摘要:

Abstract—A comparison of the transient absorption spectra from the photolysis of disulfides in solution suggests that C‐S bond breakage is a common primary photolytic process. This process becomes more important as the resulting carbon centered radical is stabilized by increasing alkyl substitution or resonance interaction with an aromatic system. The perthiyl radical product is characterized by λmax?380 nm,ε380?1700M−1cm−1and decays by second order kinetics withk2?3.7×108M−1s−1in water.In the presence of O2, the photolysis of disulfides which produce the thiyl radical give transient absorptions in the 500–600 nm region. Possible identities of these transien

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1981.tb04288.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

Abstract—The mechanism of the photooxidation of ascorbic acid by flavin mononucleotide has been studied by steady state and flash photolysis techniques. The primary reaction consists of an electron abstraction from ascorbic acid by the flavin singlet and triplet states by a diffusional process and in the latter case also by the formation of a triplet flavin‐ascorbic acid complex. Under anaerobic conditions, the secondary reactions consist primarily of a radical recombination and a ‘dark’chemical back reaction, leading to the reformation of unchanged flavin and ascorbic acid. In the presence of oxygen, the ‘back reactions’are prevented, resulting in the efficient photooxidation of as

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1981.tb04289.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

Abstract—5‐Bromouridine (BrUd) photochemically reacted withl‐tryptophan (trp) only in aqueous frozen solution giving rise to 2‐[1,2,3,4‐tetrahydro‐1‐(β‐d‐ribofuranosyl)‐2,4‐dioxo‐5‐pyrimidinyl]‐l‐tryptophan (1a). Under similar conditions irradiation of 5‐bromouracil (BrU) and trp gave the corresponding coupled product2.Addition of acetone, potassium carbonate or sodium chloride to the frozen system completely inhibited the photocoupling reaction. These photocoupling reactions are suggested to proceed via mixed aggregate formation between trp and BrUd (or BrU) in frozen aqueous solution. Stacking interactions in these aggregates are characterized by an efficient quenching of trp fluorescence by BrUd. Under the same conditions other amino acids such as phenylalanine, tyrosine, histidine and lysine did not react with BrUd, indicating that the photocoupling of B

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1981.tb04290.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

Abstract—Emission spectra of unbound reduced nicotinamide formycin dinucleotide (NFDH) revealed the presence of two major conformations of the coenzyme in solution: when examined at excitation wavelengths at or below 307 nm, emission spectra contained peaks at 343 and 447 nm; when excited above 307 nm, an additional maximum appeared at 355 nm and the peak of the dihydronicotinamide emission band shifted from 447 to 440 nm. Both conformers are probably detected at the longer wavelengths since the emission peak at 343 nm was retained. Identical changes occurred in the emission spectra of NFD+, however, the dihydronicotinamide emission between 440 and 447 nm was absent. Several mechanisms which may account for the presence of these conformers have been considered. The choice has been narrowed to conformations with ring‐ring interactions of the formycin and nicotinamide moieties resulting from (a) formycin tautomerization or (b) heterogeneity of glycosidic bond angles in the structures. The efficiency of intramolecular energy transfer from the formycin to the dihydronicotinamide moiety for free NFDH in aqueous solution was 84% and declined slightly (to 77%) when measured in 1,2‐propanediol. NFD+has coenzyme activity for NAD‐specific isocitrate dehydrogenase (Plautet al., 1979). The emission spectrum of enzyme bound NFDH was altered markedly in the presence of manganese isocitrate; emission intensity at 343 and 355 nm decreased while the emission from the dihydronicotinamide ring at 433 nm increased, when NFDH was excited at 310 nm. This shift in emission intensity was indicative of an increase in energy transfer within the NFDH molecule, caused by a change in coenzyme conformation upon binding to the enzyme‐substrat

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1981.tb04291.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

Abstract—In an attempt to study the quenching of the triplet state of acetophenone by indole, we have prepared the compounds containing these chromophores intramolecularly. The emission measurements in rigid glasses at 77 K have indicated that the quenching of the triplet acetophenone is due to intramolecular triplet‐triplet energy transfer to the indole chromophore, resulting in the sensitization of the indole phosphorescence. The efficiency of the energy transfer has reached ca. 100% in ethanol glasses, while it has been suggested that in methylcyclohexane glasses, the indole chromophore except for 1‐methyl derivative is subjected to strong interaction with the acetophenone chromophore other than electronic energy tra

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1981.tb04292.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

Abstract—The fluorescence decay kinetics of the reduced nicotinamides NMNH, NADH and NADPH in aqueous solution were investigated using an Ar ion laser, mode locked in the UV, as source of excitation and single photon counting electronics in the detection system allowing for a time resolution in the picosecond range. Analysis of the experimental fluorescence decay showed that the dinucleotides did not follow a single exponential decay law. Good fitting was accomplished with a sum of two exponentials. The mononucleotide fluorescence decay was a single exponential for at least 95% of its amplitude.The heterogeneity in lifetimes of the fluorescence of the dinucleotides was interpreted in terms of an exciplex mechanis

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1981.tb04293.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

Abstract—Quenching of the excited states of lumiflavin and 3‐methyl‐5‐deazalumiflavin by methyl‐and methoxy‐substituted benzenes and naphthalenes in methanol was investigated. The observed difference in the reactivity of acid and neutral lumiflavin triplets is explained thermodynamically by applying the Michaelis cycle, as being due to the higher reduction potential of the acid triplet. In this connection the pKvalues of lumiflavin triplet (pKM= 6.5) and semiquinone (pKM= 11.3) have also been determined in methanol. The difference in the reactivity between the singlet and triplet states of lumiflavin is found to be greater as predicted by the difference in excitation energy. The reactivities of the excited states of flavin and 5‐deazaflavin differ only slightly in contrast to the marked difference in the ground state reactivities of electron transfer reactions. This is explained in terms of the model of Rehm and Weller. The pH dependence of the electron transfer quenching of 5‐deazaflavin triplet was investigated in water, yielding a triplet pKof 2.5. In contrast to the flavin, this triplet pKdoes not significantly differ from the pKof the 5‐deazaflavin ground state. From this, different sites of protonation are deduced for the photoexcited triplet states of flavin

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1981.tb04294.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

Abstract—Reductone (HOCH2COCHO), a keto‐aldehyde produced by thermal degradation of some sugars, at alkaline pHs, blocks the excision repair of DNA lesions in uv‐irradiated wild typeEscherichia coli.This probably occurs as a result of inhibition of the exonucleolytic activity of DNA polymerase I. In addition, reductone alone induces DNA single‐strand breaks. Repair of this damage is mainly dependent on thepolAgene p

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1981.tb04295.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

Abstract—DNA and RNA syntheses were inhibited immediately after proflavine treated HeLa cells were irradiated with visible light (400–500 nm). The molecular mechanism for this photooxidation may be either a free radical‐mediated (Type I) or singlet oxygen‐mediated (Type II) reaction. Non‐toxic free radical and singlet oxygen quenchers were added to cells and sensitizer before irradiation to quench the appropriate excited state intermediate. Photooxidative damage (the inhibition of incorporation of [14C]‐thymidine) in this system was greatly reduced in the presence of free radical quenchers (glutathione, penicillamine) and not significantly affected by the presence of singlet oxygen quenchers (α‐tocopherol, β‐carotene, DABCO). This suggests that at least part of the photodynamic damage in HeLa cells is via a

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1981.tb04296.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

Abstract—HeLa cell suspensions, prelabeled with specific [14C]‐nucleosides, were treated with proflavine and irradiated with visible light (400–500 nm). The DNA was isolated from the cells (as well as from the appropriate control cells) and examined for macromolecular and molecular changes. Although the UV absorbance spectrum of DNA from irradiated HeLa cells showed no discernible change, a fluorescence spectrum (excitation/emission: 305/405) indicated a molecular change in the DNA. Isolated DNA samples were hydrolyzed with 90% formic acid and chromatographed. There were no detectable differences between the irradiated and non‐irradiated profile (Rfand radioactivity) for both guanine and adenine. However, the chromatograms of thymine and cytosine showed distinct changes. There was a loss of radioactivity in the [14C]‐thymidine labeled samples, while the [14C]‐cytidine labeled samples indicated the formation of a new compound, containing 10% of the radioactivity, running just ahead of cytosine. These data strongly suggest the formation of a new compound resulting from the photooxidation of cytosine when nuclear DNA was sensitized by

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1981.tb04297.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY