摘要:

Abstract—Kinetics of photoreactivation were studied inE. coliWP2 hcr‐, a strain deficient in dark repair. Cells in aqueous suspension were subjected to u.v.‐irradiation, then exposed to photoreactivating light for different periods. Survival curves, with samples at a minimum of six u.v. doses, were obtained at several periods of photoreactivation ranging from zero to maximum. The surviving fractions do not conform to a dose‐reduction model, but instead, they fit a ‘sequential repair’ model that assumes as a limiting condition that the number of active enzyme molecules is small. The model used assumed: (1) a single enzyme molecule is active at any one time; and (2) inactivating events are nullified consecutively around the DNA molecule. The mathematics of the model are derived and presented. Photoreactivation is attributed to the action of two processes. (1) A photochemical process, that is rate limiting below 1000 ergs mnr‐2sec‐1, was measured at a photoreactivating irradiance of 60 ergs mm‐2sec‐1. This has a rate constant of 5 × 10‐5‘events’ erg‐1mm2. (2) A dark process, measured at photoreactivating irradiances of 4000 and 6000 ergs mm‐2sec‐1, has a rat

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1970.tb06010.x

出版商:Blackwell Publishing Ltd    

年代:1970

数据来源: WILEY

摘要:

Abstract—The values of the absolute quantum yields of fluorescence and of intersystem crossing have been obtained for porphin and its derivatives by methods based on relative measurements involving flash photolysis. For a given compound the sum of these quantum yields is unity within experimental error which shows that the energetics of the molecules investigated is determined by competition of only two processes, viz., fluorescence and intersystem crossing, while internal conversion plays a negligible role. The values of the corresponding transition probabilities have been determined with the help of phase‐fluorometric measurements. The dependence of the transition probabilities on molecular structure is discus

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1970.tb06011.x

出版商:Blackwell Publishing Ltd    

年代:1970

数据来源: WILEY

摘要:

Abstract—A study of the delayed luminescence and thermoluminescence of fluorescein and others dyes in boric acid glass gives evidence of photoionisation of these compounds. To account for the energetic aspect of the photoionisation, a two photon process involving solute‐solvent proton exchange is proposed. The participation of the heterogeneous structure of the glass in the mechanism of the photoionisation is confir

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1970.tb06012.x

出版商:Blackwell Publishing Ltd    

年代:1970

数据来源: WILEY

摘要:

Abstract—A mechanistic model is proposed which describes the phototactic behavior ofEuglenaduring accumulation in an illuminated region. Measurements of the lag time occurring between illumination of the culture and net accumulation in the lighted zone as a function of culture density indicate that the relative strengths of the negative phototactic response inside and of the positive phototactic response outside this region are the prime factors controlling the lag phenomenon. Further evidence for this is provided by studies of the temperature dependence of the phototactic responses to polarized actinic light. It is shown that negative phototaxis as measured in the 7lsquo;phototaxigraph’ is not directed, but rather a shock‐mediated response. A ‘FOCAL’ computer program for simulation of experiments in the phototaxigraph has been written on the basis of our model. It correctly predicts the observed results under a variety of simulated experimental conditions. Measurements of the lag time and of the rate of accumulation in different parts of the actinic zone allow the calculation of motilities of the organisms with illumination and in the dark, the latter value being 0.08 mm/sec. For a 2–3‐weekold culture, the rate of negative phototaxis remained constant at light intensities above 40 ergs/cm2sec at 500 nm. At this wavelength, the threshold for the positive photophobic response was 10

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1970.tb06013.x

出版商:Blackwell Publishing Ltd    

年代:1970

数据来源: WILEY

摘要:

Abstract—An investigation was made of both the composition of and mechanism of photo‐protection by the carotenoid pigments ofSarcina luteaATCC 9341a and three induced mutants.The wild‐type and mutants 2a and 4b were each found to contain three major pigment fractions, each fraction consisting of a single pigment having identical absorption maxima but differing from each other in chromatographic mobility. Although the mutants contain the same kinds of pigments as does the wild‐type, the mutant cells contain less pigment per cell than does the wild‐type. The third mutant, 93a, contains no colored carotenoids.It was found that there were changes in both the absolute and relative amounts of the various pigment fractions when cultures of wild‐type, mutants 2a and 4b, grown in nutrient broth in the dark, were examined during the logarithmic and stationary phases of the growth curve. In addition, changes were observed in the pigments when the cells were exposed to light in buffer. These changes were similar in the wild‐type and in mutant 2a, but were quite different in mutant 4b. Studies of photokilling curves suggested that these changes in amounts of the various pigment fractions were not related to photoprotection, but that the important factor may be the total amount of pig

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1970.tb06014.x

出版商:Blackwell Publishing Ltd    

年代:1970

数据来源: WILEY

摘要:

Abstract—We have investigated the ultraviolet photochemistry of thymine in aqueous solution. Four isomeric dimers are produced, and the yield of each has been measured as a function of thymine concentration, oxygen concentration, and temperature. At low thymine concentration, dimerization proceedsviathe triplet state, while at high concentration it arises mainly from aggregates, probably from a singlet precursor. We have determined the ratios of rate constants for the triplet state mechanism and the quantum yield for dimerization from aggregates. The quantum yield for dimerization from the triplet state in thymine is smaller by a factor of about 10 than that in uracil, which in turn is smaller by another factor of 10 than that in orotic acid. It increases with the energy of the exciting radiation in a manner similar to the behaviour of uracil and orotic acid. On the other hand, dimer formation from aggregates is independent of photon energy. Dimerization from aggregates decreases with increasing temperature, while the total production of dimers from the triplet state is independent of temperatur

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1970.tb06015.x

出版商:Blackwell Publishing Ltd    

年代:1970

数据来源: WILEY

摘要:

Abstract—The quenching of the fluorescence of flavines by indoles in aqueous solution has been studied as a function of concentration, viscosity and temperature. The quenching shows a negative temperature dependence, and is in part diffusion controlled. Static and dynamic processes both contribute to quenching.In the presence of alcohol or denaturants static quenching is inhibited and dynamic quenching supervenes. The effect of indoles on the flavine fluorescence lifetimes and phosphorescence characteristics is consistent with the quenching mechanism propose

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1970.tb06016.x

出版商:Blackwell Publishing Ltd    

年代:1970

数据来源: WILEY

摘要:

Abstract—Using isolated chloroplasts and techniques as described by Joliot and Joliot[6] we studied the evolution of O2in weak light and light flashes to analyze the interactions between light induced O2precursors and their decay in darkness. The following observations and conclusions are reported: 1. Light flashes always produce the same number of oxidizing equivalents either as precursor or as O2. 2. The number of unstable precursor equivalents present during steady state photosynthesis is ∼ 1.2 per photochemical trapping center. 3. The cooperation of the four photochemically formed oxidizing equivalents occurs essentially in the individual reaction centers and the final O2evolution step is a one quantum process. 4. The data are compatible with a linear four step mechanism in which a trapping center, or an associated catalyst, (S) successively accumulates four + charges. TheS4+state produces O2and returns to the ground stateS0. 5. BesidesS0also the first oxidized stateS+is stable in the dark, the two higher states, S2+and S3+are not. 6. The relaxation times of some of the photooxidation steps were estimated. The fastest reaction, presumablyS*1←S2, has a (first) half time ≤ 200 μsec. TheS*2state and probably also theS*0state are processed somewhat more slowly (˜ 300

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1970.tb06017.x

出版商:Blackwell Publishing Ltd    

年代:1970

数据来源: WILEY

摘要:

Abstract—The cytosine‐thymine precursor of the U–T adduct is not subject to enzymatic photoreactivation, but can be eliminated directly from u.v.‐irradiatedE. coliDNA by exposure to wavelengths around

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1970.tb06018.x

出版商:Blackwell Publishing Ltd    

年代:1970

数据来源: WILEY

摘要:

Abstract—In stationary‐phaseEscherichia coliB/r, photoreactivation (PR) at 313 nm of ultraviolet (u.v.) killing is inefficient compared with PR at 405 nm, and can be explained solely by photoenzymatic reversal of pyrimidine dimers.InStaphylococcus epidermidis, PR shows a maximum at 313 nm, suggesting that this organism shows the Type III PR proposed by Jaggeret al.[5] forStreptomycesstrains. Reversal of pyrimidine dimers is not sufficient to explain this PR. The mechanism of Type III PR remains unknown.With bothS. epidermidisandE. coliB/r, the amount of uracil–thymine heteroadduct in DNA hydrolysates decreases if the cells are given a post‐u.v. treatment at 313 nm, but no decrease is observed if the post‐u.v. treatment is at 405 nm. The biological significance of this adduct and of its removal is not clear. It may play a role in Ty

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1970.tb06019.x

出版商:Blackwell Publishing Ltd    

年代:1970

数据来源: WILEY