bluuebunny/crossref_metadata_2025 huggingface dataset:
BIOLOGICAL AND PHYSICAL RANGES OF PHOTOPROTECTION FROM ULTRAVIOLET DAMAGE IN MICROORGANISMS
Authors: Jagger J. , Stafford R. S.
Published: January 2008

Photoprotection from biological damage by ultraviolet radiation of 2537 A has been sought in a variety of microorganisms. Its taxonomic occurrence is spotty and it is clearly not as general a phenomenon as photoreactivation.

The action spectrum has been determined for bacteria from two different taxonomic orders and found to be the same, lying in the range 3100–4000 A, with a single peak at 3400 A. These spectra are much narrower than those for photoreactivation and are confined to the near ultraviolet region. Their similarity suggests that only one chromophore is utilized in bacterial photoprotection.

Photoprotection is found to require, at an ultraviolet survival level of 2 per cent, about three times as much radiation energy as photoreactivation.

Consideration of absorption and action spectra, as well as a direct test with transforming principle, indicates that the chromophore probably is not nucleic acid. The action spectra suggest reduced diphosphopyridine nucleotide.
MEASUREMENT OF ABSORPTION SPECTRA OF CHLOROPHYLL IN ALGAL CELL SUSPENSIONS*
Authors: Murchio J. C. , Allen M. B.
Published: January 2008

The effects of instrumental geometry on the measurement of absorption spectra of scattering suspensions are considered. It is shown that the resolution of absorption spectra of cell suspensions is not improved by opal glass if a spectrophotometer receiving a large cone of light is used.

The scattering of light by suspensions of several algae is found to be approximately proportional to the fourth power of the frequency, as would be expected if the scattering is due to small particles.
DESTRUCTION OF ANTIGENICITY IN VITRO OF HUMAN SERUM ALBUMIN AND OF TOBACCO MOSAIC VIRUS BY ULTRAVIOLET RADIATION
Authors: Kleczkowski A.
Published: January 2008

Ultraviolet irradiation first makes human serum albumin unable to precipitate with antibody to the original albumin and then destroys its ability to combine with the antibody. With tobacco mosaic virus both the ability to precipitate and to combine with antibody to the original virus are destroyed simultaneously or nearly so. A rough estimate shows that about 5 J of radiation energy (at λ= 2537 Å) must be absorbed by each milligram of human serum albumin or of the protein of the virus to reduce their capacity to combine with antibodies to half. The structure of the two antigens is very considerably altered before their specific combining capacity is destroyed, which contrasts with relatively slight alterations in the structure of typical enzymes or antibodies when their specific activities are destroyed or in the virus when its infectivity is destroyed.
THE CHEMICAL TRANSFORMATION OF THE CATECHOLAMINES INDUCED BY UV IRRADIATION*
Authors: Walaas E.
Published: January 2008

Abstract— The conversion of catecholamines exposed to ultraviolet irradiation has been investigated spectrophotometricaly. The UV activation of the molecule is a direct effect and the activation is not dependent upon the presence of metals. The transformation, which includes mdolization and polymerization requkes the presence of oxygen. The rate of the oxidative transformation is of the same order of magnitude in 100 per cent oxygen as in air, provided a metal chelating agent is present.

At neutral pH photoactivated adrenaline is oxidized to adrenochrome and further polymerized to melanin. At alkaline pH U.V. irradiation results in the formation of conversion products of adrenaline, different from those observed at neutral pH, and the melanin production is low.

When noradrenaline is subjected to U.V. irradiation at neutral pH the processes of oxidation and polymerization takes place at a much slower rate than that observed for adrenaline. The radiation effect on noradrenaline at alkaline pH is characterized by increased melanin formation.

Radicals derived from U.V. irradiation of H2O2 convert the catecholamines to different products, and no melanin formation can be recorded.

The sequence of reactions leading from the activated catecholamines to indolic structures, and the participation of the indolic structures in melanogenesis from the catecholamines is discussed.
VARIATION IN ELECTRON PARAMAGNETIC RESONANCE SIGNALS OF PHOTOSYNTHETIC SYSTEMS WTTH THE REDOX LEVEL OF THEIR ENVIRONMENT
Authors: Loach Paul A. , Androes G. M. , Maksim Ann F. , Calvin Melvin
Published: January 2008

Abstract— The redox dependence of the light‐induced electron paramagnetic resonance signal at g=2 in R. rubrum, R. spheroides and Chromatium chromatophore particles and quantasonie particles from spinach chloroplasts has been determined qualitatively over the range —0.3 to +0.6 V and quantatively over the range +0.3 to ±0.6 V. A light‐induced EPR signal has been titrated and demonstrated to have a midpoint potential of +0.44 v at pH 7 and 20°C. Concentration, ionic strength and pH dependence for this transition in R. rubrum chromatophores is reported. In addition to the dark signal which replaces the light signal, in chromatophore material another dark signal, occurring in the seine location as the light signal, has been demonstrated to occur at high potential.

Selective chemical oxidation with K2lrCl6 of chromatophore particles from the three bacteria resulted in the removal of some 95 per cent of the absorbance in the near infrared and left the photoactive pigments.

Two light‐induced EPR signals were found in quantasome particles by their dependence upon the redox level. Of particular interest is a signal observed at quite high potential (e.g. + 0.60 V). It was demonstrated that oxygen evolution by these quantasonie particles in the presence of K3Fe(cN)0 occurred at the same rate at +0.55 V as at +0.40 V.
THE ACTION SPECTRUM OF AN IN VITRO DNA PHOTOREACTIVATION SYSTEM
Authors: Setlow Jane K. , Boling Maxon E.
Published: January 2008

Abstract— The wavelength‐dependence of in vitro photoreactivation of transforming DNA by yeast extract has been determined. There is an intensity‐dependent lag at the beginning of the biological reaction. There is a similar lag in the splitting of thymine dimers by the yeast extract in the light, a process known to account for most or all of the increase in transforming activity of photoreactivated DNA. The most efficient wavelengths for photoreactivation are around 3550 and 3850 Å. Although the action spectrum is not very similar to flavin absorption, riboflavin at very low concentration inhibits photoreactivation, as it also inhibits a number of flavoenzymes, suggesting that the photoreactivating enzyme might be a flavoprotein.
CHLOROPHYLL‐SENSITIZED REDUCTION OF SAFRANINE
Authors: Bannister T. T.
Published: January 2008

Abstract— The kinetics of the direct photoreduction, and the chlorophyll‐sensitized reduction, of safranine by ascorbic acid in aqueous pyridine was studied. In darkness following illumination, safranine was reformed from the colorless reduced derivative by a first order reaction. In light, however, the photostationary concentration of reduced safranine was proportional to the square root of the absorbed illumination, indicating a second order back reaction. The spectrum of reduced safranine, the inhibition of chlorophyll photoreduction by safranine, and the quenching of chlorophyll fluorescence by safranine were also measured. While other mechanisms of sensitization are conceivable, the simplest explanation of the results seems to be one involving the formation of the triplet state of safranine at the expense of the triplet state of chlorophyll.
DELAY IN GROWTH AND DIVISION INDUCED BY NEAR ULTRAVIOLET RADIATION IN ESCHERICHIA COLI B AND ITS ROLE IN PHOTOPROTECTION AND LIQUID HOLDING RECOVERY
Authors: Jagger John , Wise W. Curtis , Stafford R. S.
Published: January 2008

Abstract. Microscopic observations show that growth delay and division delay occur on nutrient agar after Escherichia coli B has been irradiated at 3341 Å. These effects also occur in nutrient broth.

A near u.v. action spectrum for growth delay in nutrient broth has been obtained. It shows a single peak at 3380 Å and is indistinguishable from the action spectrum for photo‐protection from far u.v. (2537 Å) killing in the same organism. Furthermore, photoprotected cells show a much greater growth delay than cells that have not been photoprotected. These, as well as kinetic data, suggest that the essential action of a photoprotection treatment consists in the induction of a growth‐division delay. This delay would presumably permit more time for intracellular recovery systems to operate on the far u. v. damage to nucleic acids.

Liquid holding recovery (effected by holding cells in phosphate buffer after far u. v. irradiation) shows complete overlap with photoprotection. It is concluded that photoprotection and liquid holding recovery operate on the same far u. v. damage. As with photoprotection, it is probable that the essential action of a liquid holding treatment is the induction of a growth‐division delay.

No photoprotection is observed of intracellular T2 bacteriophage or of E. coli Bs‐l (Hill).
ANGIOTENSIN ALTERATIONS INDUCED BY ULTRAVIOLET RADIATION*
Authors: Marsh Walton H.
Published: January 2008

Abstract. Angiotensin is readily inactivated by ultraviolet light presumably due to the photosensitivity of its tyrosyl and phenylalanyl residues. Alkalinity promotes and acidity delays the biological decomposition. Under conditions of oxygen exclusion (i.e. under N2 or H2) the rate of decomposition is further depressed and when irradiation is carried out under hydrogen a biologically active product is formed. This altered angiotensin, judging from its chromatographic migration and dialysis rate, represents a polymerized form of angiotensin. Under identical irradiation conditions, angiotensin was coupled with C14 labelled pheny‐lalanine which indicates that this residue might be the photoreactive species. The production of altered forms of angiotensin by ultraviolet irradiation was discussed with respect to possible utility in anti‐metabolite, antigen or tracer work.
COMPARISON TABLE OF THE PERFORMANCE OF TWENTY‐FIVE TYPES OF RADIATION DETECTOR
Authors: Clarke F. J. J.
Published: January 2008

Abstract. The paper reviews some of the difficulties associated with intercomparing the performances of different types of radiation detector, and discusses the best form of figure of merit to apply to a wide variety of detector types. A table gives current data on the best performance achieved by twenty‐five basic types of radiation detector. in terms of spectral response, usable frequency range, spatial resolution (where appropriate) and sensitivity. Sensitivity is listed under two headings: the minimum detectable signal, expressed in fundamental units (watts, watt cm‐3, joules or joules cm‐2); and the output sensitivity, which indicates the kind and magnitude of output, and which is accordingly expressed in practical units. Source references are given to cover all detector types listed.
EFFECT OF U.V. IRRADIATION ON SUSCEPTIBILITY OF SERUM ALBUMINS TO TRYPSIN
Authors: Kleczkowski A.
Published: January 2008

Abstract— Ultraviolet irradiation converts bovine serum albumin (BSA) and rabbit serum albumin (RS A) into forms much more rapidly hydrolysed by trypsin than the original proteins. The radiation energy (at 2537Å) required to be absorbed by each mg of a protein to convert half of it into a form susceptible to trypsin (E50%) is about 0.32 J for BSA and 0.62 J for RSA. The average E50% for inactivation of enzymes and antibodies is 0.7 J so the two processes may depend on similar alterations in irradiated protein molecules. The alteration can be considered as denaturation, with protein molecules becoming at least partially unfolded and making the sites susceptible to the attack by trypsin more easily accessible to the enzyme.
FLASH PHOTOLYSIS OF ROD PARTICLE SUSPENSIONS
Authors: Pratt D. C. , Livingston R. , Grellmannt K‐H.
Published: January 2008

Abstract. Flash photolytic experiments with suspensions of rod particles isolated from cattle eyes reveal that at low temperatures a transient is formed following irradiation which preceeds lumirhodopsin in the bleaching process. Kinetic studies, in the range ‐75°C to ‐50°C, suggest that the transient is composed of three intermediate species, having similar absorption spectra but changing to lumirhodopsin at different rates. The energies of activation of the decay reactions appear to be identical and equal 12.5°3 kcal/mole. Spectrophotometric experiments have demonstrated that the low temperature transient is stable at liquid nitrogen temperatures and suggest that it can be reversibly converted to a mixture of rhodopsin and isorhodopsin at ‐195°C.

A detailed comparison of light initiated reactions of rhodopsin in rod particle suspensions and in solutions reveals striking similarity in the two systems.
THE LETHAL EFFECTS OF RADIATION ON SIX SPECIES OF PROTOZOANS
Authors: Calkins John
Published: January 2008

Abstract. The lethal response of six strains of protozoa to both ionizing radiation and u.v. light has been established. Values have been determined by observation of isolated animals and death was considered to be failure of irradiated individuals to produce unlimited progeny regardless of the actual time of death following irradiation. The strains were selected to give a large variety of size. Lethal doses of u.v. radiation have been determined in the same units used for ionizing radiation (the rad). The results reported here have been supplimented by values from the literature and plotted in the form of a graph of LD‐50 value versus individual organism size.
TRIPLET STATE STUDIES OF FLAVINS BY ELECTRON PARAMAGNETIC RESONANCE—I
Authors: Shiga T. , Piette L.H.
Published: January 2008

Abstract— The triplet state of flavin derivatives and d‐amino acid oxidase was observed by electron paramagnetic resonance at 77°K.

Flavin triplets (Δm=± 2) originate from the isoalloxazine ring and are resonant at 1560 guass.The half‐life of the FMN triplet in 1N HCl is 15 nisec.This life‐time is prolonged indirectly by the presence of paramagnetic species, such as oxygen or free radicals.

The flavin triplet state is pH dependent.In neutral solution the nlaximum triplet yield is obtained and the longest life‐time is observed.The triplet state is affected by intra‐and inter‐molecular complex formation, FAD is partially quenched by indirectly substituted adenine.Tryptophan quenches completely the FMN triplet.The FAD triplet of d‐amino acid oxidase is enhanced but the life‐time is shortened relative to a pure FAD solution.
PHOTODYNAMIC DEGRADATION OF NUCLEIC ACIDS*
Authors: Bellin J. S. , Grossman L. I.
Published: January 2008

Abstract— The rate of photodynamic degradation of deoxy‐nucleic acids of various origins was followed by noting changes in their melting curves. The rate of degradation was proportional to the mole fraction of guanine residues in the nucleic acids, except for T4r+ bacteriophage DNA which showed greater susceptibility than this rule would predict. The ability of dyes to act as sensitizers is correlated with their ability to form dye molecules in a metastable excited state. In addition, the binding of dyes to specific sites on the nucleic acid molecule was shown to be of great importance. Biological activity is a much more sensitive indicator of structural integrity of DNA than is the melting curve.
BIOLOGISCHE WIRKUNGEN PHOTOCHEMISCHER WASSERSTOFFPEROXYDBILDUNG:
Authors: Berg H. , Jacob H. ‐E.
Published: January 2008

Abstract— As previously demonstrated, there is a considerable increase in the photodynamic activity of anthraquinone‐sulphonic acid on the addition of ethylene glycol, glycerol and sugars in concentrations of the order of 0.4M. This system exhibits cytostatic activity. With H2O2 at higher concentrations there is also formation of aldehydes in approximately equivalent yields as follows:

R.CH2OH+Q → QH2+R.CHO

QH2+O2→ Q+H2O2

In accordance with the recent results of Warburg this accounts for the fundamental importance of glyceraldehyde in cancer research.

Our results on the irradiation of Proteus vulgaris SG2 in buffered medium with the addition of glycerin to anthraquinone‐sulphonic acid show the existence of a similar cytostatic effect. One hour's irradiation with visible light reduces the cell count by about 50 per cent.
THE ENZYMATIC DEGRADATION OF ULTRAVIOLET‐IRRADIATED DEOXYRIBONUCLEIC ACID*
Authors: Dellweg H. , Wacker A.
Published: January 2008

Abstract— Double‐stranded DNA, after u.v. irradiation, was degraded by the action of deoxy‐ribonuclease and snake venom phosphodiesterase. In addition to mononucleotides a number of nuclease‐resistant oligonucleotides, containing photochemically altered bases, could be detected and separated by column chromatography. The phosphorus contents indicated the existence of some photoproducts other than dimers of pyrimidines, especially within the trinucleotide fraction. Comparison of the DNase action on irradiated and on unirradiated DNA suggests that DNase is hindered from splitting the chain next to a photoproduct, tetra‐nucleotides being the smallest and octanucleotides the most frequent degradation products bearing an altered base.
RELATIONS BETWEEN PHOTOCHEMISTRY AND FLUORESCENCE IN CELLS AND EXTRACTS OF PHOTOSYNTHETIC BACTERIA*
Authors: Clayton Roderick K.
Published: January 2008

Abstract— Light‐induced changes in the yield of bacteriochlorophyll fluorescence have been measured in cells and chromatophores of photosynthetic bacteria, and coordinated with light‐induced absorbancy changes. Comparisons were drawn during transitions between dark and light steady states and also between steady states established at different light intensities. Aerobic cell suspensions of Rhodospirillum rubrum, Rhodopseudomonas spheroides, Chromatium and Rhodopseudomonas sp. NHTC 133 showed a strict correspondence between changes in the fluorescence yield and the bleaching of P870 (P985 in Rps. sp. NHTC 133), as reported by Vredenberg and Duysens for R. rubrum cells. The relationship shows that singlet excitation energy in bacteriochlorophyll is quenched by P870 at a rate proportional to the concentration of unbleached P870. This implies that the photosynthetic units are not independent with respect to energy transfer.

In anaerobic cell suspensions the change in fluorescence did not follow the bleaching of P870 in the manner described by Vredenberg and Duysens. Here a change in fluorescence may have resulted from the reduction of a primary photochemical electron acceptor as well as from the oxidation (bleaching) of P870. In chromatophore preparations there were further deviations from the Vredenberg and Duysens relationship which could be attributed to changes in the rate constants for quenching of singlet excitation energy. Finally there was a light‐induced increase in the fluorescence yield which was related to a band shift of bacteriochlorophyll and not to the bleaching of P870. Aerobic cell suspensions presented a limiting case in which these complications were absent.

No change in the fluorescence was associated uniquely with the oxidation of cytochrome or band shifts of carotenoid pigments.

These results, when coordinated with earlier findings about the fluorescence of bacteriochlorophyll and P870, indicate that the singlet excitation quantum is the only energy carrier linking the absorption of light with the initiation of photochemistry in bacterial photosynthesis.
PHOTOCROISSANCE DE SEGMENTS DE COLEOPTILES D'AVOINE MAINTENUS EN SURVIE‐SPECTRES D'ACTION
Authors: Aghion Jacques , Jacques Roger , Jouglard Claude , Lourtioux Andrée
Published: January 2008

Résumé— La photocroissance des coléoptiles d'avoine in vitro est contrôlée par le phytochrome. Une thérie est proposée suivant laquelle ce pigment est sensibilisé par les pigments photosynthétiques des coléoptiles.
ELECTROSTATIC FORCES IN DYNAMIC FLUORESCENCE QUENCHING
Authors: Holmström Bertil , Tegnér Lars
Published: January 2008

Abstract— The quenching constant for the iodide quenching of the fluorescence of riboflavin phosphate was measured at pH 5 and at pH 8. The value of the constant at the lower pH (where the phosphate group is unionized), k‘= 23 M‐1 was independent of the ionic strength, μ. The ratio k“/k’= f(k” the quenching constant at pH 8, where the phosphate group carries a single negative charge) was found to be 0.70 at μ=0.025 and 0.79 at μ= 0.045. The fact that f < 1 is interpreted as due to electrostatic repulsion between the dye molecule, when ionized, and the iodide ion. Due to counter ion screening the effect is less accentuated at higher μ. The quenching constant for riboflavin (which does not contain the ionizable phosphate group) was found to have the same value, 30 M‐1 both at pH 5 and at pH 8, and with no dependence on μ.
ELECTRONIC PROPERTIES AND PHOTOCHEMICAL MONOMOLECULAR REACTIONS IN ORGANIC CONJUGATED COMPOUNDS: PHOTOCYCLIZATIONS, PHOTOISOMERIZATIONS AND PHOTOCHEMICAL REARRANGEMENTS—I
Authors: Malrieu J. P.
Published: January 2008

Abstract— It is assumed that quantum yields of monomolecular reactions are mainly determined by the probability of de‐excitation from the excited state to the ground state of the photoproduct and that a reaction will occur only if the stable molecular configuration of the excited state is close enough the molecular configuration of the photoproduct. In order to evaluate the stabilizing deformation of the excited states, the different methods of representing the excited states wave‐functions are compared. Though very different in their ability to reproduce spin densities in the triplet state, these methods appear to give very close results for the electronic repartition; particularly, unrestricted Hartree‐Fock calculations seem to give nearly the same net charges and bond indices for corresponding singlet and triplet states. A simple method for a rough evaluation of the direction and of the amplitude of the stabilizing deformation is proposed. In the Hiickel scheme, the variations in bond orders may be taken as an approximate index of that deformation.
ELECTRONIC PROPERTIES AND PHOTOCHEMICAL MONOMOLECULAR REACTIONS IN ORGANIC CONJUGATED COMPOUNDS: PHOTOCYCLIZATIONS, PHOTOISOMERIZATIONS AND PHOTOCHEMICAL REARRANGEMENTS—II
Authors: Malrieu J. P.
Published: January 2008

Abstract— Using theoretical indexes derived in Part I and Huckel and unrestricted Hartree‐Fock wave‐functions, the stabilizing deformation of the excited molecule is studied for a large number of planar conjugated compounds presenting monomolecular photo‐reactions: it is shown that in all the cases, the potential hole of the excited state of the initial molecule is strikingly displaced from its ground state configuration towards the fundamental configuration of the photoproduct. The possible influences of others electronic, steric and vibratory factors in particular are discussed in the study of photocyclization of 1–3 cis‐butadiene derivatives.
SKIN‐PHOTOSENSITIZING FUROCOUMARINS: PHOTOCHEMICAL INTERACTION BETWEEN DNA AND —O14CH3 BERGAPTEN (5‐METHOXY‐PSORALEN)
Authors: Musajo L. , Rodighiero G. , Breccia A. , Dall‘Acqua F. , Malesani G.
Published: January 2008

Abstract— For some years the mechanism of the photosensitizing effects displayed by some furocoumarins on various biological substrates (human and guinea‐pig skin, bacteria cultures, mammalian cells adapted to in vitro growth, viruses) have been studied. Recently it has been pointed out that a photoreaction occurs between the photosensitizing furocoumarins and DNA after irradiation at 3655 Å. By use of a labeled furocoumarin, i.e.—O14CH3 bergapten or 5‐methoxy‐psoralen, this has been confirmed and more extensively studied. During the irradiation a stable combination of the furocoumarin with native DNA takes place with a quantum yield of 5·2 × 10‐3. It is probable that the reactive sites of DNA are the pyrimidine bases. Yeast‐RNA and the same DNA after heat‐denaturation or in the presence of high NaCl concentration photoreact at a much reduced rate. This photoreaction may explain some various biological photosensitizing effects produced by furocoumarins.
PHOTOREACTIONS AT 3655Å BETWEEN PYRIMIDINE BASES AND SKIN‐PHOTOSENSITIZING FUROCOUMARINS
Authors: Musajo L. , Bordin F. , Caporale G. , Marciani S. , Rigatti G.
Published: January 2008

Abstract After irradiation at 3655 Å of an aqueous frozen solution containing thymine and psoralen, a new photocompound was isolated by column chromatography. It contains a furocoumarin and a pyrimidine‐moiety linked together by the formation of a cyclobutane ring (see formulas II and III). By irradiation at 2537 Å in acetic acid solution, the photocompound breaks up again yielding psoralen and thymine. From an aqueous frozen solution containing cytosine and psoralen irradiated at 3655 Å, an analogous photocompound was obtained, which, however, consists of the addition to psoralen of a uracil molecule, instead of a cytosine one (IV, V). It has been stated that the hydrolytic deamination of the cytosine moiety to the uracil one takes place during the working up of the photocompound in aqueous solution after irradiation. Substances with properties similar to those above were obtained from bergapten (5‐methoxy‐psoralen) and thymine, from psoralen and thymidine or thymidylic acid, irradiated at 3655 Å.

The new substances may be considered as model compounds in explaining the photoreactions which take place between the skin‐photosensitizing furocoumarins and DNA upon irradiation at 3655 Å.
OPTICAL EXCITATION OF THE EOSIN‐HUMAN SERUM ALBUMIN COMPLEX*
Authors: Youtsey Karl J. , Grossweiner L. I.
Published: January 2008

Abstract The excitation of the eosin‐human serum albumin complex has been investigated with spectral and photochemical techniques. Measurements of the changes in absorption and fluorescence spectra induced by complexing have shown that the first eosin molecule is held more strongly than the next few and that multiple binding leads to an enhanced fluorescence self‐quenching. Flash photolytic measurements indicate that dye‐dye quenching interactions enhance triplet eosin formation, and that oxygen quenching of the triplet is suppressed when the dye is singly bound. The quantum yield for aerobic photobleaching of the complex has been measured and is interpreted in terms of these findings.
A CALORIMETRIC DETERMINATION OF THE QUANTUM YIELD FOR THE JONIZATION OF MALACHITE GREEN CYANIDE BY ULTRAVIOLET RADIATION
Authors: Fisher G. J. , LeBlanc J. C. , Johns H. E.
Published: January 2008

Abstract The quantum yield for the conversion of malachite green cyanide (MGL) to the oxidized form MG+ has been measured over the wavelength range from 225 to 289 nm using a total absorption aluminium calorimeter to measure the flux of photons. The number of molecules of MGL converted was determined from the increase in absorbance of the solution at 622 nm. MG+ was found to have a maximum extinction coefficient of 10.63 × 104 at 622 nm. The quantum yield for the conversion of MGL to MG+ is constant over the wavelength range with a value of 0.91 ± 0.01. The use of MGL as an actinometer for photochemical studies is described.
THE REACTION OF METHYLENE WITH METHYL CHLORIDE THE EFFECT OF EXCESS ENERGY IN METHYLENE
Authors: Johnstone R. S. B. , Wayne R. P.
Published: January 2008

Abstract This paper reports studies of the reaction between methyl chloride and methylene produced by the photolysis of ketene in the two spectral regions Λ˜ 2600 Å to 3200 Å and Λ˜ 3220 Å. The course of reactin is best described by an abstraction process

CH2+ CH2CI—Ch2CI + CH3

followed by recombination of the CH3 and CH2 Cl radicals to yield C2H5, C2H5Cl, C2H4Cl2. The recombination processes are highly exothermic, and excitation of the product molecules occurs, which in the case of C2H5Cl and C2H4Cl2 leads to some unimolecular decomposition. It is shown that the rate constant for the decomposition of ethyl chloride depends upon the wavelength of the radiation used to photolyse the ketene, and it is suggested that the excess energy with which the methylene is born is handed on to the alkyl radicals. A simplified kinetic analysis of the system is given, and it is shown that the relative reactivity of methylene towards ketene and methyl chloride increases with an increase in the energy of the methylene. The rates of product formation predicted on the basis of the kinetic scheme agree satisfactorily with the measured values.

“Insertion” of methylene into C‐H and C‐CI bonds has been postulated by other workers. The present results are inconsisistent with an insertion mechanism, since such a mechanism does not account for all the observed products. The effect of wavelength of photolysis and of total pressure predicted on the basis of an insertion process is the reverse of that observed experimentally.
RADICAL INTERMEDIATES IN THE FLUORESCEIN‐AND EOSIN‐PHOTOSENSITIZED AUTOXIDATION OF l‐TYROSINE*
Authors: Kasche Volker
Published: January 2008

Abstract The Primary reactions of the cosin‐and fluorescein‐photosensitized autoxidation of L‐tyrosine were studied in aqueous media (pH = 8.6) by the flash‐photolysis technique. The dye molecules were quantitatively converted to their triplet states in a single flash. The triplet dye molecules were found to react with tyrosine or oxygen. Ground state or radical dye molecules were formed in these reactions. Some 40 per cent of the triplet‐tyrosine reactions yielded radicals, in triplet dye‐oxygen reactions the corresponding yield was less than 10 per cent. The ground state dye was regenerated from the semireduced dye in reactions with oxygen and from the semioxidized dye in reactions with tyrosine. In the absence of oxygen the radicals formed in the photoinduced electron‐transfer between the triplet dye and tyrosine recombined to a large extent.
PHOTOREVERSAL OF ACRIFLAVINE INHIBITION OF DEOXYRIBONUCLEIC ACID REPLICATION IN ULTRAVIOLET LIGHT‐EXPOSED BACTERIA
Authors: Doudney C. O.
Published: January 2008

Abstract With a strain of Escherichia coli lacking the capacity for photoenzymatic splitting of the pyrimidine dimer, acriflavine prevents restoration of DNA replication after u. v. exposure even after photoreversal treatment. With a revertant strain possessing capacity for photoreversal response, light exposure prior to addition of the dye eliminates a large part of the blockage of DNA synthesis by the dye. These results show that the presence of dimercontaining regions in the DNA prevents restoration of DNA replication when the dye is present. However, the non‐photoreversibility of some of the u.v.‐damage responsible for blockage of DNA synthesis by the dye suggests that acriflavine also interacts with lesions not restituted by the photoreactivating enzyme.
COMPLEXING OF RIBOFLAVIN AND ITS 2‐SUBSTITUTED ANALOGS WITH ADENOSINE AND OTHER 6‐SUBSTITUTED PURINE DERIVATIVES*
Authors: Roth Jerome A. , McCormick Donald B.
Published: January 2008

Abstract Fluorescence properties of riboflavin and its 2‐substituted analogs and the quenching of fluorescence which occurs upon complexing with adenosine and other 6‐substituted purine derivatives have been examined at different concentrations of interactants and hydrogen ion. The 6‐substituted purines and their ribosides show varying degrees of association with riboflavin. Successive N‐alkylation of the 6‐amino group of purine ribosides leads to more stable complexing in the order ethyl > methyl > propyl with riboflavin and its analog. These results suggest that such groups, including the 6‐amino group of adenine or adenine moiety of FAD, do not directly interact with flavins by hydrophobic or hydrogen bonding. Lowering the pH to above that required to protonate the isoalloxazine system disrupts the intermolecular complexes. Hence, only the unprotonated bases can form such complexes. The 2‐substituted aminoriboflavins, as well as FAD, exhibit fluorescence optima upon changing pH. The species involved appear to be a non‐fluorescent form with protonated isoalloxazine and amine or adenine portions at low pH, a fluorescent form with only the amine or adenine portions protonated at moderately acid pH, and a non‐fluorescent form which is intramolecularly quenched at higher pH. The general electron donating properties of the amines may be the common reason for their quenching of fluorescence in inter‐ and intramolecular complexes with flavins.
PHOTOBIOLOGY OF RNA BACTERIOPHAGES—I. ULTRAVIOLET INACTIVATION AND PHOTOREACTIVATION STUDIES*
Authors: Werbin Harold , Valentine Raymond C. , McLaren A. D.
Published: January 2008

Abstract— Biologically active f2‐RNA, Obtained from bacteriophage f2, was inactivated by ultraviolet (u.v) light (2537 Å) with a quantum yield of 3.3 ± 0.3 times 10‐3 when assayed in the dark with protoplasts of an F‐ strain of E. coli k12. Assay under “black light” gave a quantum yield of 2.7 ± 0.5 times 10‐3 which was just enough lower to suggest that 17 per cent photorecovery of the u.v. lesions has taken place.

Intact phage f2 was inactivated by u.v. radiation with a quantum yield of 0.7 ± 0.12 times 10‐3, Thus the whole phage is much less sensitive than the free RNA.

No evidence of photorecovery was found in u.v.‐irradiated RNA phage 7S assayed in its host Pseudomonas aeruginosa.
ULTRAVIOLET PHOTOLYSIS OF RIBOFLAVIN IN PYRIDINE: A PRELIMINARY STUDY
Authors: Song Pill‐ Soon
Published: January 2008

Abstract— Ultraviolet photolysis of riboflavin in pyridine leads to the formation of a red photoproduct. This photoreaction is apparently attributed to the photochemical interaction of pyridine with riboflavin. It is found that the rate of the photoreaction follows 1/2 order kinetics with respect to pyridine. ESR spectrum of the irradiated reaction mixture appears to be identical with that of riboflavin semiquinone obtained from visible light irradiation. Whether or not such a flavin radical is the responsible intermediate in the photoreaction is not established. Possible modes of the reaction are discussed. Absorption, excitation, and emission spectra of the red photoproduct strongly suggest that the structure of the photoproduct is considerably different from that of riboflavin or pyridine, indicating an extensive photolytic process involving covalent bond alterations.
A COMPARISON OF DECAY KINETICS OF PHOTO‐PRODUCED ABSORBANCE, EPR, AND LUMINESCENCE CHANGES IN CHROMATOPHORES OF RHODOSPIRRILLUM RUBRUM*
Authors: Loach Paul A. , Sekura Diane L.
Published: January 2008

–Quantitative comparison of the decay rate for absorbance photochanges in chromatophores of R. rubrum at the major wavelengths of peaks and troughs (280, 365, 385, 433, 605, 763, 790, 810, 850, 865, 890 nm) reveal no major differences under a variety of sample conditions. In addition, the decay kinetics of EPR phtochange at the two environmental potentials used in this study are identical with the absorbance ph;otochange decay.

Decay curves for fresh chromatophores, aged chromatophores, and fresh chromatophores at high ionic strength display a variety of half times and curve shapes. All of these data however, may be fit (within 10 per cent) by the equation x=αe8.2t+βe0.18t by merely varying the values of α and β. This is interpreted as meaning that any single trapping site may exist in one of two major forms each of which decays following a first order or pseudo‐first order reaction. Although the change in decay pattern upon aging is not reversible, that due to high ionic strength is. The time dependencies for two light emission phenomena have been measured as well as their response to the oxidation state of molecules at the trapping site. A major component of long‐lived luminescence follows pseudo‐second order kinetics with a half time of 38 msec. A very long lived luminescence shows an increse with time which is nearly proportional to the decay of absorbance photochanges.
ULTRASTRUCTURAL STUDIES OF THE LIGHT‐INDUCED CHLOROPLAST SHRINKAGE*
Authors: Dilley R. A. , Park R. B. , Branton D.
Published: January 2008

Both Class I (intact) and Class II (without the outer plastid membrane) chloroplasts of Spinacea oleracea exhibit a shrinkage of the thylakoid volume under conditions which lead to the well known light‐induced light scattering increases. In the present report this shrinkage has been measured on micrographs prepared by the freeze‐etch technique. In cloroplasts kept in darkness through the freezing or in those treated with DCMU prior to exposure to red light, the thylakoids are in a slightly swollen condition: in plastids exposed to red light and no inhibitor, the thylakoid membranes are closely appressed, giving the thylakoid a shrunken appearance relative to the control.

It is further shown that Class I chloroplasts which are actively fixing CO2 do not give appreciable light scattering changes, but lowering the pH away from the optimum for ATP formation (and CO2 fixation) or adding the uncoupler quinacrine restores the light‐induced scattering increases.
PHOTOREACTIVATING ENZYME FROM NEUROSPORA CRASSA*
Authors: Terry Claude E. , Setlow Jane K.
Published: January 2008

Abstract— Extracts of Neurospora crassa contain photoreactivating enzyme by the criteria of ability to split thymine‐containing dimers and to increase the transforming ability of u.v.‐irradiated Hemophilus influenzae DNA. The latter activity is heat‐labile and is destroyed by trypsin. The action spectrum of such in vitro photoreactivation is a simple one (with a single maximum at 405 nm in the range 313 to 436 nm), differing from the more complicated in vitro spectra for yeast and Escherichia coli. However, the in vitro Neurospora spectrum coincides closely with the in vivo spectrum for this organism, suggesting that there is little or no “indirect” photoreactivation in Neurospora. It is concluded that the Neurospora photoreactivating enzyme is probably of a different type than those of yeast and Escherichia coli.
INTERFERENCE BY NEAR ULTRAVIOLET AND GREEN LIGHT WITH GROWTH OF ANIMAL AND PLANT CELL CULTURES*
Authors: Klein Richard M. , Edsall Pamela C.
Published: January 2008

Abstract— Near ultraviolet (365 nm) and green‐yellow (546‐579nm) radiations repress the growth of liquid cell cultures of Ginkgo pollen and of monolayer cultures of HeLa. The deleterious effects of green‐yellow wavelengths can be negated by red radiation; near‐u.v.‐induced growth repressions are insensitive to visible light photorestoration. These wavelengths do not interact synergistically and evoke different kinetics of response.
PHOTOCHEMISTRY OF URACIL. INTERSYSTEM CROSSING AND DIMERIZATION IN AQUEOUS SOLUTION
Authors: Brown I. H. , Johns H. E.
Published: January 2008

Abstract— Ultraviolet irradiation of 14C‐uracil in aqueous solution results in the formation of hydrate and dimer photoproducts. The rate of dimerization increases with increasing uracil concentration, and decreases with increasing concentration of oxygen in solution. The kinetics are in agreement with a model previously proposed to account for the reactions, in which dimerization occurs by a reaction involving the triplet state of uracil, but hydration occurs from an excited singlet state. Oxygen reduces dimer formation by quenching the triplet. The quantum yield for intersystem crossing (ISC) to the triplet depends on the irradiation wavelength, increasing from 0.0014 at 280 nm to 0.016 at 230 nm. The ratio of rate constants for reaction of the triplet with oxygen and for dimerization is 1.1; the ratio of rate constants for triplet decay and for dimerization is 5.9 × 10‐5 M. The increase in ISC with photon energy suggests that ISC is favoured from excited vibrational levels. The quantum yield for hydration is about 0.002 at pH 4.5 for all wavelengths, but increases as the pH is decreased.
SENSITIZATION OF PHOTOCONDUCTIVE EFFECTS IN ZINC OXIDE*,†
Authors: Grossweiner Leonard I.
Published: January 2008

Abstract— The current status of research on sensitized photoconductive effects in zinc oxide is reviewed. The various types of chemical and spectral photosensitization processes observed in polycrystalline specimens can be explained qualitatively with a generalized electron acceptor molecule (EAM) model. in which electronegative surface agents trap zinc oxide conduction electrons in the dark and illumination causes a release of the trapped electrons. The EAM may be an adsorbed gas or vapor, adsorbed dye molecules, or an aggregated dye film, depending on the particular photoeffect.
ELECTRONIC ENERGY STATES OF CERTAIN SENSITIZERS AND DIELECTRICS*
Authors: Komp Richard J. , Fitzsimmons Thomas J.
Published: January 2008

Abstract— The electron affinities and ionization potentials of amorphous sulfur, vitreous selenium and several phthalocyanines were measured by Nelson's electron beam retardation method and photoelectric emission, respectively. The electron affinities (Ac) measured were: 4.5 eV for sulfur, 3.4eV for selenium, and approximately 4.3 eV for all the phthalocyanines measured. Photoemission thresholds (Ic) were 7.0 eV for sulfur and 6.0 eV for selenium. Two photoionization thresholds (at about 5.2 and 6.1 eV) were found for both α‐copper and α‐metal‐free phthalocyanine; the former is attributed to impurity levels, the latter to bulk states. In each case, the band gap calculated from Ic and Ac was found to be approximately equal to the optical threshold for photoconductivity.
ENERGY LEVELS IN CHLOROPHYLL AND ELECTRON TRANSFER PROCESSES*,†
Authors: Nelson R C.
Published: January 2008

Abstract— A brief account of the localized model for charge carrier generation in solid dyes is given, and generalized to include transfers of electrons and holes between unlike molecules. A part of the experimental basis for the localized model is the finding that the dye molecule sees its environment as a classical dielectric medium, even when it is composed of like molecules; since the effects of the environment on the ionization energy of chlorophyll are small, a close relationship can be inferred between laboratory energy levels and those effective in the chloro‐plast. The measured energy diagrams suggest a very low yield of charge carriers in either pure chlorophyll, but leave open the possibility of a substantial yield at an interface between chlorophylls a and b. The energy levels are consistent with the idea that charge carrier transfers between excited chlorophyll molecules and acceptors take place in the primary process.
FLASH PHOTOLYSIS OF FLAVINS‐III. REOXIDATION OF PHOTOREDUCED FLAVINS BY QUINONE*,†
Authors: GREEN MAURICE , TOLLIN GORDON
Published: January 2008

Abstract— The three‐component system, glycerol: flavin: p‐benzoquinone, was examined as a model for mitochondrial electron transport. A sequential electron transfer was observed in which photoproduced FMNH is reoxidized by benzoquinone.
THE FLUORESCENCE OF ADENINE. THE EFFECTS OF SOLVENT AND TEMPERATURE ON THE QUANTUM YIELD
Authors: EASTMAN J. W. , ROSA E. J.
Published: January 2008

Abstract— To investigate the mechanism of the solvent and thermal quenching of fluorescence, the absolute fluorescence quantum yield of adenine has been determined in several alcoholic glasses as a function of the concentration (10‐5‐10‐4M) and temperature (77°‐298°K). The yield is independent of the concentration but increases with decreasing temperature and increasing bulk cohesion and rigidity of the solvent.

The environmental effects on the yield are attributed to radiationless electronic relaxation processes caused by solute‐solvent interaction. Increasing the temperature and decreasing the intra‐solvent cohesion cause increases in the interaction and therefore accelerate the relaxation. The rate determining step behaves like a diffusion in the limit of low viscosity.
KINETICS OF CHLOROPLAST‐MEDIATED PHOTOXIDATION OF DIKETOGULONATE*
Authors: HABERMANN HELEN M. , HANDEL MARY ANN , McKELLAR PENELOPE
Published: January 2008

Abstract— Illuminated chloroplasts can mediate a photoxidation of diketogulonic acid (DKGA) with rates of oxygen uptake equivalent to rates of Hill reactions with ferricyanide or quinone. The photoxidation of DKGA is sensitive to dichlorophenyl dimethylurea (DCMU) and exhibits the drop in quantum yield at long wavelengths characteristic of photosystem II. Still, the reaction is only partially inactivated by heating chloroplasts at 50° for 10 min (which destroys oxygen evolution). The photoxidation is inhibited by copper and detergents; and is stimulated by added flavin (or methyl viologen) and manganous ions.

A model system containing Mn3+ (as manganipyrophosphate) and DKGA, mimics the chloroplast system. Pre‐illuminated chloroplast suspensions can be substituted for Mn3+ in the model dark reaction.

It seems that a light‐dependent oxidation of Mn2+ to Mn3+ by photosystem II is the essential contribution of the chloroplasts. Electrons from Mn2+ move through the electron transport system to ferricyanide or to photosystem I where, via flavin (or methyl viologen), oxygen is reduced to H2O2.
CHARACTERIZATION OF THE FAST LIGHT‐EVOKED ELECTRICAL RESPONSES FROM LEAVES
Authors: EBREY THOMAS G.
Published: January 2008

Abstract— A fast electrical response with a millisecond time course and no detectable latency has been recorded from the front surface of a bean leaf uniformly illuminated by an intense flash. The response is biphasic with the tip of the leaf always going negative with respect to the base of the leaf during the first msec regardless of the electrode positions and direction of light. Careful checks were made to eliminate the possibility that the response is due to some type of electrode artifact or to a photovoltaic, photogalvanic or photoconductive process in the leaf. Several properties of this response, including temperature dependence, flash energy dependence, polarity of the response with respect to leaf orientation and the effect of external polarizing voltages were investigated in order to compare it with fast photoresponses recently detected from other biological materials such as the vertebrate eye, pigment epithelium, and skin.
FLUORESCENCE STUDIES ON PHOTOSYNTHETIC PIGMENT DEVELOPMENT IN RHODOPSEUDOMONAS SPHEROIDES*,†
Authors: CELLARIUS R. A. , PETERS G. A.
Published: January 2008

Abstract— When bleached, aerobically grown cells of Rhodopseudomonas spheroides are transferred to semi‐aerobic conditions to induce bacteriochlorophyll synthesis, a new fluorescence band, with a maximum at 790 nm, is observed in addition to the 885 nm emission maximum normally seen in pigmented cells. The 790 nm fluorescence may be due to bacterio‐chlorophyll which has not been bound into the chromatophore membrane. The quantum yield of the 885 nm fluorescence is at first relatively high and then, about 1 hour after transfer, drops to the level found in pigmented photosynthetic cells. The coupling to the rest of the photo‐synthetic apparatus, as indicated by the effect of dithionite on the fluorescence, also seems to occur during the first hour of pigment development, which suggests that the onset of fluorescence quenching is due at least in part to the synthesis of photochemical reaction centers. Continuation of these studies should provide new information on the formation, structure and molecular interactions of the pigments and the photosynthetic membranes.