Primary Photochemical Reaction Research Articles

Investigation of the photolysis of polyurethanes based on 4,4′-methylene bis(phenyldiisocyanate) (MDI) using laser flash photolysis and model compounds

Mechanistic evidence is given for the primary pathways leading to the primary photochemical reactions in aromatic diisocyanate based polyurethanes. Laser flash photolysis studies on 4,4′-methylene bis( phenyldiisocyanate ) ( MDI) based polyurethanes and appropriate model compound analogs are presented. Transient species are produced by both direct photolysis (248nm) and indirect photolysis (351 nm) via tert-butyl peroxide generated radicals in solution. Results present strong evidence supporting a dual mechanism for photodegradation involving both hydrogen peroxide formation and photo-Fries rearrangement.

Effect of Temperature on Photoinhibition and Recovery in Actinidia deliciosa

Photoinhibition of photosynthesis was induced in intact leaves of kiwifruit (Actinidia deliciosa) grown in natural light not exceeding a photon irradiance (PI) of 300 �mol m-2 s-1 by exposing them to a PI of 1500 �mol m-2 s-1. The temperature was held constant during the high-light exposure between 5 and 35°C. Recovery was followed at temperatures between 10 and 35°C, after photoinhibition was induced by a 240 min exposure to high light. The kinetics of photoinhibition and recovery were followed by chlorophyll fluorescence at 692 nm and 77K. Photoinhibition occurred at all temperatures but was greatest at low temperatures. Temperature affected the severity of photoinhibitory damage but not the kinetics of photoinhibition. Recovery was also temperature-dependent with little or no recovery occurring below about 20°C and rapid recovery at 30-35°C. The extent of photoinhibition also affected the rates of recovery which were reduced as the severity of photoinhibition increased. An analysis of the rate constants for energy transfer within photosystem II indicated that kiwifruit leaves have some capacity to prevent photoinhibition by increasing the amount of non-radiative energy dissipation. However, the analysis also indicates that this protection mechanism was not wholly effective since the primary photochemical reactions apparently become inactivated during exposure of these leaves to high light.

Magnetic resonance methods in the mechanistic photochemistry of ketones, olefins, and oximes

Abstract

The electron spin resonance study on primary photochemical reaction of hematoporphyrin derivative.

The rate of oxygen consumption and the yield of free radical anion of hematoporphyrin derivative (HPD) in aqueous solutions of HPD and pyrocatechol were measured by the probe 2,2,6,6-tetramethyl-4-piperidone-1-oxyl. It has been found that both singlet oxygen and free radical mechanisms exist simultaneously in primary photochemical reactions, and there is a competition between both mechanisms. When the oxygen concentration in solutions comes down to 12-14% of the stanting level, the predominant mechanism can be changed from the singlet oxygen to the free radical. Whether HPD exists in aggregation state is very important to photosensitization mechanisms. In the presence of the aggregation state of HPD, the predominant mechanism is the free radical, and photosensitization effects of HPD are all the better.

Hydrogen peroxide photoproduction by the semicarbazide—tris(2,2′-bipyridine)ruthenium(II)—oxygen system

Abstract A light-driven system consisting of tris(2,2′-bipyridine)ruthenium(II) (Ru(bpy)32+) as the photosensitizer, semicarbazide as the electron donor and molecular oxygen as the electron acceptor has been employed for hydrogen peroxide production. The efficiency of this photosystem markedly depends on pH: while the peroxide yield is almost negligible at acid, neutral or slightly alkaline pH, it reaches significant values at high hydroxide concentrations, the initial rate of H2O2 formation drastically increasing from pH 12 to pH 14. In 1 M NaOH solutions containing Ru(bpy)32+ and semicarbazide at optimum concentrations, the number of catalytic cycles (or turnover number) undergone by the ruthenium complex over the complete course of the photochemical reaction is as high as 1.1 × 104. Spectrofluorometric and laser flash photolysis techniques were used to study the primary photochemical reactions involving the excited state of the ruthenium complex as well as the photochemically generated species Ru(bpy)33+ and Ru(bpy)3+. It is proposed that at pH 14 a sequence of reactions leading to O2 photoreduction by electrons from semicarbazide takes place, with the concomitant formation of H2O2; the excited state of Ru(bpy)32+ appears to react via oxidative quenching by oxygen rather than via reductive quenching by semicarbazide. At neutral pH, in contrast, there is no H2O2 formation owing to the fact that semicarbazide is unable to reduce (Ru(bpy)33+ to Ru(bpy)32+, although the photoexcited ruthenium complex is quenched equally by oxygen.

Light-induced Changes in the Infrared Spectra of Reaction Centers from Rhodopseudomonas sphaeroides in H2O and D2O Solutions

Abstract Light-induced changes were observed in the infrared spectra of the reaction centers from Rhodopseudomonas sphaeroides in aqueous solutions. These changes were suppressed in the presence of ferricyanide, demonstrating that the light-induced changes were due to the primary photochemical reactions in photosynthesis.

Modified Light-Induced Absorbance Changes in dim Y Photoresponse Mutants of Trichoderma

A brief pulse of blue light induces the common soil fungus Trichoderma harzianum to sporulate. Photoresponse mutants with higher light requirements than the wild type are available, including one class, dim Y, with modified absorption spectra. We found blue-light-induced absorbance changes in the blue region of the spectrum, in wild-type and dim Y mutant strains. The light-minus-dark difference spectra of the wild type and of several other strains indicate photoreduction of flavins and cytochromes, as reported for other fungi and plants. The difference spectra in strains with normal photoinduced sporulation have a prominent peak at 440 nm. After actinic irradiation, this 440 nanometer difference peak decays rapidly in the dark. In two dim Y photoresponse mutants, the difference spectra were modified; in one of these, LS44, the 440 nanometer peak was undetectable in difference spectra. Detailed study of the dark-decay kinetics in LS44 and the corresponding control indicated that the 440 nanometer difference peak escaped detection in LS44 because it decays faster than in the control. The action spectrum of the 440 nm difference peak is quite different from that of photoinduced sporulation. The light-induced absorbance changes are thus unlikely to be identical to the primary photochemical reaction triggering sporulation. Nevertheless, these results constitute genetic evidence that physiologically relevant pigments participate in these light-induced absorbance changes in Trichoderma.

Photostability of Anthraquinone and Azo Dyes in N‐Ethylacetamide (Nylon Model)

Measurements of the quantum yields of photodegradation of anthraquinone and azo dyes in N‐ethylacetamide (nylon model) and triplet sensitisation of dye fading showed that the photochemical reactions are initiated by an upper excited n‐π* triplet state. The primary photochemical reaction with both anthraquinone and azo dyes involves hydrogen abstraction from the amide so/vent. In some cases oxygen retards dye fading by reoxidation of the reduced structures. In other cases oxygen accelerates the photochemical reaction via free‐radical initiated oxidation reactions. The quantum yields of dye fading are strongly wavelength dependent. On exposure to a simulated sunlight spectrum photodegradation is mostly caused by radiation in the region 300–400 nm. While unsubstituted anthraquinone is very photoreactive, amino‐substituted anthraquinones are much more photostable in N‐ethylacetam/de. Photostability of mono‐ and di‐substituted anthraquinone derivatives increases with the electron‐donating power of substituent groups. Azo dyes with increased conjugation such as diazo and naphthazo structures are more photostable than simple azobenzene derivatives. Certain electron‐withdrawing substituents, which do not affect dye colour, improve the ‘oxidative’ photostability of both anthraquinone and azo dyes. A cobalt premetallised azo dye (C.I. Acid Red 182) is very photostable in N‐ethylacetamide, showing the effect of metal chelates on photostability. The low values of quantum yields for the dye solutions are comparable with those of acid and disperse dyes in nylon films, indicating that N‐ethylacetamide is a suitable nylon model for mechanistic studies of dye fading.

Excited states and primary photochemical reactions in the photosynthetic bacterium Heliobacterium chlorum

The charge separation and excited states of antenna bacteriochlorophyll in membrane fragments of the recently discovered photosynthetic bacterium Heliobacterium chlorum were studied by absorbance-difference spectroscopy. Formation of singlet excited states of bacteriochlorophyll g with a lifetime of 200 ps or less was observed as the disappearance of the ground state absorption bands. From the absorbance-difference spectra, it was concluded that the primary photochemical reaction consists of the transfer of an electron from the primary donor P-798 to a possibly bacteriochlorophyll c-like pigment absorbing at 670 nm. Electron transfer to the secondary acceptor occurred with a time constant of about 500 ps. The midpoint potential of this acceptor (between -450 and -560 mV) and the absence of significant absorbance changes in the near-infrared upon its reduction suggest that this acceptor is an iron-sulfur center. It is concluded that the primary photochemistry of H. chlorum is similar to that of green sulfur bacteria.

The effect of Cd2+ on photosynthetic reactions of mesophyll protoplasts

Photosynthetic CO2‐fixation of mesophyll protoplasts of lambs lettuce [Valerianella locusta (L.) Betcke] was inhibited by short time exposure to Cd+. Inhibition was due to uptake of the metal ion into the protoplasts and increased with increasing Cd2+ concentrations and the time of preincubation. A 10 min pretreatment at 2 mM Cd2+ reduced CO2‐fixation by 40–60%. Inhibition of photosynthesis was independent of the light intensity to which the protoplasts were exposed. Measurement of the lightinduced electrochromic pigment absorption change at 518nm and chlorophyll fluorescence studies revealed that primary photochemical reactions associated with the thylakoid membranes were not affected by the metal ion. Also, light activation of the ribulose‐1,5‐bisphosphate carboxylase (EC 4.1.1.39) was not inhibited by Cd2+. Under rate‐limiting CO2 concentrations, inhibition of CO2‐fixation was smaller than at Vmax of CO2 reduction indicating that the carboxylation reaction of the Calvin cycle is not susceptible to Cd2+. Cd2+ treatment of protoplasts significantly extended the lagphase of CO2‐supported O2‐evolution and partly inhibited light activation of the glyceraldehyde‐3‐phosphate dehydrogenase (EC 1.2.1.13) and the ribulose‐5‐phosphate kinase (EC 2.7.1.19). Measurement of relative concentrations of [14C]‐labeled Calvin cycle intermediates showed that Cd2+ caused a decrease in the 3‐phosphoglycerate/triose phosphate ratio and an increase in the triose phosphate/ribulose‐1,5‐bisphosphate ratio. It is concluded that in protoplasts Cd2+ affects photosynthesis mainly at the level of dark reactions and that the site of inhibition may be localized in the regenerative phase of the Calvin cycle.

Primary photochemical reaction of rhodopsin.

Recent progress in the primary photochemical reaction of rhodopsin studied by picosecond laser photolysis has been reviewed along with a summary on the Visual transduction mechanism in the vertebrate rod receptor cell on the basis of a cGMP hypothesis.Since picosecond laser photolysis was first applied to the primary photochemical reaction of rhodopsin at 1972, numerous conflicting results have been published about the process for formation of bathorhodopsin or hypsorhodopsin. The disagreement of the results was now explained by considering the energy density of the laser pulse used in these experiments.Excitation of rhodopsin with a very weak laser pulse, with which the amount of the intermediate produced was proportional to the laser intensity, yielded a new intermediate, photorhodopsin, an immediate precursor of bathorhodopsin. Hypsorhodopsin was not produced under these conditions. It was produced by excitation of rhodopsin with a highly intense laser pulse with which a multiphoton reaction was occurred.Thus the primary photochemical reaction of rhodopsin at physiological temperature is summarized as follows: On absorption of a photon in the visible region, rhodopsin converts to photorhodopsin through the excited state of rhodopsin. Photorhodopsin decays to bathorhodopsin and then to lumirhodopsin. Hypsorhodopsin may not be an intermediate of rhodopsin under a light-intensity of physiological environment but be produced by photoreaction of photorhodopsin.

Picosecond Spectroscopy on Reverse Photoreaction from Batho-Intermediate of Bacteriorhodopsin at 6.5 K

The reverse photoreaction from batho-intermediate of bacteriorhodopsin has been investigated by picosecond laser flash photolysis at 6.5 K. Upon exciting batho-intermediate with 694 nm light pulse of 31 ±4 ps width, the increase of absorbance around 570 nm and concomitant decrease of that around 630 nm are observed without delay time. Based on the above result, the models for the primary photochemical reaction of bacteriorhodopsin are discussed.

Formation of aquated electrons and the individual quantum yields for photoactive species in the Cu(I)—KCN—H 2O system

The photoredox behaviour of the Cu(I)—CN system in aqueous solutions was studied. The cyanide content decreased on irradiation at 253.7 nm and the quantum yield Φ CN for cyanide conversion was determined at various pH values. The individual quantum yields for [Cu(CN) 2] − and [Cu(CN) 3] 2− were calculated. The results give an indication of the photoctivity of the tri-coordinated complex. The aquated electron was identifed as a product of the primary photochemical reaction by scavengers and flash photolysis. The quantum yield Φ e of this reaction is 0.06.

Wavelength controlled production of SO4- radicals in the ultraviolet photolysis of ceric sulfate solutions

Solutions of cerium(III)/(IV) and formic acid in 0.4 M sulfuric acid have been photolysed under 254 nm and 365 nm light. Marked differences in the reaction kinetics and quantum yields are observed at the two different wavelengths. At 365 nm, the reactions leading to cerium(IV) reduction are caused almost exclusively by the SO4- radical. The ratio of rate constants, k(SO4- + CeIII)/ k(SO4- + HCOOH), is 116 � 11 and the quantum yield of sulfate radicals, ф(SO4-), is 0.023 � 0.002. At 254 nm, the reactions leading to cerium(IV) reduction are caused mainly by the OH radical, but approximately 35% of the oxidizing radicals formed in the primary photochemical reaction are SO4-. Cerium(III) species, excited at 254 nm, transfer energy to cerium(IV) and this results in an additional yield of OH and SO4- radicals. Fluorescence measurements confirmed the efficiency of the energy transfer reaction. The ratio of rate constants, k(OH+CeIII)/k(OH+HCOOH), is 2.22 � 0.18 and ф(CeIV*) and ф(CelIII*) giving oxidizing radicals are 0.116 � 0.010 and 0.0083 � 0.0008 respectively. Thus about 5 times more total oxidizing radicals are produced from excited cerium(IV) species at 254 nm than at 365 nm.

二, 三のp-置換ベンゼンジアゾニウム塩の電子構造と光化学反応の初期過程の機構についての一考察

二, 三のp-置換ベンゼンジアゾニウム塩の電子構造と光化学反応の初期過程の機構についての一考察

Photosynthetic Membranes of Porphyridium cruentum

Three chlorophyll-protein complexes (CP I, CP III, CP IV) were electrophoretically separated from thylakoids of the eukaryotic red alga Porphyridium cruentum. CP I contained the primary photochemical reaction center of photosystem I as judged by its light-induced reversible absorbance change at 700 nanometers, by its fluorescence emission maximum at 720 nanometers (-196 degrees C), and by the molecular weight of its apoprotein (68,000 daltons). CP III and CP IV appeared to belong with photosystem II as suggested by the absence of light-reversible absorbance at 700 nanometers, by their fluorescence maximum at 690 nanometers (-196 degrees C), and by the presence of a chlorophyll-binding polypeptide with a molecular weight of about 52,000 daltons. CP IV when completely denatured had two additional polypeptides of about 40,000 and 48,000 daltons. All three chlorophyll-protein complexes contained carotenoids: the chlorophyll/carotenoid molar ratio of 15:1 for CP I, and 20:1 for CP III and CP IV. The thylakoid membranes of P. cruentum contained four cytochromes, detected by heme-dependent peroxidase activity, but there was no observed association with the electrophoretically separated chlorophyll-protein complexes.

Photopolymerization reactions initiated by copper (II)–amino acid chelates: Investigation of the initiating species by flash photolysis. I

AbstractFlash photolysis of copper (II)–bis(amino acid) complexes (amino acids: glutamic acid, serine, or valine) in deaerated aqueous solution produces transient species having absorption maxima at around 350 nm. The transient species are identified as copper (II)–alkyl complexes. In the case of Cu(valine)2 at pH > 6.5 formation of Cu(II)‐alkyl complex is not observed; this is interpreted to be due to the presence of two bulky methyl groups of the coordinated valine ligand, which hinders the rearrangement. Pseudo‐first‐order rate constants for the decay of the transients are determined at different pH with varying concentration of amino acid ligand. The free‐radical species of the complexes responsible for the initiation of the vinyl polymerization reactions are identified as Cu(I)‐coordinated amino acid radicals which are formed in the primary photochemical reaction of the complex. A mechanism for the secondary reactions involving the initiating species consistent with the nature of the product formed and the pH dependence of the decay of the transients is proposed.

Ageing of chloroplasts in vitro ? III. Comparison of the effects of ageing in vitro and senescence on the red absorption band and primary photochemical reactions of sunflower chloroplasts

A comparison of changes in absorption properties and electron transport activities of chloroplasts ageing in vivo and in vitro is made. Chloroplasts from sunflower leaves senescing in vivo during 7 days in dark do not show a blue shift of the red absorption band; in contrast, the shift becomes apparent within 24 h of in vitro ageing of isolated organelles. Photosynthetic activity by chloroplasts is lost much faster during in vitro than in vivo ageing. During in vitro ageing, the rate of degradation of thylakoid membranes as characterised by the shift in the red absorption band and loss in Hill reaction is further accelerated in chloroplasts isolated from dark-induced senescing leaves, suggesting the influence of the in vivo status of the chloroplasts on their in vitro stability.

Ageing of chloroplasts in vitro ? III. Comparison of the effects of ageing in vitro and senescence on the red absorption band and primary photochemical reactions of sunflower chloroplasts

A comparison of changes in absorption properties and electron transport activities of chloroplasts ageing in vivo and in vitro is made. Chloroplasts from sunflower leaves senescing in vivo during 7 days in dark do not show a blue shift of the red absorption band; in contrast, the shift becomes apparent within 24 h of in vitro ageing of isolated organelles. Photosynthetic activity by chloroplasts is lost much faster during in vitro than in vivo ageing. During in vitro ageing, the rate of degradation of thylakoid membranes as characterised by the shift in the red absorption band and loss in Hill reaction is further accelerated in chloroplasts isolated from dark-induced senescing leaves, suggesting the influence of the in vivo status of the chloroplasts on their in vitro stability.

One-electron photooxidation of carbazole in the presence of carbon tetrachloride. Part II. Carbon tetrachloride as a reaction medium. Use of ammonia after irradiation and during irradiation

In part I of this series of papers we proposed the mechanism of electron transfer as the primary photochemical reaction in the carbazole – carbon tetrachloride system along with a secondary photochemical reaction initiated by transformations of the radical cation of carbazole in the solvent cage resulting in intermediates:[Formula: see text]In this paper we discuss the influence of ammonia, used after and during irradiation, on the mechanism of secondary transformation and the formation of thermodynamically stable products in the system studied. Such compounds as N-cyanocarbazole, 1-cyanocarbazole, and 3-cyanocarbazole have been formed as the main products during neutralization of the photolyte solution by ammonia gas. The mechanism of formation of these compounds has been explained by the chemical reaction of ammonia with cations α and γi. If ammonia is present in the solution of carbazole in CCl4 during irradiation, such products as N,N′-dicarbazyl and N-cyanocarbazole are mainly formed along with 3-(N-carbazyl)carbazole, 3,9-di-(N-carbazyl)carbazole, and N-cyano-3-(N-carbazyl)carbazole. In such a case, reactions of radicals β are the determining factors in the secondary photochemical transformations. Radicals β are formed by the reaction involving ammonia with radical cations of carbazole. All the results in this paper have been discussed taking under consideration the influence of the reaction media on the mechanism of photochemical transformation of carbazole.