bluuebunny/crossref_metadata_2025_split huggingface dataset:
MITOTIC ACCELERATION INDUCED IN GRASSHOPPER NEUROBLASTS BY SMALL DOSES OF MONOCHROMATIC ULTRAVIOLET RADIATION
Authors: Carlson J. Gordon , Stephens Ralph E. 
Published: January 2008

Abstract—Neuroblasts of the grasshopper, Chortophaga viridifasciata (DeGeer), in culture were exposed to 4–15 J/m2 of monochromatic ultraviolet radiation in middle prophase and timed to prometaphase (breakdown of the nuclear membrane). At wavelengths 265 and 280 nm the times recorded for the treated cells were significantly less than those for the matched control cells. Wavelength 225 nm produced neither significant acceleration nor deceleration of mitotic rate.Evidence from the present experiment does not implicate any particular chromophore in this response, though similar results of a previous microbeam study of neuroblasts point to the nucleic acid or protein of the nucleolus as the possible effective material.
FLUORESCENCE INDUCTION IN ALGAE AND CHLOROPLASTS
Authors: Lavergne Jerome 
Published: January 2008

Abstract—The first phases of the fluorescence transient elicited by illumination of dark‐adapted algae or isolated chloroplasts (biphasic rise φ***v ‐ φI ‐ P) have previously been shown to be controlled by two quenchers: Q, the primary acceptor of Photosystem 2 interacting with the secondary acceptor pool A; and R, a non‐photochemical quencher which goes into a non‐quenching state as A is reduced.The dependence of the kinetics of φ decay after illumination upon the redox state of A was studied. It is suggested that some of the centres are in a disconnected state, where electron transfer between Q and A is hindered, the amount of such centres being correlated to the reduction state of A. The implications of this hypothesis on the problem of the variation of the Q‐A‘equilibrium constant’ under different experimental conditions, and on Murata's ‘weak light effect’ are discussed.The effect of 3‐(3,4 dichlorophenyl)‐1,1 dimethylurea (DCMU) on R is shown to depend upon the redox state of A. A DCMU‐induced shift of the midpoint potential of R may account for this dependence.Evidence is given suggesting that the transient reduction of A which occurs in algae during the φI‐φP rise is controlled by an induction process on the acceptor side of Photosystem 1.
KINETIC ANALYSIS OF THE FLUORESCENCE INDUCTION IN 3‐(3,4‐DICHLOROPHENYL)‐1,1‐DIMETHYLUREA POISONED CHLOROPLASTS
Authors: Melis Anastasios , Homann Peter H. 
Published: January 2008

Abstract— The size of the area over the fluorescence rise curve of chloroplasts is a measure of the total number of quanta utilized in photosystem II during the fluorescence induction, while the growth of the area reflects the progress of photochemical events. In the presence of 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU), the growth kinetics of the area are affected by the reoxidation of the primary acceptor Q‐ with stored oxidizing charges on the donor side of system II.At low light intensities, a slow component of this back reaction may limit the steady state fluorescence emission. At higher intensities, however, the fluorescence rise is limited solely by photochemical events, although fast thermochemical reactions like the immediate recombination of photochemically separated charges may affect the efficiency of the photochemistry.A kinetic analysis of the area growth at moderate light intensities revealed that it occurred in two first order phases which were described by the rate constants kα and kβ. The biphasic nature suggested a sequential two‐electron reduction of the primary acceptor Q, or the presence of two different types of photochemical centers in system II. The rate constants were light intensity dependent. They also were affected by changes in pH, by an addition of NH2OH, or by a preillumination with short flashes prior to addition of DCMU. It is suggested that the pH of the medium, the presence of NH2OH, and the flash induced state Sn of the water splitting enzyme, control the values of kα and kβ by changing the rate constants of electron carrier interactions in the reaction center complex, with a resulting modification of the frequency of back reactions between the primary donor and the primary acceptor.
PHOTOCONTROL OF PHYTOCHROME DESTRUCTION IN GRASS SEEDLINGS. THE INFLUENCE OF WAVELENGTH AND IRRADIANCE
Authors: Schäfer E. , Lassig T.‐U. , Schopfer P. 
Published: January 2008

