bluuebunny/crossref_metadata_2025 huggingface dataset:
A STUDY OF THE SAFRANINE‐ASCORBIC ACID PHOTOCHEMICAL REDOX REACTION
Authors: WONG H. K.
Published: January 2008

Abstract— Flash‐photolysis of safranine in deoxygenated 5 per cent aqueous pyridine produces a transient bleaching, which disappears by a second order process. Very probably this transient species consists of a pair of radicals or radical ions and not of the safranine in the triplet state. These radicals are also produced in chlorophyll sensitized reactions. Present results combined with previous work by us[I] indicate that the chlorophyll sensitization of safranine bleaching involves the formation of these radicals.

The direct photochemical reduction of safranine to its leuco form occurs through the intermediate formation of the radicals. The appearance and disappearance of these radicals is fast compared to the role of photochemical formation and thermal disappearance of leuco safranine. Ascorbic acid influences but is not essential to the photo‐bleaching of safranine in 5 per cent aqueous pyridine. An increase in ascorbic acid concentration increases the steady‐state yield of the radicals but decreases the steady‐state yield of the leuco safranine. In the absence of ascorbic acid or other added substance, safranine photobleaches irreversibly in deoxygenated 5 per cent aqueous pyridine. A plausible mechanism is proposed in explanation of the direct photochemical reduction of safranine.
TRIPLET STATE DE‐ACTIVATION RATES IN GLASSES
Authors: Crystals And , Hunter T. F.
Published: January 2008

Abstract— The temperature dependence, from 4·2° to 77°K, of naphthalene fluorescence and phosphoresence in a glass was measured, and the observed small increase in the rate of the radiationless process with lowered temperature is attributed to an increase in the density of lattice states. Measurements on the triplet deactivation of p‐bromonaphthalene in a naphthalene crystal between 4·2° and 77°K showed both the presence of a similar unimolecular radiationless process to that observed in glasses, and a guest‐guest triplet annihilation proced‐ing through thermal excitation into the host exciton band.
MOLECULAR MECHANISMS IN PHOTODYNAMIC ACTION†
Authors: Grossweiner L. I.
Published: January 2008

Abstract— Photodynamic action can be considered at the molecular level as a dye‐sensitized photoautoxidation of the biological substrate. Recent work has shown that interactions of a sensitizing dye, such as occur in aggregation or binding, can lead to significant changes in its photochemical reactivity. The possible role of such effects in the primary photodynamic mechanism is reviewed.
COMPARATIVE DECAY CHARACTERISTICS OF THE LIGHT GENERATED FREE RADICAL IN CHROMATOPHORES AND CHLOROPLASTS*
Authors: Cost K. , Bolton J. R. , Frenkel A. W.
Published: January 2008

Abstract— Chromatophores of Rhodospirillum rubrum when illuminated exhibit a free radical which ordinarily shows a biphasic decay. When chromatophores are dehydrated at room temperature the time course of the appearance of the light‐induced free radical is unaffected, but the decay pattern has been altered. Only the fast component remains, the slow component is no longer evident. Scanning the magnetic field reveals the presence of a dark signal which is stable as long as the preparation remains dehydrated. This signal has the same peak‐to‐peak line width of ˜ 10 G and the same g factor as the signal evident in the light. The amplitude of this signal is equal to the amplitude of the slow decay component seen in aqueous chromatophore suspensions. Chromatophores frozen in an aqueous medium at —150°C exhibit a behavior identical with dehydrated preparations. The effects produced by lyophilization or by freezing at low temperatures are entirely reversible. When a lyophilized preparation is re‐hydrated, the stabilized portion of the signal now decays in the dark; the same is observed when preparations frozen at —150°C are thawed. When such thawed or re‐hydrated preparations are illuminated again, they exhibit the usual light‐induced ESR signal showing a biphasic dark decay.

A comparison was made between the behavior of the light‐induced ESR signal of chromatophores and that of system I of chloroplasts. This comparison revealed that there is a greater similarity in some of the decay characteristics of these signals than had been recognized previously. In chloroplasts, temperature insensitive, non‐enzymatic back‐reactions of the light‐induced free radical appear to be nil, and in chromatophores a distinct portion of the light‐induced free radicals exhibit the same characteristics. Another portion of the chromatophore free radicals must be able to back‐react by electron tunneling, a mechanism which appears to be absent in the chloroplast system.
DIFFUSE SPECTRAL REFLECTANCE MODIFICATION OF SKIN BY U.V. IRRADIATION
Authors: Anglin J. H. , Raz A. I.
Published: January 2008

Abstract— Irradiation of guinea pig skin in vivo with light shorter than 300 nm produced a significant decrease in 330–400 nm reflectance. A similar decrease in reflectance was seen by irradiation of excised guinea pig and human skin. Chemical or physical vasodilation had no effect on the 240–400 nm reflectance. Irradiation of guinea pig skin in vivo with light longer than 300 nm produced no change in 330–400 nm reflectance unless photosensitized with 8‐MOP. U.V. irradiation of skin (in the presence of suitable endogenous or exogenous photo‐sensitizers) may result in the formation of new products that effectively absorb 330–400 nm photons.
A FLASH PHOTOLYSIS STUDY OF INTERMEDIATES IN PHOTOCHEMICAL REACTIONS OF CHLOROPHYLL
Authors: Chibisov A. K.
Published: January 2008

Abstract— The photochemical reactions of chlorophyll intermediates in vitro have been studied by the flash photolysis method. The flash excitation of pigment solutions has been shown to involve the population of a chlorophyll triplet state where the oxidation‐reduction processes occur. The mechanism and kinetics of pigment triplet decay have been investigated from 20°to — 50°C and the ability of chlorophyll molecules to carry out triplet‐triplet energy transfer has been established. The latter phenomenon has been used to show up the role of chlorophyll triplets in the reversible photooxidation reaction with P‐quinone. There have been studied initial products of pigment photoreduction with ascorbic acid and phenylhydrazine. Experimental data of the mechanism of the initial oxidation and reduction in chlorophyll photosensitized reactions have been analysed. There have been also obtained the differential spectra of chlorophyll triplets and radicals. A calculation has been made of rate constants for a few elementary reactions.
CHEMILUMINESCENCE AND ENERGY TRANSFER FROM SINGLET OXYGEN PAIRS
Authors: Balny Claude , Canva Jennifer , Douzou Pierre , Bourdon Jean
Published: January 2008

