUCTOR POLYCRYSTALLITES SEPARATING TWO AQUEOUS SOLUTIONS

Solar cells using polycarbonate membranes, with CdS deposited on them, were made by a very simple way; the CdS-containing membrane separates a Lucite cell into two compartments. On illumination, about 150 mV photovoltage ( V _op) and 0.5 μA cm^-2 photocurrent ( I _sc) could be produced; one side of the membrane acted as photoanode, and the other side as photocathode. By means of coating Victoria Blue B (VBB) onto the membrane before CdS deposition, the maximum V _op and I _sc of the CdS-deposited membrane could reach 500 mV and 3.0 (μA cm^-2, respectively. A mixture of CdS and CdSe deposited membrane has also been tested and found to have both the advantages of high photovoltage (over 400 mV) and good stability after modification. Even more interesting results were also obtained with CdSe pellets in place of the CdS-deposited membrane, in which V _op and I _sc of the cell were 1.2 V and 6 mA cm^-2, respectively. The essential aspect of the system, modelled after the photosynthetic thylakoid membrane, contains an asymmetrical, ultrathin semiconductor crystallite layer separating two aqueous solutions.     

FLUORESCENCE INDUCTION IN THE RED ALGA PORPHYRIDIUM CRUENTUM

Abstract— The intensity dependence and the spectral changes during the fast (sec) and the slow (min) transient of chlorophyll (Chl) a fluorescence yield, measured at 685 nm, have been analyzed in the red alga Porphyridium cruentum . Both the fast and the slow fluorescence yield changes are affected differently by the inhibitors of electron transport ( e.g ., DCMU) and by the uncouplers of phosphorylation (atebrin and FCCP). Fixation of Porphyridium cells with glutaraldehyde abolishes most of the fluorescence yield changes except for the so-called very fast ( OI ) phase. The same fixed cells, however, reduce DCPIP (a Hill oxidant) but do not evolve O₂ when CO₂ is used as electron acceptor. We interpret these and other results by the hypothesis that fluorescence transients in intact cells are linked to both electron transport and the energy dependent structural changes in the thylakoid membrane.     

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.     

Mediated Electron Transfer Across Supported Bilayer Lipid Membrane with TCNQ‐Based Organometallic Compounds

Supported bilayer lipid membrane (s‐BLM) containing one‐dimensional compound 1, TCNQ‐based (TCNQ=7,7,8,8‐tetracyanoquinodimethane) organometallic compound {(Cu₂(μ‐Cl)(μ‐dppm)₂)(μ₂‐TCNQ)}_∞, was prepared and characterized on the self‐assembled monolayer (SAM) of 1‐octadecylmercaptan (C₁₈H₃₇SH) deposited onto Au electrode. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) results showed that the compound 1, dotted inside s‐BLM, can act as mediator for electron transfer across the membrane. Two redox peaks and the charge‐transfer resistance of 400 kΩ were observed for compound 1 inside s‐BLM. The mechanism of the electron transfer across s‐BLM by TCNQ is by electron hopping while TCNQ‐based organometallic compound is by conducting. Further conclusion drawn from this finding is that the TCNQ‐based organometallic compound embedded inside s‐BLM exhibits excellent electron transfer ability than that of free TCNQ. This opens a new path for the development of s‐BLM sensor and/or biosensor by incorporation with TCNQ‐based organometallic compounds.     

PORPHYRIN-SENSITIZED PHOTOREACTIONS IN THE PRESENCE OF ASCORBATE: OXIDATION OF CELL MEMBRANE LIPIDS AND HYDROXYL RADICAL TRAPS

Abstract— Photooxidation reactions in ascorbate (AH)-containing erythrocyte membrane suspensions have been studied in broad perspective by simultaneously monitoring lipid peroxidation in the membrane compartment and formation of hydrogen peroxide (H₂O₂) and hydroxyl radical (OH) in the aqueous compartment. Non-bound uroporphyrin (UP) and membrane-bound protoporphyrin (PP) were used as sensitizers. Photoreduction of UP to the radical anion (UP^-) was detected by electron spin resonance when UP/AH/membrane mixtures were irradiated anaerobically. Aerobic irradiation resulted in a strong AH^--stimulation of lipid peroxidation, H₂O₂ formation, and OH^- generation (detected with 2-deoxyribose (DOR) and the spin trap 5,5-dimethyl-l-pyrroline-N-oxide). Use of diagnostic agents (e.g. catalase, desferrioxamine, mannitol) revealed that OH^- is involved in light-stimulated DOR oxidation, but not in lipid peroxidation. Similar irradiation in the presence of PP resulted in far greater lipid peroxidation than observed with UP, but less DOR oxidation, and insignificant accumulation of H₂O₂. This suggests that photoreduction of membrane-bound PP is less efficient, possibly due to hindered access of AH^-.     

Na+/K+ TRANSPORT AND PHOTOSENSITIVITY OF THE COLORLESS FLAGELLATE Peranema trichophorum (Euglenida)

Abstract— Perimycin, ouabain and elevation of extracellular K⁺ concentrations cause an increase in the fluence rate thresholds (white light) for the step-up photophobic response in Peranema trichophorum . Elevation of extracellular Na⁺ concentration decreases the thresholds for this response in comparison to the control level. The fluence rate threshold of perimycin-treated cells increases before the side effect of an antibiotic action appears. Removal of K⁺ ions from the medium of K⁺-treated cells to a concentration of 1 mM depresses the threshold for the step-up response to the control level. By addition of K⁺ or Na ⁺ ions to perimycin- or ouabain-treated cells the threshold returns to the control value. It is suggested that the flagellar and cell membrane are responsible for changes of P. trichophorum photosensitivity.     

EFFECT OF TRANSMEMBRANE POTENTIAL ON CHARGE PHOTOSEPARATION IN LIPOSOMES

Abstract— An unexpected transmembrane potential effect on the recombination rate of the pheophytin or bacteriopheophytin anion-radicals (dissolved in membrane) and ascorbic ion-radicals (dissolved in aqueous interior) has been established in liposomes. The influence of transmembrane potential on the recombination rate of Ru³⁺ (dissolved in inner volume) and (C₁₈H₃₇)V⁺ or (C₁₄H₂₉)V⁺ (dissolved in membrane) was observed. The potential was created by a potassium concentration gradient between inner and outer volumes of liposomes in the presence of valinomycin. The effect of the potential was considered on the bases of: (1) it was determined by the diffusional drift of the hydrophobic radicals in a radial direction in the membrane, according to the direction of the electric field; (2) the electric field changed the rate constant of the electron transfer, owing to the effects on the free energy and electronic coupling. Our results show the first explanation to be preferable.     

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.     

THE QUANTUM EFFICIENCY FOR THE INTERPHOTOCONVERSION OF THE BLUE and PINK FORMS OF PURPLE MEMBRANE

When the cations bound to purple membrane are removed it turns blue, and when this blue membrane is irradiated its color changes to pink. Irradiation of pink membrane leads to the reformation of blue membrane. We have determined that the quantum efficiency for the formation of pink membrane from deionized blue membrane is 1.6 ± 0.6 ± 10 ⁴ at 0^oC, pH 5.0. We also found that the quantum efficiency for the back photoconversion, i.e. the formation of blue membrane from pink membrane, is 8.8 ± 1.6 ± 10^-3 at 0^oC, 55 times greater than that of the forward photoconversion reaction. The extinction coefficients of the pink membrane and blue membrane were determined to be 44 500 ± 670 cm^-1 M^-1 at 491 nm and 54 760 ± 830 cm^-1 M ^-1 at 603 nm, respectively, assuming light-adapted purple membrane is 63 000 cm^-1 M ^-1 at 568 nm. The quantum efficiency for forming pink membrane from blue membrane is much lower than that for forming the photointermediate of the blue membrane's photocycle. Their relationship is similar to that of light-adaptation and photocycle of the dark-adapted purple membrane.     

265 NM LASER FLASH PHOTOLYSIS OF SOME ORTHO-SUBSTITUTED ANILIDES AND RELATED N-FORMYLKYNURENINE DERIVATIVES

Abstract— The physical and chemical properties of the triplet state of eight ortho-substituted anilides including N -formylkynurenine (FK), the major trp UV-photooxidation product and a remarkable photodynamic agent, have been investigated using both pulse radiolysis and 265 nm laser flash photolysis techniques. The molar extinction coefficient, the intersystem-crossing quantum yield and the oscillator strength of the T ₁→ T _n absorption band (Λ_max˜ 450 nm) have been determined. It is shown that anilides having n,π* triplets readily react with most solvents whereas those having π ,π* triplets slowly react with alcohols. In both cases, the semi-reduced species are formed. In water, the formation of the semi-reduced. species most probably involves the first excited singlet state. The triplet state properties of the FK derivatives (i.e. ortho-substituted anilides having a side chain bearing charged groups such as carboxylic or amino groups) are strongly modified by the ionization state of the charged side chain. In the case of the FK derivatives possessing an uncharged amino group, quenching of the triplet state occurs via a fast reversible electron transfer reaction from the NH₂ to the triplet anilide.     

SALT AND pH-DEPENDENT CHANGES OF THE PURPLE MEMBRANE ABSORPTION SPECTRUM

Abstract —Purple membrane suspensions change their color to blue and the absorption maximum shifts to 608 nm when the membrane is deionized on a cation exchange column or when it is washed first with < 2N NaCl followed by deionized water. The deionized chromophore is essentially identical with the chromophore produced by lowering the pH of the native membrane to < 4.0 (p K < 3.0). However, the deionized membrane does not aggregate and can be obtained in the pure state. The original purple color of the membrane is restored by addition of around 1 m M Na⁺, K⁺ or 10 μ M Mg²⁺, Ca²⁺, Sr²⁺, Mn²⁺, Pb²⁺ or La²⁺ when the protein concentration is 5μ M . The required salt concentrations decrease with decreasing pH. Direct measurement of bound Ca²⁺ by atomic absorption spectroscopy yields a ratio of Ca²⁺ to protein of <2 and a binding constant of 1.4 × 10⁶. Titration of the spectral change with salts at different pH values shows a linear relation between the pH and the logarithm of the salt concentration, with a 1:1 ratio for Na⁺ and 1:2 ratio for Ca²⁺. These relations are well predicted by Gouy-Chapman theory; however, the accompanying release of protons, changes of the CD spectrum, the complex kinetics of the spectral change during reconstitution with salt and preliminary X-ray diffraction results all suggest that conformational changes may be occurring in the protein.     

LIGHT-INDUCED FOURIER TRANSFORM INFRARED SPECTROSCOPIC INVESTIGATIONS OF THE INTERMEDIARY ELECTRON ACCEPTOR REDUTION IN BACTERIAL PHOTOSYNTHESIS

Abstract— Molecular changes associated with the light-induced reduction of the intermediary electron acceptor I (bacteriopheophytin, BPh) in bacterial photosynthesis were studied by means of Fourier transform infrared (FTIR) difference spectroscopy. Chromatophore membranes and reconstituted reaction centers (RCs) of Rhodopseudomonas viridis were prereduced with sodium dithionite and illuminated in order to trap photochemically the state I⁻. Fourier transform infrared spectra of these samples were recorded before, during and after illumination, with an accuracy better than 10⁻³ absorbance units. Difference spectra of I⁻ in chromatophores and in RCs closely correspond to each other. In the carbonyl stretching frequency region between 1640 and 1750 cm⁻¹, bands are tentatively attributed to a shift (from 1713 to 1683 cm⁻¹) of a keto carbonyl group, a change of an acetyl carbonyl grou at 1656 cm⁻¹ and a decrease in absorbance strength of ester carbonyl groups (at 1746 and 1732 cm⁻P) after reduction of I. These groups likely belong to the BPh molecule, although at least one of the ester carbonyls could be assigned to an amino acid side chain. The absence of strong bands in the amide I and amide II region excludes large protein conformational changes associated with I reduction.     

KINETIC STUDIES ON THE STABILIZATION OF THE PRIMARY RADICAL PAIR P680+ Pheo- IN DIFFERENT PHOTOSYSTEM II PREPARATIONS FROM HIGHER PLANTS*

Abstract— The stabilization of the primary radical pair P680+ pheophytin (Pheo)^- through rapid electron transfer from Pheo to the special plastoquinone of photosystem II (PS II), Q_A, was analyzed on the basis of time-resolved (40 ps) UV-absorption changes detected in different PS II preparations from higher plants. Lifetime measurements of¹Chi* fluorescence by single photon counting and a numerical analysis of the redox reactions revealed (1) at exciton densities required for light saturation of the stable charge separation, annihilation processes dominate the excited state decay leading to very similar lifetimes of ¹Chi* in systems with open and closed reaction centers and (2) the difference of absorption changes induced by actinic flashes of comparatively high photon density in samples with open and photochemically closed reaction centers, respectively, provides a suitable measure of the rate constant of QA formation. Conclusion 2 was confirmed in PS II membrane fragments by measurements at three wavelengths (280 nm, 292 nm and 325 nm) where the difference spectrum of Q^-_A formation exhibits characteristic features. The numerical evaluation of the experimental data led to the following results: (1) the rate constant of Q^-_A formation was found to be (300 ± 100 ps)^-1 in PS II membrane fragments and PS II core complexes deprived of the distal and proximal antenna and (2) an iron depletion treatment of membrane fragments does not affect these kinetics. The implications of these results are briefly discussed in terms of the PS II reaction pattern.     

ELECTRON AND ENERGY TRANSFER IN ILLUMINATED POROUS VYCOR GLASS

Abstract— Porous Vycor glass samples containing adsorbed molecules were illuminated at 77 K by a mercury lamp jacketed by a filter cutting off wavelengths below 250 nm. Oxygen or carbon dioxide on Vycor produces an asymmetric electron paramagnetic resonance (EPR) signal best described as holes trapped in the glass. Methyl bromide produces an identical EPR signal plus four other lines due to methyl radicals. Evidence is presented that the products result from excitonic energy transfer from the Vycor to the adsorbed materials. Triphenylamine (TPA) adsorbed on Vycor can also be photoionized by similar illumination, and the cation radical TPA⁺ can be stabilized at 77 K if an electron acceptor is also adsorbed. Attachment of the photoejected electron by carbon dioxide forms CO₂^-, and that by methyl bromide leads to methyl radicals. The CH₃ radical yield is dependent on the surface separation between the electron donor (TPA) and the acceptor (CH₃Br). By monitoring the relative quantum yield of the methyl radicals as a function of distance separating the TPA and CH₃Br, it is shown that the photoelectron is capable of migrating on the Vycor glass surface.     

CHLOROPHYLL PHOTOSENSITIZED ELECTRON TRANSFER IN PHOSPHOLIPID BILAYER VESICLE SYSTEMS: SECONDARY OXIDATION OF REDUCED VIOLOGEN ACCEPTORS BY Ru(NH3)63+

Abstract— The kinetics of the oxidation of a homologous series of 4,4'-di(n-alkyl)-bipyridinium (viologen) radicals by Ru(NH₃)₆³⁺ in vesicle suspensions was studied using laser flash photolysis. The viologen radicals were produced photochemically in the bilayer membrane phase of the vesicles by electron transfer from the triplet state of chlorophyll-α. At high concentrations of Ru(NH₃)₆³⁺, the rate of oxidation of the viologen radicals in the aqueous phase was limited by the rate at which the radicals diffused from the membrane to the aqueous phase. The exit rate constant decreased from 2 × 10⁵ s⁻¹ for the methyl viologen radical to 4 × 10³ s⁻¹ for the pentyl viologen radical. Both the exit rate constants and the calculated values for the equilibrium association constants of the viologen radicals were unexpectedly insensitive to the length of their alkyl substituents. This, as well as other data, suggests that the radicals that diffused into the aqueous phase tended to remain associated with the membrane-water interface.     

Photosensitized reduction of methyl viologen using eosin-Y in presence of a sacrificial electron donor in water-alcohol mixture

Photoreduction of methyl viologen (MV²⁺) by eosin-Y (EY²⁻) in the presence of triethanolamine (TEOA) has been investigated in water–methanol mixture by means of steady-state photolysis and laser-flash photolysis in the visible/near-infrared regions. The complete conversion to the persistent methyl viologen radical cation (MV^·+) was observed in the presence of lower concentrations of EY²⁻ and excess TEOA. By laser-flash photolysis measurements, electron transfer was confirmed to occur from the triplet state of EY²⁻ [³(EY²⁻)*] to MV²⁺ in the rate constants of ca 2.0 × 10¹⁰ M ⁻¹ s⁻¹. The rates and efficiencies of production of MV^·+ were found to be dependent on solvent compositions and concentrations of MV²⁺ ionic salt and TEOA. The back electron transfer reaction from MV^·+ to EY^·− was retarded in the presence of TEOA, which supports that EY²⁻ is reproduced by accepting an electron from TEOA. In the presence of excess TEOA, the indirect formation of MV^·+ from EY^·3− which was produced by accepting an electron from TEOA, was confirmed. The contributions of both the oxidative and reductive routes of ³(EY²⁻)* for the MV^·+ formation have been confirmed.     

INHIBITION OF OXYGEN EVOLUTION IN CHLOROPLAST PHOTOSYSTEM II BY THE PROTEIN-MODIFYING AGENT TETRANITROMETHANE

Abstract— The protein-modifying agent tetranitromethane (TNM) reacts with tyrosine residues and -SH groups. It was found to inhibit photo synthetic electron transport on the water splitting side of photosystem II (P. V. Sane and U. Johanningmeier, Z. Naturforsch. 35c, 293–297, 1979). In the present work the inhibition by TNM is studied in detail using photosystem II submembrane fractions. It is shown that the action of TNM with membrane-bound proteins could imply the modification of tyrosine residues. At concentrations below 30 μ M and with short incubation periods (<2 min), TNM produces the release of the extrinsic polypeptides involved in the stabilization of the water-splitting complex, this being correlated with inhibition of electron transport at a site prior to H₂O₂ electron donation even though the inhibition cannot be prevented by the addition of Cl or Ca²⁺, which are known cofactors for oxygen evolution. As the incubation period or the concentration of TNM is increased, photosynthetic pigments are bleached, starting with aggregates absorbing at relatively long wavelengths. The inhibition by low concentrations of TNM differs from the effect of most of the previously reported inhibitors acting at the oxygen-evolving complex of photosystem II.     

PREPARATION AND PHOTOPHYSICAL STUDIES OF PORPHYRIN-C60 DYADS

Abstract Porphyrin-C₆₀ dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels-Alder reaction. The porphyin singlet lifetimes of both the zinc (Pz_n-C₆₀) and free base (P-C₆₀) dyads, determined by time-resolved fluorescence measurements, are ≦17 ps in toluene. This substantial quenching is due to singlet-singlet energy transfer to C₆₀ The lifetime of Pzn-¹C₆₀ is -5 ps in toluene, whereas the singlet lifetime of an appropriate C₆₀ model compound is 1.2 ns. This quenching is attributed to electron transfer to yield P_zn^bull;+-C₆₀^bull;-. In toluene, P-¹C₆₀ is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 ran and a lifetime of-10 μs was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C₆₀ triplet. In the more polar benzonitrile, P-¹C₆₀ underoes photoinduced electron transfer to give P^•+-C₆₀^bull;-. The electron transfer rate constant is −2 × 10¹¹ s⁻¹.     

Contribution of the cAMP-Dependent Signal Pathway to Circadian Synchrony of Motility and Resting Membrane Potential in Paramecium

It is known that the ciliated protozoan Paramecium multimicronucleatum has synchronized circadian rhythms of motility, resting membrane potential and cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) concentrations. The present study shows that (1) extracellularly added 4 m M tetraethylammonium (TEA)⁺ (a K⁺ channel blocker) almost completely abolishes the diurnal oscillation of intracellular cAMP concentrations; (2) even 32 m^M TEA⁺ fails to abolish the circadian motility rhythm; but (3) the motility rhythm is highly damped when 4 m^M TEA⁺ and 100 μ^M CdCl₂ (a Ca²⁺ channel blocker) are added simultaneously. A cAMP analogue ( N ⁶-monobutyryl-cAMP) added extracellularly accelerates swimming velocity. Both a K⁺ channel blocker ( e.g . TEA⁺) and an inhibitor (trifluoperazine) of adenylate cyclase (AC) suppress cAMP formation, supporting the hypothesis that AC in Paramecium has dual functions, as a K⁺ channel and as an enzyme for cAMP formation. It is hypothesized that the circadian synchrony is due to circadian fluctuations of AC causing separate circadian changes both in ciliary motion and membrane potential through a cAMP-dependent signal pathway that forms a sophisticated network of second messengers to govern the synchrony together with Ca²⁺- and cGMP-dependent pathways in a manner antiphasic and/or complementary to one another.