Abstract— –The kinetics of phytochrome destruction in vivo of coleoptiles and mesocotyls of etiolated grass seedlings (Avena sativa L., Zea mays L.) in continuous light were investigated using wavelength and irradiance as experimental variables. In contrast to dicotyledonous seedlings, the destruction reaction of these monocotyledons is saturated at very low levels of the far‐red absorbing form of phytochrome, Pfr (e.g. at 1% of total phytochrome, corresponding to the photostationary state established by 727 nm light, in 2.5‐day‐old dark‐grown Avena). On the other hand, the first‐order rate constant of monocotyledon destruction may be at least one order of magnitude larger than in dicots, as indicated by the zero‐order rate measured in the presence of saturating amounts of Pfr (τl/2 1.5 min in Avena). At sub‐saturation Pfr levels, the destruction rate was found to be determined by the rate constants of the photoreactions over a wide range of wavelengths and irradiances. These results can be interpreted in terms of a destruction enzyme with high catalytic efficiency but limited availability.Analysis of in vivo binding of phytochrome to a pelletable cell structure during destruction revealed that both the pelletable and the non‐pelletable fraction lose photoreversiblility with similar rates and thus provide no useful information with respect to a causal relationship between the two processes. However, due to the short half‐life of Pfr at sub‐saturation levels (which make the photoreactions and intermediary processes rate‐limiting for destruction even at relatively high irradiances) the existence of a similarly rapid dark‐reaction between the photoreactions producing Pfr and the destruction reaction could be demonstrated. This dark reaction displays the properties of Pfr binding to a receptor site.
THE PHOTOPEROXIDATION OF UNSATURATED ORGANIC MOLECULES—XV. O21δg QUENCHING BY BILIRUBIN AND BILIVERDIN
Authors: Stevens B. , Small R. D. 
Published: January 2008

Abstract—The use of competitive methods with rubrene as reference acceptor provides values of the rate constants kBQ= 2.3 times 109/ mol‐1 s‐1 and kvq= 3.3 times 109 mol‐1 s‐1 for the physical quenching of O21δg, by bilirubin (in CCl4) and biliverdin (in CHCl3) respectively attributed to electron and energy transfer processes.The rate constant kBR= 1.7 times 108/ mol‐1 s‐1 for bilirubin reaction with O21δg is considerably greater than that kvR 3 times 108 mol‐ s‐1 for the reaction of biliverdin which has identical O21δa, acceptor sites, and is attributed to a primary Type I oxidation process followed by a slower Type II addition of O21δg to the primary products.
TRIPLET ELECTRONIC STRUCTURE AND PHOTOREACTIVITY OF 8‐METHOXYPSORALEN*
Authors: Moore Thomas A. , Montgomery Alan B. , Kwiram Alvin L. 
Published: January 2008

SummaryPsoralen and its derivatives are implicated in a variety of photobiological processes including skin‐sensitization in mammals, the experimental photochemotherapy of psoriasis, and photomutagenesis in bacteria. Although the various derivatives differ markedly in photoactivity, their excited triplet states as characterized by conventional luminescence spectroscopy are very similar and closely resemble that of the parent compound, coumarin, which is inactive as a skin sensitizer. Employing a more sensitive probe of triplet electronic structure, we have utilized optical detection of magnetic resonance to measure the zero‐field splittings in the triplet state of several psoralens and find a striking variation among the derivatives. D*, taken as a composite measure of the dipole‐dipole interaction of the unpaired electrons in the triplet state was found to be anomalously large, greater than 0.140 cm‐1, for the very active compounds, psoralen and 8‐methoxypsoralen, while D*= 0.124cm‐1 for the inactive, though smaller, coumarin molecule. Although 8‐hydroxypsoralen has a large D* value its inactivity as a skin‐sensitizer may be explained by the dissociation of the hydroxyl proton. The resulting (excited) triplet state anion has D*= 0.120 cm‐1.
IN VIVO INDUCTION OF 4‐THIOURIDINE‐CYTIDINE ADDUCTS IN tRNA OF E. COLI B/r BY NEAR‐ULTRAVIOLET RADIATION*
Authors: Ramabhadran T. V. , Fossum T. , Jagger J. 
Published: January 2008

Abstract— Near‐ultraviolet (near‐UV; 320–405 nm) irradiation of Escherichia coli B/r induces the formation in vivo of 4Srd‐Cyd adducts in transfer RNA, as evidenced by (1) fluorescence spectrum changes of tRNA extracted from irradiated cells and reduced with NaBH4, (2) thin‐layer chromatography on cellulose of hydrolysates of trichloroacetic acid‐precipitable extracts of irradiated cells, and (3) comparison of these findings with adduct formation induced by near‐UV irradiation of purified mixed tRNA from E. coli.The kinetics of induction of the 4Srd‐Cyd adduct in vivo, and the near‐UV fluences required, provide strong support for our earlier hypothesis that formation of these adducts is responsible for near‐UV‐induced growth delay in E. coli.
HETEROGENEITY OF THE PHOTOCHEMICAL CENTERS IN SYSTEM II OF CHLOROPLASTS*
Authors: Melis Anastasios , Homann Peter H. 
Published: January 2008

Abstract— In 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) poisoned chloroplasts of algae and‘ higher plants the area over the fluorescence induction curve increases with biphasic first order kinetics (Melis and Homann, 1975). Two possibilities are considered to explain the biphasic nature of the area growth. The first is a sequential double reduction of the primary electron acceptor in system II while the second envisages a heterogeneity of its photochemical centers. The kinetic properties of the area growth after firing a single saturating flash proved to be incompatible with the predictions of the “sequential double reduction” model. This conclusion was corroborated by results obtained from a kinetic analysis of the area restoration process in the dark, and an analysis of the partially restored areas.Assuming an existence of a heterogeneous pool of photochemical centers, the growth of the area over the fluorescence curve could be further analyzed to yield two components, a fast a‐component, and a relatively slow β‐component. The kinetic characteristics of these components, and the effect of a short saturating flash on their respective size, led to the conclusion that one type of photochemical center had a faster recombination rate of the photochemically separated charges and was less efficient in trapping excitation energy.
ELECTROPHORETIC AND SPECTROPHOTOMETRIC STUDIES OF CHLOROPHYLL‐PROTEIN COMPLEXES FROM TOBACCO CHLOROPLASTS. ISOLATION OF A LIGHT HARVESTING PIGMENT PROTEIN COMPLEX WITH A MOLECULAR WEIGHT OF 70,000
Authors: Remy R. , Hoarau J. , Leclerc J. C. 
Published: January 2008

Abstract— …After a short‐term solubilization with sodium dodecyl sulphate, chloroplast membranes of tobacco were separated by polyacrylamide gel electrophoresis into three chlorophyll‐protein complexes. In addition to the two major complexes termed I and IIc corresponding respectively to P700 chlorophyll a‐protein and light‐harvesting chlorophyll a/b‐protein described by Thornber (1975), a relatively stable complex termed IIa has been observed. This new complex has an apparent molecular weight of 70,000 daltons and possesses Chl a and b.Complexes I, IIa and IIC have been isolated and precise spectroscopic analyses have been performed. Fourth derivative analyses of low temperature absorption spectra suggest that complex IIa seems more representative than IIC of chlorophyll a forms present in intact thylakoid membranes.Moreover, the electrophoretic study reveals that CPI and CPII are composed of only one polypeptidic subunit with respective molecular weights of 68,000 and 24,000 daltons.
CHLOROPHYLL METABOLISM IN ETIOLATED GHERKIN SEEDLINGS I. PHOTOINHIBITION OF CHLOROPHYLL ACCUMULATION
Authors: Dorsman A. , Elgersma O. , Meuer G. , Steuten L. 
Published: January 2008

Abstract. Cotyledons of etiolated gherkin seedlings do not turn green upon transfer to high intensity red light (about 25 W/m2). A pre‐irradiation with high intensity red light has an after‐effect as chlorophyll accumulation during a subsequent exposure to white light (20 W/m2) is inhibited.The capacity of protochlorophyll regeneration during a dark period depends on the length of a previous light period but is hardly affected by the light intensity. At high intensity light the rate of protochlorophyll regeneration, which also depends on the length of the foregoing irradiation, is lower than that at low intensity light only during the first 1.5h of the light period. It is concluded that high intensity red light inhibits chlorophyll accumulation mainly by photo‐bleaching of chlorophyll. The after‐effect is the result of a photooxidation which may lead to photo‐bleaching of newly formed chlorophyll in relatively low intensity light.Photoinhibition of chlorophyll accumulation is accompanied by a disturbed development of etioplasts into chloroplasts.
THE FUNCTION OF PHYTOCHROME IN THE NATURAL ENVIRONMENT—III. MEASUREMENT AND CALCULATION OF PHYTOCHROME PHOTOEQUILIBRIA
Authors: Smth Harry , Holmes M. G. 
Published: January 2008

Abstract— Phytochrome photoequilibria have been measured in dark‐grown Phaseolus uulgaris L. and Cucurbita pepo I. hypocotyl hooks which had been exposed to various natural and artificial radiation sources. Mean phytochrome photoequilibria (φ) varied from 0.20 within a wheat canopy to 0.54 above, although lower values were occasionally observed in densely shaded areas. Greater variation in phyto chrome photoequilibria and lower levels of Pfr were recorded within a sugar beet canopy. The range of photoequilibria was φ= 0.04 in dense shade to φ= 0.54 above the canopy. Photoequilibrium was achieved within 5 s in mid‐day sunlight and approximately 30 s in dense canopy shade.A close correlation was found between φ and the ratio of the quantum flux in the red and far‐red wavelength bands (ζ) in broad spectrum (400–800 nm) radiation. This relationship allows direct prediction of φ from a knowledge of ζ. Phytochrome showed greatest sensitivity to spectral changes in the range ζ= zero to ζ= 1.0, which is the range found in the natural environment.The observations provide support for the hypothesis that phytochrome is involved in the detection of shading by plants.
PROTONATION AND DEPROTONATION EQUILIBRIA OF LUMICHROME
Authors: Laser N. , Feitelson J. 
Published: January 2008

Abstract— In order to investigate the possibility of the tautomerization of alloxazine to isolloxazine in its ground state, the parameters affecting the redistribution of charges in the lumichrome molecule were studied. The absorption and emission spectra of lumichrome as a function of pH in the range H0= ‐ 6 to pH = 12 were recorded. At extreme pH conditions the spectra of lumichrome are similar to those of the isoalloxazine system. At high acid concentration (H0 < ‐ 3.0) the absorption spectrum of lumichrome protonated at N10, is practically identical to that of lumiflavin. The fluorescence quantum yield of the two cations is negligible at room temperature.At pH = 10.5 lumichrome is deprotonated at positions N1 or N3, The two monoanions have different excitation spectra. Except for slight differences in the extinction coefficients, the absorption of the anion deprotonated at N3 is very similar to the lumichrome spectrum. The absorption spectra of the N1 monoanion and of the di‐anion are similar to the spectrum of lumiflavin except for a blue shift of about 20 nm. Furthermore, the emission spectrum of the N1 ‐monoanion is identical to that of the isoalloxazine system. These results indicate that the charge distribution in the lumichrome molecule depends on the protonation and deprotonation of the nitrogen atoms at positions 10 and l. Both processes cause a redistribution of charges so that an isoalloxazine ring system is formed.
THE EOSIN‐SENSITIZED PHOTOOXIDATION OF SUBSTITUTED PHENYLALANINES AND TYROSINES*
Authors: Rizzuto Frank , Spikes John D. 
Published: January 2008

Abstract— The eosin‐sensitized photooxidation of tyrosine and a number of compounds related to tyrosine (substituted phenylalanines) was studied by steady‐state kinetic and flash photolysis techniques. In particular, the role of the phenolic group and the amino and carboxyl groups of the alanyl side chain in the photooxidation mechanism was investigated in detail. Several relationships between substrate structure and susceptibility to photooxidation as well as effects of substrate structure on photooxidation mechanisms were found.For example, phenylalanine is not photooxidizable, hut substitution of electron‐donating (activating) groups such as‐OH (as in tyrosine) or‐NH2 (as in p‐aminophenylalanine) results in rapidly photooxidized derivatives. However, substituting deactivating groups such as—C1 (as in p‐chlorophenylalanine) or weakly activating groups such as ‐ OCH3 (as in 4‐methoxyphenylalanine) result in non‐photooxidiz‐able derivatives. Substitution of additional activating groups to the ring of hydroxy‐substituted phenylalanines results in increased rates of photooxidation, whereas additional deactivating groups result in decreased photooxidation rates.The rate‐determining step in the photooxidation mechanism is shown to be dependent on the presence and position of an electron‐donating substituent on the benzenoid ring. Only minor involvement of the side chain amino and carboxyl groups was found. Both singlet oxygen and hydrogen abstraction mechanisms are involved in the eosin‐sensitized photooxidation of hydroxy‐substituted phenylalanines (e.g. tyrosine). The hydrogen abstraction mechanism probably predominates at both pH 8 and 11.
OXYGEN‐INTERACTION LUMINESCENCE SPECTROSCOPY*
Authors: Khalil Gamal‐Eddin , Kasha Michael 
Published: January 2008

Abstract— A new spectroscopic method has been developed in our laboratory for perturbation of singlet‐triplet transitions in organic molecules by O2. A dilute solution (10‐2‐10‐4M) of the molecule to be studied in various solvents is pressurized in a bomb with O2 at 100 atm (10MPa) for 3–10h. The pressure is released, the solution pipetted into a sample tube, sealed, and frozen to a rigid glass state at 77 K. Then the luminescence spectra and excitation spectra of the sample are scanned. For benzene‐O2 we have evidence for a weak complex with a mild perturbation and a strong complex, with various new combination states. For the strong complex, the T1→ S emission and excitation spectrum is observed, and also the T2← S excitation spectrum. The T1→ S emission for the benzene‐O2 complex is seen as a highly resolved emission at 77 K in the glass solvent, but also at 298 K as a poorly resolved emission (induced triplet–triplet fluorescence in the complex). Various upconversion simultaneous transitions and downconversion simultaneous transitions are observed, e.g. (3Blu+1Σ+g), (3Blu+1Δg) upconversions, and the (3Blu+1Σ+g), (3Blu+1Δg) downconversions; thus, in the latter, the third π, π* triplet is revealed for the first time. A novel structure for the benzene–O2 strong complex is proposed as an anf jamel model.For pyridine–O2 and perdeuteropyridine–O2 we observe the T1→ S emission for the first time; it is π, π* in character, analogous to benzene.
CYTOTOXIC REACTIONS OF FREE RADICAL SPECIES OF OXYGEN*
Authors: Borg Donald C. , Schaich K. M. , Jr J. J. Elmore , Bell J. A. 
Published: January 2008

Abstract— We report on autooxidation reactions related to selective cell killing in vivo and present oxygen uptake and EPR data on the molecular nature of these changes, with a note on a reaction used to test for singlet oxygen.Selective obliteration by 6‐hydroxydopamine, 6‐aminodopamine and their congeners of beta‐adrenergic cell receptors and of adrenergic and dopaminergic neurons in vivo and in vitro have been reported recently. These effects devolve from autoxidation of the compounds in situ after appropriate biological concentration, but it is unclear whether cytotoxicity is mediated by the reactive oxygen species themselves or by nucleophilic reactions of the product quinones. Because of the possibility of harnessing these reactions for a generalized “chemical surgery” if the former mechanism is operative, and because many other oxidative damage reactions involving unsaturated lipids of biomembranes seem to share similar autoxidative initiating and chain‐carrying steps, we have studied the autoxidation of ascorbate and other reductants catalyzed by polyhydroxy quinols related to 6‐hydroxydopamine. We find that superoxide anion mediation may or may not be important, depending on redox potentials and reaction kinetics of particular compounds. To the extent that the cellular toxicity of these compounds is oxidative in nature, it is facilitated by—and may depend upon—hydroxyl radical production due to Fenton‐type reactions. Likely Fenton reactants in vivo are hydrogen peroxide produced from the quinol‐catalyzed autoxidations and non‐heme iron reduced cyclically by superoxide anions from the same autoxidations.Studies using flow systems adapted for EPR at 35 GHz have provided support for some of the reaction mechanisms proposed. In nonaqueous solvents eerie oxidation produced transient free radicals from hydrophobic diphenyl and isobenzo furans, which are often cited as “specific” probes for singlet oxygen detection in biochemistry.
CONTROL OF CHLOROPHYLL b BIOSYNTHESIS BY PHYTOCHROME
Authors: Oelze‐Karow H. , Mom H. 
Published: January 2008

AbstractIn the mustard seedling cotyledons, chlorophyll b appears from the very beginning in white light provided that a red light pulse pretreatment was given 12 h prior to the onset of white light. The red light pulses act through phytochrome. Without pretreatment no chlorophyll b is detectable at least during the first 60 min after the onset of white light (25°C). Biogenesis of chlorophyll b specifically depends on the action of phytochrome during the pre‐steady state period as well as during the steady state period of chlorophyll accumulation. In light pulse experiments, it was found that formation of chlorophyll b takes place stoichiometrically at the cost of chlorophyll(ide) a.
BIOSYNTHETIC PATHWAYS TO 6‐AMINOLEVULINIC ACID INDUCED BY BLUE LIGHT IN THE PIGMENT MUTANT C‐2A‘ OF SCENEDESMUS OBLIQUUS*
Authors: Klein Otto , Senger Horst 
Published: January 2008

AbstractThe X‐ray induced mutant C‐2A of Scenedesmus obliquus grows heterotrophically but forms only traces of chlorophyll in the dark. Upon illumination, baminolevulinic acid (ALA) is synthesized and chlorophyll is formed. These processes are blue light dependent and cease immediately when the cells are transferred back into darkness.Addition of levulinic acid (LA) inhibits the light‐dependent formation of chlorophyll and causes accumulation of ALA by competitive inhibition of the ALA dehydratase (EC. 4.2.1.24). By feeding specifically labelled 14C precursors to the pigment mutant, inhibiting the ALA dehydratase with LA, accumulating, extracting and analyzing the ALA, two pathways leading towards ALA could be established: glycine and succinyl CoA can be condensed to ALA and the 5 carbon skeleton of glutamate can completely be incorporated into ALA via a second pathway.The glycine‐succinyl CoA pathway dominates over the glutamate pathway, but both lead to chlorophyll formation.