Abstract— Chemiluminescence and photochemiluminescence of systems generating excited (singlet) molecular oxygen has been studied at low temperature in fluid media. A possible interpretation is proposed: that dimers of singlet oxygen are directly or indirectly responsible for the observed emission, the species [1δg +1σg+] predominating. Energy transfer to a fluorescent molecule is possible only when conditions of spectral overlap, required for transfer by inductive resonance, are fulfilled. Evidence is also presented for the formation of a transient complex between oxygen and a sensitiser or acceptor molecule [S…O2], which is the precursor of singlet oxygen in these systems. However, other possible mechanisms cannot be excluded.
THE EFFECT OF A uvr MUTATION ON RESCUE OF U.V. IRRADIATED LON STRAINS OF ESCHERICHIA COLI*
Authors: Green M. H. L. , Donch J. , Greenberg J.
Published: January 2008

Abstract— Treatments such as plating at elavated temperature, plating in the presence of pantoyl lactone, plating on minimal medium, plating on complete medium after intense white illumination, or plating on complete medium after holding in buffer, are known to increase survival of U.V. irradiated Lon strains of Escherichia coli. The effect of these treatments on Hcr Lon+ and Hcr Lon strains was studied. Pantoyl lactone and elavated temperature recovery seem specifically related to the Lon defect. Holding in buffer does not enhance survival in an Hcr strain, and only exerts a small effect in an Hcr Lon strain. White irradiation and minimal medium may greatly enhance recovery of both Hcr and Lon strains. However, an Hcr Lon strain is sensitive to white irradiation itself.
A METHOD OF TREATING PHOTOCHEMICAL TRIPLET QUENCHING DATA
Authors: Shetlar Martin D.
Published: January 2008

Abstract— A modified Stern‐Volmer plot method is proposed for treatment of quantum yield data where both reactive photoexcited singlet and triplet states are present in solution. The method may be applied when the reactions occurring are zeroth order in ground state photochemical substrate and allows the separation of singlet and triplet state contributions to the total quantum yield and the calculation of kinetic lifetimes without going to very high quencher concentrations. The method is illustrated through its application to literature data on the Type II photoelelimination reaction of 2‐hexanone.
PROPERTIES OF THYMINE DIMERS
Authors: Herbert M. A. , Leblanc J. C. , Weinblum D. , Johns H. E.
Published: January 2008

Abstract— The photochemical and chemical properties of the four dimers of thymine have been studied. The extinction coefficients, reversal cross‐sections and quantum yields for reversal are presented as a function of wavelength in the range 200–289 nm. At any wavelength, the dimers have different reversal cross‐sections but also different extinction coefficients. The quantum yields for reversal are nearly the same for all four dimers, the values ranging from 0·6 to 0·9 over the wavelength range 200–289 nm. Titration curves for the four dimers show that for each dimer, two groups are involved with two pK's at about 10·5 and 12·2. Dimers A and B are stable at alkali and acid pH's. Dimers C and D are stable from pH 1 to 3 and unstable at other pH's, with thymine the main degradation product.
EXCITATION ENERGY TRANSFER IN BILIRUBIN‐CHLOROPHYLL AGGREGATES
Authors: MIEDZIWKO E. , FRACKOWIAK D.
Published: January 2008

Abstract— Solutions of chlorophyll a (chl a) with different concentrations of bilirubin (blr) were used as a model system for studying the mechanism of energy transfer between bili‐proteins and chl in blue‐green algae. Several optical properties such as the absorption spectra, emission spectra for different wavelengths of excitation, degree of polarization and lifetime of fluorescence were measured. The measurements were carried out in several solvents (of different hydrogen bond‐forming capacities) and at various dye concentrations.

The results indicate that aggregates of chl with blr are formed. Energy absorbed by blr in the aggregates is emitted as chlorophyll fluorescence, indicating energy transfer from blr to chl within the aggregate. The fluorescence yield and fluorescence polarization of aggregates are lower than those of isolated chl.
ESR STUDIES OF CHLOROPHYLL ONE‐ELECTRON PHOTOCHEMISTRY IN SOLUTION: KINETICS OF REVERSIBLE QUINONE REDUCTION AND GENERAL REDOX MECHANISMS†
Authors: MUKHERJEE DULAL C. , CHO DUCK H. , TOLLIN GORDON
Published: January 2008

Abstract— ESR studies have been made of the kinetics of semiquinone radical formation and disappearance resulting from the reversible photosensitization of reduction or oxidation, by chlorophyll, pheophytin or hematoporphyrin, of several quinone‐hydroquinone pairs in various solvents. The rate of radical decay was found to be second order with respect to the radical concentration in all systems. Radical formation rates were determined by the initial production rate minus the decay rate. The kinetic constants for single electron transfer between triplet porphyrins and quinones or hydroquinones were determined usingβ‐carotene as a quencher in aqueous pyridine, and by measuring the initial rate of radical formation at various concentrations of quinones and hydroquinones in methanol and ethanol. These constants were found to be approximately the same in a given solvent for benzoquinone and hydroquinone with all porphyrins, though the rates differed in different solvents: pyridine‐water ∼ 106I./mole sec, and methanol and ethanol ∼ 5X 104l./mole sec. Trimethylquinone and its hydroquinone also give similar rate constants for radical formation in pyridine‐water, ∼ 106 l./mole sec. The second order radical decay constants for both benzoquinone and hydroquinone in pyridine‐water were the same, ∼ 105 I./mole sec, with either chlorophyll, pheophytin or hematoporphyrin as sensitizer. The same activation energy, 6900 cal/mole, was found for chlorophyll‐benzoquinone and hydroquinone in aqueous pyridine; 5500 cal/mole was obtained for these systems in ethanol. In methanol and ethanol solutions of chlorophyll, the same radical decay rate constants, ∼106 I./mole sec, were observed for both benzoquinone and hydroquinone. Also, the same decay constants, ∼ 106 I./mole sec, were found for trimethylquinone and its hydroquinine in pyridine‐water. These latter two compounds gave extremely small steady‐state ESR signals in ethanol compared with aqueous pyridine. We have also observed that the steady‐state signal obtained with chlorophyll‐menadione in ethanol‐water was much enhanced by the presence of NADH. In contrast, NAD+ was found to decrease radical production, by increasing the decay rate, in the chlorophyll‐hydroquinone system in aqueous pyridine. These results are discussed in terms of possible mechanisms for radical formation and disappearance. The most likely possibility is considered to be a one‐electron oxidation or reduction of the porphyrin triplet, followed by radical disproportionation and redox reactions between the disproportionation products.
ON THE REQUIREMENT AND MODE OF ACTION OF LONG‐CHAIN ALDEHYDES DURING BACTERIAL BIOLUMINESCENCE*
Authors: CORMIER MILTON J. , ELEY M. , ABE S. , NAKANO Y.
Published: January 2008

Abstract— We have reinvestigated the role of aldehyde in bacterial bioluminescence relative to its absolute requirement for light emission. We have found that aldehyde is an absolute requirement for light emission at 25°C as well as in the frozen state (—3° to — 9°C). As found by earlier workers, certain luciferase preparations isolated from Ph. Jischeri do not appear to require aldehyde for bioluminescence from the frozen state. We can now attribute this behavior to contaminating levels of aldehyde in those preparations. The results suggest that reactant puddling exist in the frozen state in which micro regions of liquid form within the ice crystals resulting in enormous increases in reactant concentration.
PHOTOTAXIS IN EUGLENA—V. PHOTOSUPPRESSION OF PHOTOTACTIC ACTIVITY BY BLUE LIGHT*,†
Authors: TOLLIN GORDON , ROBINSON MARY I.
Published: January 2008

Abstract— The phototactic response of Euglena gracilis is suppressed when the organisms are exposed to bright light of wavelength less than 650 nm. Activity returns in the dark, approximately 40 min being required for full restoration. Action spectral measurements demonstrate that the receptor pigment which mediates the suppression response may be a flavoprotein. Evidence is presented which indicates that photosuppression involves a direct action on the photo‐tactic apparatus, perhaps a photobleaching of the tactic photoreceptor pigment.
PHOTOREACTIVATION ACTION SPECTRUM OF TOBACCO MOSAIC VIRUS‐RIBONUCLEIC ACID INACTIVATED BY U.V. RADIATION*
Authors: HIDALGO‐SALVATIERRA OSCAR , McLAREN A. D.
Published: January 2008

Abstract— Initially photoreactivation of irradiated (2537 Å) nucleic acid on pinto bean increases linearly with time of illumination with white light of 250 ft‐c. Maximum amounts of photo‐reactivation depend on the quality of light used. The action spectrum shows a peak in the ‘black light’ region, where the greater amount of photoreactivation is found, and a shoulder in the blue light region. Maximum repair is obtained with ‘black light.’ Photoreactivation does not occur at wavelengths above 550 nm. Photoprotection by illumination of leaves prior to inoculation by irradiated RNA was not found. The action spectrum for photoreactivation does not resemble the action spectrum for photosynthesis.
QUININE SULFATE AS A FLUORESCENCE QUANTUM YIELD STANDARD
Authors: FLETCHER AARON N.
Published: January 2008

Abstract— Samples of quinine sulfate or bisulfate from a number of vendors have been examined. The light absorptivity and fluorescence excitation spectra of quinine bisulfate prepared from six sources of hydrated quinine sulfate had satisfactory agreement with each other. The solid samples of quinine sulfate or bisulfate showed good agreement (2.2 per cent relative difference) in their fluorescence quantum yields but the one commercial solution of quinine sulfate was 20 per cent low. The relative fluorescence quantum yield of quinine sulfate was evaluated with excitation from 240 through 400 nm and was found to have no unexplainable deviations from a constant value. It is suggested that some of the conflicting results in the literature may be due to the common practice of using a different spectral bandwidth for absorbance measurements than that used for excitation.
ACTION SPECTRUM FOR POLAROTROPISM IN THE CHLORONEMA OF THE FERN DRYOPTERIS FILIX‐MAS (L.) SCHOTT
Authors: STElNER A. M.
Published: January 2008

Abstract— A series of dose response curves was worked out under steady state conditions for mediation of the polarotropic response of the chloronema of the fern Dryopreris filix‐mas (L.) Schott. The dose response curves show changes in slope with wavelength. In red and u.v. the slope values are higher than in blue. As a consequence the relative height of the peaks in red, blue, and u.v. and the fine characteristics in blue of the action spectra calculated on the basis of these dose response curves change decisively with different response levels taken for calculation. Therefore, a final decision cannot be made as to what photoreceptor(s) might be involved besides phytochrome. However, at low response levels the action spectrum looks similar to those action spectra described for a variety of other blue‐u.v.‐mediated photoresponses, which generally are believed as being indicative for a flavin. The findings clearly indicate that in cases such as Dryopteris and in cases, where action spectra were done only at one arbitrarily chosen dose, action spectroscopy may not allow the unequivocal identification of the photoreceptor(s) involved.
CHANGE IN THE BASE COMPOSITION OF MESSENGER RNA OF ESCHERICHIA COLI BY ULTRAVIOLET IRRADIATION AND ITS RECOVERY
Authors: YONEI SHUJI , TOKUNAGA FUMIO , NOZU KEIICHI
Published: January 2008

Abstract— The base composition of messenger RNA in Escherichia coli B/r and B8–1 irradiated with ultraviolet (u.v.) light has been examined. The experimental results are as follows: (1) the synthesis of rapidly labeled RNA does not stop in ultraviolet irradiated bacteria. (2) The rapidly labeled RNA in irradiated cells shows a change in base composition corresponding to the formation of pyrimidine dimers in DNA molecules. The mole per cent of adenine component is increased with ultraviolet dose. The ratio of purine/pyrimidine becomes larger and the GC content smaller. (3) The base composition of the rapidly labeled RNA in irradiated bacteria reversed to that in unirradiated cells, when the irradiated cells were reactivated by experimental procedures for photoreactivation or dark reactivation. The reversion in the base composition corresponds well to the decrease in the amount of thymine dimers in DNA molecules. (4) The mechanism of the change in the base composition of rapidly labeled RNA caused by ultraviolet irradiation is discussed.
PHOTOCHEMICAL REACTIONS OF PYRIMIDINE DERIVATIVES WITH ACRYLONITRILE IN AQUEOUS SOLUTIONS
Authors: HELENE CLAUDE , BRUN FRANCINE
Published: January 2008

Abstract— Ultraviolet excitation of pyrimidine derivatives in aqueous solutions containing acrylonitrile can lead to polymerisation of acrylonitrile and/or photoaddition reactions of acrylonitrile and the pyrimidine. Polymerisation of acrylonitrile induced by excitation of orotic acid occurs even in aerated solutions and is very likely due to radicals derived from excited orotic acid molecuies. Using 14C‐labelied orotic acid, the polymer is shown to contain the pyrimidine. With other pyrimidine derivatives, polymerisation is observed only in deaerated solutions and its low yield did not allow us to determine whether the polymerisation is induced by radicals or by photosensitisation. The role of the lowest excited singlet and triplet states of the pyrimidine derivatives in the photoaddition reaction is discussed. In the case of orotic acid, photoaddition is shown to proceed entirely by way of the triplet state.
LASER SATURATION OF OPTICAL TRANSITIONS IN A STARCH COMPONENT: THE AMYLOSE‐IODINE‐IODIDE COMPLEX*
Authors: HUFF L. , DeSHAZER L. G. , SCHNEIDER F. W.
Published: January 2008

Abstract— Saturation of optical absorption by an aqueous solution of the amylose‐iodine‐iodide complex was observed. Transmission of ruby laser radiation as a function of incident laser power density was measured and fitted to the steady‐state solution of a double‐absorption rate‐equation model. This study gave an indirect determination of the lifetime of the first excited energy level. The material behaved as a passive Q‐switch for a ruby laser.
THE INFLUENCE OF SEMI‐DEHYDRATION ON THE RESPONSE OF STREPTOCOCCUS LIQUEFACIENS TO ULTRAVIOLET LIGHT
Authors: WEBB S. J. , MALWINSKA KRISTINA
Published: January 2008

Abstract— The effect of relative humidity on the survival and sensitivity to radiations of Streptococcus liquefaciens has been studied. The micro‐organism was found to be little affected by dehydration in aerosols and its sensitivity to the lethal action of 2537 Å light to be unaffected by changes in the relative humidity at which the cells were held during irradiation. The cells were more stable to the lethal action of 3200–4000 Å, however, when they were held at 70% relative humidity than at 50 or 30% relative humidity. Mutant cells unable to liquify gelatine were induced by semi‐dehydration at 50% relative humidity and their numbers were increased by concomitant irradiation with 3200–4000 Å light. This type of mutant was not observed when the cells were irradiated with 2537 Å light. Mutant cells which had a different response from that of the parent cell to the presence of oxygen in their growth medium were produced by both wavebands of light, but only when the cells were held at 50% relative humidity. It is proposed that semi‐dehydration stresses the cell membrane and damages those parts of the bacterial DNA associated with the membrane. Concomitant irradiation is suggested to enhance this particular effect.
A RECONSIDERATION OF THE PROPERTIES OF A PULSED RUBY LASER IN RELATION TO MICROSURGERY
Authors: RAJARAMAN R. , KAMRA OM P.
Published: January 2008

Abstract— The properties of a pulsed ruby laser beam focused through a triocular microscope were studied in relation to microsurgery. Quantitative studies on ‘model tissues’ with different absorbencies at 6943 Å revealed that the spot diameter of the focused microbeam was dependent upon several factors. It decreased with (1) a decrease in the absorption by the object at 6943 Å; (2) a decrease in the energy of the microbeam; and (3) an increase in the magnification of the microscope. The existence of the ‘tube of effect’ in the optical path of the focused micro‐beam was realized by irradiating the object at different planes. The depth of the tube of effect increased with (1) increase in laser energy or (2) decrease in magnification. By employing certain plant species (Spirogyra praetensis Transeau, Ulva lactuca Linnaeus, Porphyra miniata (Lyngbye) C. Agardh and Elodea sp.) the reaction of the protoplasm to the laser microbeam in terms of ‘primary damage’ and ‘secondary damage’ was defined. Based on these experimental data the concept of minimal visible spot size in laser microsurgery and its limitations are discussed.
FLUORESCENCE OF 5‐METHYLCYTOSINE
Authors: GILL JAMES E.
Published: January 2008

Abstract— 5‐Methylcytosine and 5‐methyldeoxycytidylic acid are fluorescent in aqueous solution at room temperature and neutral pH. 5‐Methylcytosine, 10‐3M, pH 8.5, 25°C, has a quantum yield of 5 ×10‐4, 5‐Methyldeoxycitydylic acid, 10‐4M, pH 7.5, 20°C, has a quantum yield of 8 × 10‐4. Emission maxima are 2.91 and 2.80μ‐1. At pH 14, the quantum yield of 5‐methylcytosine is 1.6 × 10‐2; the emission maximum is 2.82μ‐1. At pH I, the quantum yield of both compounds is less than or equal to 10‐4. Both compounds were chromatographically homogeneous, had absorption spectra which agreed with published data, and excitation spectra which agreed closely with absorption spectra.
MECHANISM AND COMPUTER SIMULATION OF THE PHOTOTACTIC ACCUMULATION OF EUGLENA IN A BEAM OF LIGHT*‡
Authors: DIEHN BODO
Published: January 2008

Abstract— A mechanistic model is proposed which describes the phototactic behavior of Euglena during accumulation in an illuminated region. Measurements of the lag time occurring between illumination of the culture and net accumulation in the lighted zone as a function of culture density indicate that the relative strengths of the negative phototactic response inside and of the positive phototactic response outside this region are the prime factors controlling the lag phenomenon. Further evidence for this is provided by studies of the temperature dependence of the phototactic responses to polarized actinic light. It is shown that negative phototaxis as measured in the 7lsquo;phototaxigraph’ is not directed, but rather a shock‐mediated response. A ‘FOCAL’ computer program for simulation of experiments in the phototaxigraph has been written on the basis of our model. It correctly predicts the observed results under a variety of simulated experimental conditions. Measurements of the lag time and of the rate of accumulation in different parts of the actinic zone allow the calculation of motilities of the organisms with illumination and in the dark, the latter value being 0.08 mm/sec. For a 2–3‐weekold culture, the rate of negative phototaxis remained constant at light intensities above 40 ergs/cm2sec at 500 nm. At this wavelength, the threshold for the positive photophobic response was 100 ergs/cm2 sec.
EXCITED STATES OF FLAVINES – V. THE QUENCHING OF FLAVINE LUMINESCENCE BY INDOLES
Authors: BOWD A. , BYROM P. , HUDSON J. B. , TURNBULL J. H.
Published: January 2008

Abstract— The quenching of the fluorescence of flavines by indoles in aqueous solution has been studied as a function of concentration, viscosity and temperature. The quenching shows a negative temperature dependence, and is in part diffusion controlled. Static and dynamic processes both contribute to quenching.

In the presence of alcohol or denaturants static quenching is inhibited and dynamic quenching supervenes. The effect of indoles on the flavine fluorescence lifetimes and phosphorescence characteristics is consistent with the quenching mechanism proposed.
ACTIVATION KINETICS OF PHOTOSYNTHETIC OXYGEN EVOLUTION UNDER 20–40 NANOSECOND LASER FLASHES
Authors: WEISS CHARLES , SAUER KENNETH
Published: January 2008

Abstract— Chlorella samples, incubated for varying periods in darkness, were exposed to a series of 20–40 nsec flashes, spaced 15 sec apart, from a Q‐switched ruby laser. A stationary Teflon‐covered platinum electrode measured the microjet of oxygen produced by each flash. After a dark preincubation exceeding 3 min, at 23°C, little or no oxygen is evolved until the third flash in a sequence. The yields from subsequent flashes increase monotonically until a constant value is reached. If low levels of background light are supplied, or if the interval between series of flashes is decreased, oxygen is evolved on the second or even the first flash. Very similar results were obtained from analogous experiments with tailless 28 μsec flashes from a xenon flashtube. In particular, very little oxygen was evolved from the second flash following a long dark period whatever the spacing between flashes. This means that there are no systematic differences between the effects of saturating 20 nsec and 28 μsec light flashes on the activation processes during the first few flashes following a long dark period. No oscillations of flash yield with successive flashes were observed because of the long interval between flashes. These results are consistent with the idea that the reaction center of Photosystem II must undergo a dark process lasting considerably longer than 28 μsec before it can absorb a second photon.
DETECTION DE FAIBLES LUMINESCENCES EN SPECTROPHOTOMETRIE: COMPARAISON ENTRE DEUX TECHNIQUES DE MESURE PAR PHOTOMULTIPLICATEUR (‘EN COURANT’ ET ‘EN IMPULSIONS’)
Authors: DUQUESNE M. , VIGNY P. , GABILLAT N.
Published: January 2008

Abstract— Both photomultiplier detection techniques (anodic current method and pulse method) are recalled, and the expression for the signal‐to‐noise ratio is discussed.

In pulse detection, the role of time of observation appears clearly, as well as that of the discrimination threshold, the latter allowing a lower background noise.

Both techniques are used for spectrophotometric detection of weak luminescences. Experimental results show that pulse technique allows a better sensitivity of the spectrophotometer.
A COMPARISON OF LIGHT‐TRIGGERED SPECTRAL CHANGES IN RHODOPSIN AND ISORHODOPSIN
Authors: LAM P. B. , PRATT D. C.
Published: January 2008

Abstract— This report describes spectral changes associated with the transformation of metarhodopsin I to metarhodopsin II following light excitation of isorhodopsin and rhodopsin. Irradiated isorhodopsin gives rise to an equilibrium mixture of metarhodopsin I and metarhodopsin II which at 2°C and pH 6.8 favors the former. Isorhodopsin and rhodopsin are converted to metarhodopsin II via metarhodopsin I at very similar rates and activation parameters for the conversions are essentially identical. It is concluded that the initial cis to trans isomerization erases all differences in the two pigments.
THE EFFECT OF ULTRAVIOLET LIGHT ON RNA METABOLISM IN BROMOURACIL DEOXYRIBOSIDE‐GROWN HeLa CELLS*
Authors: PAINTER R. B. , GOODMAN M. G. , REISNER B. L.
Published: January 2008

Abstract— The presence of 5‐bromouracil deoxyriboside (BrUdR) in the DNA of HeLa cells has profound effects on RNA metabolism after u.v. irradiation. In normally grown cells 200 ergs/mm2 depresses RNA synthesis by about 30 per cent while in BrUdR‐grown cells the same exposure to u.v. depresses RNA synthesis by 95 per cent. When BrUdR‐grown cells are u.v. ‐irradiated after being labeled with 3H‐uridine, the normal autoradiographic pattern, where label shifts from nucleus to cytoplasm, fails to occur. Also, in lieu of the increase in RNA specific activity that is observed in unirradiated cells for a few hours after 20‐min pulse‐labeling, there occurs a constant decrease in specific activity after the irradiation.
EFFECTS OF ULTRAVIOLET RADIATION ON THE MITOTIC CYCLE AND DNA, RNA AND PROTEIN SYNTHESIS IN MAMMALIAN EPIDERMIS IN VIVO
Authors: EPSTEIN JOHN H. , FUKUYAMA KIMIE , FYE KEN
Published: January 2008

Abstract— Acute effects of ultraviolet radiation on the mitotic cycle and macromolecular synthesis were investigated on hairless mouse epidermis in vivo. Colcemid was used to arrest mitoses in metaphase and thus allow more accurate mitotic counts. The radioactive tracers, TdR‐3H, cytidine‐3H, and the amino acids, histidine‐3H and methionine‐3H were used to examine DNA, RNA and protein synthesis, respectively. Using these techniques, we found that wavelengths shorter than 320 nm markedly inhibited mitosis, increased the basal cell turnover time and depressed DNA, RNA and protein synthesis within the first few hours post‐irradiation. By 24hr, recovery and acceleration of these functions were in progress, reaching a peak by 48–72 hr and persisting though to a lesser degree for 7 days. This stage of acceleration was associated with epidermal hyperplasia and most likely represented post‐injury cell renewal.
DIFFERENTIAL, LETHAL AND MUTAGENIC ACTION OF 254 nm AND 320–400 nm RADIATION ON SEMI‐DRIED BACTERIA
Authors: WEBB S. J. , TAI C. C.
Published: January 2008

Abstract— The effect of culture conditions on the lethal and mutagenic action of 254 nm (u.v.) and 320–400 nm (b.l.) light has been examined. Ten strains of Escherichin coli were used in these investigations. It was found that semi‐dehydration in aerosols greatly enhanced the lethal and mutagenic actions of both U.V. and b.l., Mutations induced by U.V. were found to be of a random kind, while those produced by b.l. were specific and of a particular biochemical type depending on the strain of cell and its stage of development. The presence of oxygen during irradiation enhanced b.l. effects but had no effect on U.V. damage while anaerobic growth endowed the cells with added resistance to b.l. and u.v., Stationary phase cells of E. coli B/r were found to be mutated by b.l. specifically at a thymine locus and to be more sensitive than E. coli B to the inhibition by b.l. of respiration. Some mutations induced by b.l. in E. coli B/r were found to hinder the cells ability to carry out the photoreversal of U.V. damage. It is suggested that b.1. affects a specific piece of DNA which is in contact with the cytochrome chain of the cytoplasmic membrane and that this contact point between the cytochrome chain and DNA alters sequentially as the cell proceeds through its life cycle.
RED LIGHT INTERACTIONS WITH BLUE AND ULTRAVIOLET LIGHT IN POLAROTROPISM OF GERMLINGS OF A FERN AND A LIVERWORT
Authors: STEINER A. M.
Published: January 2008

Abstract— In polarotropism of the chloronema of the fern Dryopteris filix‐mas (L.) Schott and of the germ tube of the liverwort Sphaerocarpos donnellii Aust. a phytochrome action in blue and u.v. was presumed[1, 2]. In the present paper this assumption was tested by simultaneously irradiating with red and blue, and red and near u.v. Red energy is given to shift the phytochrome photoequilibrium in favour of high Pfr/Ptotal concentrations. The data obtained by simultaneous irradiation are consistent with the predictions made under the assumption of a phytochrome involvement in the blue‐ and u.v.‐mediated polarotropic response.
ON THE MECHANISM OF RADIOSENSITIZATION BY 5‐BROMOURACIL. THE OCCURRENCE OF DNA STRAND BREAKS IN U.V.‐IRRADIATED PHAGE T4 AS INFLUENCED BY CYSTEAMINE
Authors: HOTZ G. , WALSER R.
Published: January 2008

Abstract— U.V.‐irradiation of phage T4Bor results in a decrease in sedimentation rate of BU‐DNA which is attributed to single‐ and double‐strand breaks. No breaks could be observed in unsubstituted DNA. Cysteamine present during u.v.‐irradiation is able to prevent double‐strand breaks but does not influence the production of single‐strand breaks measured by alkaline sucrose gradient centrifugation. The biological importance and nature of DNA strand breaks due to BU‐incorporation as well as the action of the protective agent on these breaks and on the biological activity are discussed.
A MOLECULAR MODEL FOR A PHOSPHOLIPID‐RETINALDEHYDE COMPLEX ABSORBING AT 500 nm
Authors: SHARPLESS NORMAN E. , ADAMS RALPH G. , JENNINGS WILLIAM H.
Published: January 2008

Abstract— –The structure of a complex formed from retinaldehyde and ethanolamine phosphoglyceride has been proposed as a Schiff's base stabilized byπ–πinteraction between the double bond of an oleic acid group (Δ9) and the 9–10 double bond of the retinaldehyde chain. Considerations of stereochemistry and simple MO theory indicate the proposed structure to be energetically favored, and its absorption maximum is predicted by free electron theory.
LIGHT‐MEDIATED REGULATION OF TMV‐RNA PHOTOREACTIVATION
Authors: MURPHY TERENCE M. , GORDON MILTON P.
Published: January 2008

Abstract— –Nicotiana tabacum var. Xanthi n.c. plants placed in the dark lose their ability to photoreactivate u.v.‐irradiated tobacco mosaic virus RNA over a course of approximately 1 week. When such plants are returned to the light, they recover their photoreactivation ability. The recovery occurs after a lag of at least 3 hr and is complete in 12–24 hr. Three hours or less of cool‐white illumination (2400 ft‐c) are necessary to induce recovery. Blue light is effective in inducing recovery; green light is less effective; red and near‐u.v. are not effective.
THE FORMATION OF HYDRATED ELECTRONS FROM THE EXCITED STATE OF INDOLE DERIVATIVES
Authors: FEITELSON JEHUDA
Published: January 2008

Abstract— –Hydrogen atoms can be observed in u.v. irradiated aqueous solutions of indole derivatives. These H‘ atoms are produced in a reaction between H+ and solvated electrons which are formed in the excited state of indole. Protons are also known to be good quenching agents for the fluorescence of indole. However the pH dependence and effect of oxygen on the yield of hydrogen atoms indicates clearly that although both fluorescence and electron ejection originate in the excited singlet state the fluorescence quenching by protons is not caused by a transfer of electronic charge from the excited ring to H+. The temperature dependencies of both fluorescence and electron ejection yield an abnormally large “activation energy”. It is proposed that this temperature dependence is to a large extent determined by a process characteristic of water as a solvent.
RETARDATION OF ULTRAVIOLET LIGHT ACCELERATED LEAF SENESCENCE BY A CYTOKININ: N6 BENZYLADENINE
Authors: WU J. H.
Published: January 2008

One clear evidence of u.v. injury is the manifestation of chlorosis after leaf irradiation, and such chlorosis can be photorepaired by blue light[1]. We found that such u.v.‐accelerated chlorosis could be prevented in the dark by applying N6 benzyladenine (BA), a synthetic cytokinin, either before or after u.v. irradiation. Cytokinins are known to retard natural senescence of detached leaves kept in the dark [2]. The present finding suggests that u.v. damage may accelerate the natural senescence process of detached leaves kept in the dark. N. glutinosa leaves were used in this experiment, because chlorosis develops quickly (within one week) when the mature leaves are detached and incubated in the dark. The plants were grown in a glasshouse without white‐wash. The light intensity at noon often reached 9000 ft‐c., and the temperature ranged from 19–35°C. The leaves shown in Fig. 1 were from mature plants, i.e. plants with the terminal growing point developed into a flower bud. The floral buds were cut off several days before the leaves were detached for exposure to u.v. light.
THE ABSOLUTE YIELD OF BACTERIOCHLOROPHYLL FLUORESCENCE IN VIVO*
Authors: WANG RICHARD T. , CLAYTON RODERICK K.
Published: January 2008

Abstract— –The method of Weber and Teale for determining absolute fluorescence quantum yield of dyes in solution was modified for determination of the yield of bacteriochlorophyll fluorescence from chromatophores and whole cells of photosynthetic bacteria. Measured yields ranged from about 1–6 per cent. The yield depended on intensity and wavelength of the exciting light. The higher yield at higher light intensity was interpreted as due to saturation of photosynthesis. The lower yield in some strains when excited at 810 nm was attributed to preferential excitation of the reaction center pigment P800.

From this study and the lifetime measurements of others, the relation τ=Q.τ0 was substantiated for the fluorescence of bacteriochlorophyll in vivo, τ being the actual lifetime, τ0 the intrinsic lifetime as estimated from the absorption band area, and Q the quantum yield of fluorescence.
DNA REPAIR IN ULTRAVIOLET LIGHT IRRADIATED HeLa CELLS AND ITS REVERSIBLE INHIBITION BY HYDROXYUREA
Authors: BEN‐HUR E. , BEN‐ISHAI R.
Published: January 2008

Abstract— –The repair of u.v. damaged DNA in HeLa cells can be detected using the alkaline sucrose gradient technique. As a result of pyrimidine dimer excision single strand breaks are produced in DNA of irradiated cells. Rejoining of these breaks occurs during an 8 hr post‐irradiation incubation period and is prevented by hydroxyurea and acriflavine. The inhibition of repair by hydroxyurea can be reversed by a mixture of all 4 deoxyribonucleosides at a concentration that does not reverse the inhibition of total DNA synthesis.
THEORETICAL CALCULATIONS OF THE TRANSITIONS OF PHOTOCOLORED FORMS OF CHROMENES
Authors: EDWARDS LAWRENCE , KOLC JAROSLAV , BECKER RALPH S.
Published: January 2008

Abstract— –Theoretical prediction (MO‐SCF‐CI) of numbers, energies and intensities of transitions are compared with experimental data for the photocolored products of three types of chromenes. Generally good results are obtained. Several conclusions resulted from variation of parameters: (1) When all carbon‐carbon bonds are considered equal in length, there is good agreement between experiment and theory regarding band locations and intensities, however, very poor correlation exists between theory and experiment when calculations are done on a model where the carbon‐carbon bonds are alternately single and double, (2) theoretical calculations employing the Mataga approximation for the two‐center repulsion integrals gives superior correlation with the experimental spectra in comparison with the Pariser‐Parr approximation, and (3) variation of certain parameters (as I.P., E.A., β) did produce changes in the results, especially in the oscillator strengths, but there was no clear best choice, and therefore, conventional parameters were used.
A MECHANISM FOR THE METHYLENE BLUE SENSITIZED OXIDATION OF NUCLEOTIDES
Authors: KNOWLES A.
Published: January 2008

Abstract— An investigation of the methylene blue sensitized oxidation of isolated nucleotides has been carried out, based on the behaviour of the excited dye molecule in long‐term irradiation and flash photolysis experiments. It was necessary first to establish the effect of the phosphate buffer upon the triplet excited state of the dye and the consequent effect upon its photo‐fading. The buffer has a salt effect which accelerates the protonation of the dye triplet, and also the photoreduction of the dye which takes place through the resulting protonated triplet. Xanthosine monophosphate (XMP), which is the nucleotide most sensitive to the photodynamic action of methylene blue, is also effective in reducing the triplet of the dye, while other nucleotides which are resistant to photooxidation do not act as reducing agents. The quantum yield of the sensitized oxidation of XMP was found to correspond to that of the anaerobic photo‐reduction of methylene blue in its presence, which leads to the conclusion that the primary step of the photooxidation is electron or hydrogen abstraction from the nucleotide by the triplet excited methylene blue molecule.
GLOW CURVES FROM PHOTOSYNTHETIC BACTERIA*
Authors: FLEISCHMAN DARRELL E.
Published: January 2008

Abstract— Cells and chromatophores of the photosynthetic bacterium Rhodopseudomonas viridis were frozen in the dark and illuminated while frozen or were illuminated during freezing. Upon being heated they emitted light. The conditions under which glow curves could be produced and the kinetics of the light emission indicate that the glows resulted from the thermal release of electrons from secondary electron acceptors and their recombination with oxidized reaction center bacteriochlorophyll b.

Glow curves could not be obtained with Rhodospirillum rubrum.
PHOTOREACTIVATION OF KILLING IN STREPTOMYCES‐III. ACTION SPECTRA FOR PHOTOLYSIS OF PYRIMIDINE DIMERS AND ADDUCTS IN S. GRISEUS AND S. GRISEUS PHR‐l*
Authors: IKENAGA M. , PATRICK M. H. , JAGGER J.
Published: January 2008

Abstract— Photolysis of tritium‐labelled thymine‐derived photoproducts by 254‐nm ultraviolet radiation (u.v.) in conidia of Streptomyces griseus was measured by chromatography of cell hydrolysates. The relative photolysis cross‐sections of uracilthymine dimer (UT○) at various wavelengths are the same as those of thymine‐thymine dimer (TT○), and their ratios at 313, 365, 405 and 436 nm are 2:1:2:3. Except at 436 nm, these relative values agree very well with cross‐sections previously reported for photoreactivation of u.v. killing in this organism, leading to the conclusion that photoreactivation in the wild type is due to repair of cyclobutane‐type pyrimidine dimers. In a mutant showing restricted photoreactivation (S. griseus PHR‐1), post‐u.v. treatments at the above wavelengths did not affect UT○ and TT○ in the conidia, supporting the earlier suggestion that this organism does not contain active PR enzyme. Another u.v. photoproduct, the precursor of a pyrimidine adduct (PO‐T) that appears in cell hydrolysates, was removed from both wild‐type and mutant cells very efficiently at 313 nm. This is presumably a direct photochemical reaction. In addition, in wild‐type cells, the precursor of PO‐T appeared to be inefficiently removed photoenzymatically at all wavelengths. Removal of the precursor of PO‐T appears to be biologically significant, however, only in the mutant.
ABSORPTION AND EMISSION STUDIES OF ELECTRONIC STATES OF N‐ARYLBENZAMIDES
Authors: O'CONNELL. JR. E. J. , DELMAURO M. , IRWIN J.
Published: January 2008

Abstract— Absorption and emission spectra of several N‐arylbenzamides have been measured. The quantum yields for their fluorescence were found to be dependent on matrix viscosity and temperature. Singlet‐triplet splittings for these compounds were determined from their emission spectra and found to be abnormally small forπ. π* states (˜ 1500 cm‐1). Indeed, the phosphorescence maxima of N‐arylbenzamides occur slightly to the blue relative to their fluorescence maxima. Intersystem crossing efficiencies were determined for several of these compounds and are consistent with S1→S0 radiationless decay.
STUDIES OF SYSTEM II PHOTOCENTERS BY COMPARATIVE MEASUREMENTS OF LUMINESCENCE, FLUORESCENCE, AND OXYGEN EMISSION*
Authors: JOLIOT P. , JOLIOT A. , BOUGES B. , BARBIERI G.
Published: January 2008

Abstract— New results are presented on the emission of oxygen by algae and chloroplasts illuminated by a sequence of short saturating flashes. These results favor the four‐state hypothesis of Kok and co‐workers, in which formation of oxygen requires the accumulation of four oxidants produced by four successive photoreactions. Deactivation of the more oxidized precursor states in the dark is studied under different conditions of preillumination. Our results suggest that both a one step and a two step mechanism of deactivation exist. In order to understand the biological significance of Kok's parameter α—the fraction of photochemical centers unable to react on each flash (“misses”)‐we study reoxidation of acceptor Q after one flash by fluorescence techniques. It appears that a fraction of Q‐ is reoxidized by a back reaction which cancels the effect of the preilluminating flash and is probably responsible for the misses. The results of some luminescence experiments are also reported. These experiments demonstrate that delayed emission of light is associated with the deactivation of states S2 and S3. It is possible that excitons produced by deactivation can be reabsorbed by active photochemical centers, which can modify considerably the deactivation process.
LIGHT‐INDUCED ABSORBANCE CHANGES IN CHLOROPLASTS AT ‐196°C*
Authors: ERIXON K. , BUTLER W. L.
Published: January 2008

Abstract— Absorbance changes induced by irradiating chloroplasts at — 196°C were measured in the region of 525–575 nm with a single‐beam spectrophotometer. Irradiation at low temperature caused a bleaching at 556 nm due to oxidation of cytochrome b559 but little or no change of cytochrome f. There occurred in addition a loss of absorbance at 547 nm and an increase at 543 nm. The bleaching at 547 nrn (and possibly the increase at 543 nm) could be induced chemically with dithionite or borohydride but not ascorbate. Subchloroplast particles with only Photosystem I activity showed no light‐induced absorbance changes, while particles containing combined Photosystem I and Photosystem II activities showed the same changes as whole chloroplasts. Scenedesmus mutant No. 11 cells showed no absorbance changes while mutant No. 8 and wild‐type cells showed the normal changes. It is concluded that the photooxidation of cytochrome b559 and the photoreduction causing the bleaching at 547 nm are both mediated by Photosystem II.