摘要:

Abstract.Pulsed laser photolysis at 347nm has been used to study the transient spectroscopy of alloxazine, lumichrome, lumiflavin, and riboflavin in acidic (pH 2.2) aqueous solution and in ethanol. Intersystem crossing quantum yields (φISC) were determined by a modification of the comparative laser excitation method which utilizes the variation of the triplet yield with intensity in conjunction with a kinetic model for the various photophysical and photochemical processes occurring during the pulse. Fluorescence quantum yields and lifetimes are also reported. Correction for quenching of the excited singlet state by H+ions shows that, in neutral aqueous solution, intersystem crossing for flavins is an efficient process (φISC˜ 0.7) which, in conjunction with fluorescence, accounts for the fate of all absorbed photons. For alloxazine (φISC˜ 0.45) and lumichrome (φISC˜ 0.7) the results are more difficult to interpret owing to interconversion betweenalloxazineandisoalloxazinestructures in the singlet excited state. For all four compounds, the quantum yield of products derived from the singlet excited state is estimated as ˜0.04. There is evidence of biphotonic product formation at high laser energies. In ethanol, where φISCfor lumichrome is about twice that of lumiflavin, internal conversion between the excited singlet and ground states appears to be a significant process. Complete triplet‐triplet absorption spectra in the region 260–750nm are reported. For lumichrome at pH 2.2 there is spectral evidence for isomeric triplet states which appear to be in

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1977.tb07495.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

Abstract.Using the CNDO/S method, we have studied the lowest singlet ← singlet and triplet ← singlet transition energies of phenazine and its two conjugate acids. The spectra of the conjugate acids are not well reproduced by the calculations unless a water molecule is introduced to hydrogen bond to each

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1977.tb07496.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

Abstract.5,5‘‐Diuridinyl and 5,5’‐dideoxyuridinyl have been isolated from UV irradiation (254nm) in a frozen state of 5‐bromouridine and 5‐bromodeoxyuridine, respectively. These compounds were characterized by UV, IR, NMR, and mass spectra. They are formed in relatively high quantum yields which are comparable to thymidine dimerization and were found to be unaffected upon further irradiation. This photostability suggests that these 5,5‘‐coupled products, if formed, may accumulate in biological systems. Experiments carried out in “puddles” indicate, by negative inference, that the main reaction pathway is probably the recombination of two radicals to yield the coupled products in a frozen state. Therefore, the coupled products formation should be explored as a possible mechanism for the increased radiation and photosensitivity of biological systems when thymine in DNA is replaced with 5‐halouracil. In addition, these compounds may be synthesized chemically, though, in

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1977.tb07497.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

Abstract.The photochemical reaction of mercuric sulfide was studied in aqueous solutions. The photo‐solubilization of mercuric sulfide occurred under irradiation with light of ≤650nm and was enhanced in the presence of oxygen and electrolytes. The resultant mercuric ions were easily methylated in the presence of acetate ions via the photosensitized reaction by the simultaneously produced “photo‐sulfur” under irradiation with light of ≤420nm. The photoelectrochemical studies of mercuric sulfide electrode revealed that black mercuric sulfide was ann‐type semiconductor and the photo‐current due to the dissolution as mercuric ions was observed on anodic polarization. The photo‐solubilization of mercuric sulfide may be interpreted in terms of the loc

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1977.tb07498.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

Abstract.The present communication is concerned with the effects of near‐UV radiation (300–380 nm) on yeastCandida guilliermondii.It was found that certain doses of 313 nm irradiation caused inactivation of the yeast which was exhibited in a way different from the lethal action of far‐UV radiation. It was also found that the cells inactivated by 313 nm are capable of recovering vitality, if incubated for some time in a non‐nutrient medium. The yeast inactivated by far‐UV radiation also proved to be capable of recovering, though to a lesser degree. Both 334 nm radiation and non‐lethal doses at 313 nm induced the photoprotective effect against far‐UV damage. The effect was exhibited if there was a certain time interval (2–4 h) between the exposures to photoprotective light and subsequent far‐UV radiation. Within this time interval the extent of photoprotection was depende

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1977.tb07499.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

Abstract.Thiols and disulfides protect both φX174 phage and its isolated DNA from the lethal action of proflavine plus light. The protective ability of these compounds appears to be attributed to the ‐SH or the ‐S‐S‐ group and the property to interact with the proflavine‐phage DNA complex. The phage inactivation efficiency per proflavine bound to DNA is reduced by 50 to 30% upon addition of cysteine or cystamine. Substances that affect the lifetime of singlet oxygen modify the rate of phage photoinactivation in the presence of proflavine; the inactivation rate is decreased by N‐3and increased by D2O. Irradiation under N2atmosphere markedly decreases the phage photosensitization by proflavine. Irradiation with monochromatic light of 440 nm is less efficient than irradiation with light of 440 nm plus 360 nm, and the difference is more pronounced in N2than in air. These results are discussed in relation to various possible photochemi

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1977.tb07500.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

Abstract.Both S‐ and G2‐phase cells of amoeba are found to be sensitized by coirradiation of cells with Tryptophan (Trp). At a fluence of 1.5 times 105Jm‐2to which G2phase cells are resistant, they show a drastic (8‐fold) increase in sensitivity when irradiated in 0.5% Trp solution. Freshly irradiated Trp solution is found to be lethal to G2cells that have received a sublethal fluence of near‐UV light. Irradiated Trp, however, does not kill unirradiated amoeba.Nuclear transplantation experiments have shown that the lethality of cells when coirradiated with Trp is due to damage located predominantly in the cytoplasm. It is suggested that sublethal fluence of near‐UV light makes the cells “leaky” to the toxic photooxidation products

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1977.tb07501.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

Abstract.The ability of UV‐irradiated African green monkey kidney cells (CV‐1) to support the growth of unirradiated herpes simplex virus type 1 as measured by plaque forming ability has been investigated. The lowering of plaque formation by the virus when the host cell was irradiated was examined at thirteen different wavelengths. An action spectrum for this cellular parameter (capacity) was obtained in the wavelength region of 235–302 nm. This action spectrum points to nucleic acid as the critical target molecule for this e

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1977.tb07502.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

Abstract.Previous work showed that the bacteriumMicrococcus roseusis killed by photodynamic action when an exogenous photosensitizer is present, but when a photosensitizer is not added the organism survives long exposure to high intensity (22,000ft‐c, 348 J/s/m2) white light. Experiments designed to demonstrate the presence of a mechanism to repair damage caused by photodynamic action failed to reveal such a mechanism. However, the organism was killed by light of a very high intensity (32,000ft‐c, 506 J/s/m2) in the absence of added photosensitizer, indicating that cells have an effective endogenous photosensitizer(s). Two carotenoid‐deficient mutants were killed via photodynamic action more rapidly than the fully pigmented wild‐type in the presence or absence of an exogenous photosensitizer. Thus, resistance ofM. roseusto photodynamic action is not due to a repair mechanism, nor to lack of an effective endogenous photosensitizer, but to the protective action of carotenoid p

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1977.tb07503.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

Abstract.Photosynthetic reduction of nitrite to ammonia with type C chloroplasts from the heterocont algaBumilleriopsis filiformiswas investigated using 3,6‐diaminodurene/ascorbate and 3,6‐diaminodurene/dithioerythritol (DAD/DTE) as electron donor couple. Rates approach 6–10 μmol NO‐2reduced/mg chlorophyll/h and are steady for up to 30 min. The presence of oxygen or NADP+only slightly diminished the rates of nitrite reduction obtained with DAD/DTE. Illuminated chloroplasts reduce oxygen in the presence of DAD/DTE at 135 μmol/mg chlorophyll/h without acceptor supplied. Photosynthetic oxygen uptake by this system in the presence of ferredoxin and NO‐2, however, is inhibited to 42% by nitrite reductase with concurrent nitrite reduction. NO‐3and NO‐2have no effect on photosystem I‐mediated NADP+reduction, NO‐2(10 mM) inhibits ferricyanide‐mediated oxygen evolution to 72%. Also photosystem II reactions assayed e.g. with silicomolybdate are inhibited significantly by NO‐2(1 mM), but only slightly by NO‐3. Nitrite reductase is inhibited byp‐chloromercuribenzoate (pCMB), and this inhibition is prevented by DTE. Results suggest that photosynthetic nitrite reduction can cope with low concentrations of either compound, provided relevant

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1977.tb07504.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY