ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1996.tb02988.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1996.tb02989.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract—The current work concerns investigation of the polarization properties of complex molecular ensembles exhibiting threefold (C3) rotational symmetry, particularly with regard to the interplay between their structure and dynamics of internal energy transfer. We assume that the molecules or chromophores in such complexes possess strongly overlapped spectra both for absorption and fluorescence. Such trimeric structures are widely found in biological preparations, as for example the trimer of C‐phycocyanin (C‐PC). Higher order aggregates,e.g.hex‐amers and three‐hexamer rods, are also investigated and compared with the trimer case. The theory addresses both steady‐state and 8‐pulse excitation and establishes some links between them. Monochromophoric, bichro‐mophoric and trichromophoric molecular complexes are individually examined.For steady‐state excitation, analytical formulas are reported for the degree of fluorescence polarization and absorption anisotropy. It is shown that the polarization is dependent on the chromophore inclination relative to the symmetry axis, the relative efficiencies of absorption and fluorescence by chromophores of different spectral types, and the rates of energy equilibration. To assess the validity of the theory, it has been applied to C‐PC aggregates. Here it was found that different C‐PC aggregates provide practically identical polarization response. For S‐pulse excitation we give analytical formulas for determination of the fluorescence depolarization, and also the depolarization associated with absorption recovery, both for a monochromophoric trimer and some particular cases of bichromophoric trimer. More complicated systems are analyzed by computer modeling. Thus it transpires that the initial polarization anisotropy r(t = 0) takes the value 0.4 for all considered aggregates; the long‐time limit r(t →∞) has about the same value as is associated with steady‐state excitation. We also show that with steady‐state excitation the degree of fluorescence polarization is practically equal for various C3aggregates of C‐PC, and that the major factor determining the polarization is the chromophore orient

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1996.tb02990.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract—Reaction kinetics of radicals produced by the nanosecond laser flash photolysis of 2,2′‐dithiobis(pyridineN‐oxide) and related compounds have been studied. The transient absorption band at 360 nm was attributed to the radical in which the unpaired electron mainly localizes on the S atom (N‐oxypyridyl‐2‐thio radical). The reactivities of the radical for conjugated dienes are lower than those of the pyridyl‐2‐thio radical, suggesting that a considerable unpaired electron density on the S atom delocalizes onto theN‐oxypyridine moiety. The addition reaction rate of the radical to the conjugating diene was accelerated with hydrogen‐bonding solvents and with addition of the cation, which may stabilize the N+‐O‐canonical structure, increasing the unpaired electron density on the S atom. By the photolysis ofN‐hydroxypyridine‐2‐thione, the N‐O bond was predominantly dissociated producing a pyr‐idyl‐2‐thio radical. By the photolysis of its anion, photoejection took place followed by the N‐O

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1996.tb02991.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract—The combination of steady‐state and time‐resolved quenching experiments was employed to study the aggregation behavior of sodium cholate at concentrations below 50 mAf. Naphthalene, anthracene and pyrene were used as fluorescent probe molecules, and protection by the aggregates from aqueous quenchers, as well as the onset of aggregation at low sodium cholate concentrations, was dependent on the shape of the probes. Protection from aqueous quenchers was inferred by comparing the efficiency for dynamic quenching in the absence and presence of sodium cholate and was best for naphthalene followed by pyrene and anthracene. Static quenching was observed, suggesting that probe molecules are located in an aggregate environment that also contains iodide. The incorporation of pyrene at low sodium cholate concentrations, as well as the small degree of static quenching observed, suggest that the shape complementarity,i.e.hydrophobic surface and packing, between pyrene and sodium cholate is optimum for aggregate form

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1996.tb02992.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract—In contrast to the damage caused by far‐UV, the damage caused by UVA (320‐400 nm) is largely oxygen dependent, suggesting near‐UV‐mediated DNA damage involves reactive oxygen species. The DNA repair enzymes that recognize oxidized bases may, therefore, be an important part of the cell's near‐UV defense repertoire. To evaluate the relative importance of Fpg (Fapy) glycosylase (an enzyme known to remove oxidized bases) and the DNA damage‐inducible UvrABC excinuclease in recovery from near‐UV‐induced stress, we have constructed,fpganduvrAderivatives ofEscherichia coliand tested the response (survival) of these strains to both UVA and far‐UV radiation. Relative to control strains,the fpg‐derivatives were found to be consistently more sensitive to the lethal effects of UVA, but not far‐UV radiation. In contrast,uvrAmutants were more sensitive than control strains to both UVA and far‐UV radiation. Thymine dimers, known to be produced by far‐UV and corrected by UvrABC, were not generated by the UVA fluences used in this study, suggesting that some other UVA‐induced lesion(s) is recognized and

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1996.tb02993.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract—The biologically effective dose of solar UV radiation was estimated from the inactivation of UV‐sensitiveBacillus subtilisspores. Two types of independent measurements were carried out concurrently at the Aerological Observatory in Tsukuba: one was the direct measurement of colony‐forming survival that provided the inactivation dose per minute (ID/min) and the other was the measurement of the spectral irradiance by a Brewer spectrophotometer. To obtain the effective spectrum, the irradiance for each 1 nm wavelength interval from 290 to 400 nm was multiplied with the efficiency for inactivation derived from the inactivation action spectrum of identically prepared spore samples. Integration of the effective spectrum provided the estimate for ID/min. The observed values of ID/min were closely concordant with the calculated values for the data obtained in four afternoons in 1993. The average ratio (±SD) between them was 1.24 (±0.16) for 14 data points showing high inactivation rates (<0.05 ID/min). Considering difficulties in the absolute dosimetry of UV radiation, the concordance was satisfactory and improved credibility of the two types of monitoring systems of biologically effective dose of solar UV ra

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1996.tb02994.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract—Damage to lens epithelial cells is a probable initiation process in cataract formation mediated by UV radiation. In these experiments, we investigated the effects of exposure to 254 nm radiation on cell cycle progression and gene expression in the rabbit lens epithelial cell line N/N1003A. The RNA was harvested at various times following exposure to UV (254 nm) radiation and analyzed by dot‐blot and northern blot hybridizations. These results revealed that during the first 6 h following exposure of the cells to UV, there was, associated with decreasing dose, a decrease in accumulation of transcripts specific for histones H3 and H4 and an increase in the in RNA encoding protein kinase C and β‐ and γ‐actin. Using flow cytometry, we detected an accumulation of cells in G1/S phase of the cell cycle 1 h following exposure to 254 nm radiation. The observed changes in gene expression, especially the decreased accumulation of histone transcripts reported here, may play a role in UV‐induced inhibition of cell cycle

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1996.tb02995.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract—Laser flash photolysis spectroscopy has been used to investigate the kinetics of oxidation and reduction of pho‐tosynthetic plastocyanin (Pc) and cytochromec6(Cyt) from the cyanobacteriaAnabaenaPCC 7119 andSyne‐chocystisPCC 6803 by lumiflavin, riboflavin and flavin mononucleotide (FMN). For both redox reactions similar steric and electrostatic effects were observed withSyne‐chocystisproteins andAnabaenaCyt, thus suggesting that the same (or closely adjacent) sites are being used for electron entry and removal. In the case ofAnabaenaPc, however, both the steric and electrostatic effects suggest that FMN‐dependent protein oxidation and reduction may occur at different sites, that is oxidation at the hydrophobic patch and reduction at the hydrophilic one. KineticpKavalues of 5.8 and 6.2 have been determined for the lumiflavin‐dependent reduction ofAnabaenaandMonoraphidiumPc, respectively, whereas the oxidation reactions appear to be pH independent. Not only the reduction kinetics but also protein tyrosine fluorescence quenching by iodide ions suggest the occurrence of pH‐induced conformational changes in Pc. In conclusion, kinetic and fluorescent studies indicate that there are considerable quantitative similarities between Cyt and Pc when isolated from the same organism, consistent with the identical physiological role that these two proteins play ins

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1996.tb02996.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract—A comparison of the spectra ofin vitro(3‐hydroxymethyl‐131‐oxometallochlorin) andin vivochlorosomal (bacterio‐chlorophyll‐c) aggregates suggests a similar supramolecular structure for the artificial oligomers and the bacte‐riochlorophyll‐caggregates in the extramembranous antenna complexes (chlorosomes) of green photosynthetic bacteria. Synthetic zinc and magnesium chlorins have been found to aggregate in 1 % (vol/vol) tetrahydrofuran and hexane solutions and in thin films to form oligomers with the Qyabsorption bands shifted to longer wavelengths by about 1900 (Zn chlorins) and 2100 cm−1(Mg) relative to the corresponding monomer bands. Visible absorption and circular dichroism spectra of various zinc chlorins establish that a central metal, a 31‐hydroxy and a 131‐keto group are functional prerequisites for the aggregation. Vibrational bands measured by IR spectroscopy of solid films reveal two characteristic structural features of the oligomers: (1) a five‐coordinated metallochlorin macrocycle with an axial ligand (bands at 1500‐1630 cm−1), and (2) a hydrogen bond between the keto oxygen of one chlorin and the hydroxy group of a second chlorin, the oxygen of which is chelated to the metal atom of a third molecule

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1996.tb02997.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY