共查询到20条相似文献,搜索用时 15 毫秒
1.
2.
Abstract— Laser flash photolysis of subtilisin BPN'at 265 nm has shown that photoionization of tryptophanyl (Trp) and tyrosinyl (Tyr) residues are the principal initial photochemical reactions. The initial products are the corresponding oxidized radicals. Trp and Tyr, and hydrated electrons (eaq ) which react with the enzyme at: k (eaq + subt. BPN') = 2.1 × 1010 M−1 s−1 . The photoionization quantum yield was 0.032 ± 0.005 at 265 nm, which was enhanced 3.5-fold by simultaneous excitation at 265 and 530 nm. The photoionization yields were unchanged by 3 M bromide ion and 8 M urea. which did affect the enzyme fluorescence excited at 265 and 295 nm. A similar lack of correlation between the effects of perturbants on the photionization yields and fluorescence yields was found for subtilisin Carlsherg. The results indicate that the monophotonic and biphotonic ionization of the Trp residues does not involve the thermally-equilibrated. lowest excited singlet state and that singlet energy transfer from Tyr to Trp does not contribute to Trp photoionization. The photoinactivation quantum yield was 0.014 for 265 nm laser excitation. which was not changed by simultaneous 530 nm excitation. The corresponding quantum yield was 0.009 for low intensity 254 nm radiation, indicative of a biphotonic contribution to photoinactivation. The results are explained by postulating that photolysis of Trp-113 leads to disruption of hydrogen bonding to Asn-117 and a shift in the primary chain sequence associated with the aromatic substrate binding sites. The photoionization quantum yields in subtilisin BPN'and subtilisin Carlsberg agree with a model based on the assumption that exposed Trp and Tyr residues contribute independently at intrinsic photoionization efficiencies characteristic of the chromophores. 相似文献
3.
Edward J. Land Suppiah Navaratnam† Barry J. Parsons Glyn O. Phillips 《Photochemistry and photobiology》1982,35(5):637-642
Abstract— The spectra have been measured of the transient species formed in the nanosecond flash photolysis of aqueous solutions of sulphacetamide under a variety of conditions. In addition to the excited triplet state, the cation radical and the solvated electron were observed. The ionisation of aqueous sulphacetamide was found to occur by a biphotonic process. The extinction coefficient of the cation radical of sulphacetamide was determined by both laser flash photolysis and pulse radiolysis techniques, a value of 1.9 times 103 dm3 mol-1 cm-1 being obtained. The rate of electron reaction with sulphacetamide and the anion radical spectrum were also determined by the two techniques, good agreement being obtained. The spectrum of the product of the reaction of the superoxide anion radical and the corresponding rate constant have also been determined. A possible mechanism of photosensitized skin reaction due to sulphacetamide is discussed. 相似文献
4.
H. I. Hamoudi P. F. Heelis R. A. Jones S. Navaratnam B. J. Parsons G. O. Phillips M. J. Vandenburg W. J. C. Currie 《Photochemistry and photobiology》1984,40(1):35-39
Abstract Laser flash photolysis and pulse radiolysis have been carried out on the diuretic drug amiloride. The primary photochemical processes in aqueous solution were found to be photoionisation and triplet formation. Photoionisation was found to occur by a biphotonic process for 265 nm excitation but by a monophotonic process for excitation at 353 nm. The spectral properties of the resulting cation radical of amiloride were determined by pulse radiolysis using one electron oxidation by the radical anion Br2 ·¯ Photoexcitation of amiloride in isopropanol did not result in photoionisation but instead a semireduced radical of amiloride was observed. The spectral properties of the semireduced radical of amiloride were determined using one electron reduction by the CO2 ·¯ radical. 相似文献
5.
Abstract— The transient absorption spectra of aqueous solutions of eosin and of the lysozyme-eosin complex have been examined after excitation with a Q -switched frequency-doubled (347 nm) ruby laser pulse. Eosin itself gives three broad, intense short-lived absorption bands with maxima at wavelengths of 410, 460 and 580 nm, which other workers have identified with the semi-reduced and semi-oxidized radicals and the triplet state of the dye, respectively. In the complex with lysozyme, the yield and lifetime of the eosin triplet are greatly reduced in comparison with the free dye. It is suggested that excited eosin, when bound to lysozyme, decays mainly from the singlet state by pathways such as charge transfer which are not available to the free molecule. 相似文献
6.
Abstract. Nanosecond flash photolysis of rhodopsin with 530 or 353 nm light produces an initial transient absorption spectrum with peaks at ˜57O and ˜420nm, and a subsequent transient species with a maximum absorption at 480 nm. These results are interpreted as the initial formation of prelumi-rhodopsin (570 nm) followed by its conversion to lumirhodopsin (470 nm). The peak at 420 nm in the first transient may be due to either hypsorhodopsin or isorhodopsin. 相似文献
7.
Abstract— Laser flash photolysis studies of DNA-complexed ethidium bromide were undertaken. We have observed a singlet-singlet (S1 -Sn ) absorption process for DNA-complexed ethidium bromide. The observed lowest singlet excited state lifetime was 21 ± 2 ns. The molar difference extinction coefficient was measured to be 2.4 ± 0.4 × 103 M-1 cm-1 at 370 nm. The assignment of this transition was confirmed by time resolved fluorescence measurements. 相似文献
8.
Abstract— The influence of nucleotides or polynucleotides on the photophysics and the photochemistry of tryptophan (Trp) derivatives has been investigated in aqueous solutions using the 265 nm laser flash photolysis technique. In solutions containing mixtures of N -acetyltryptophanamide and uridine monophosphate (UMP) or mercurated dUMP, the Trp triplet and the hydrated electron (eaq ) are quenched at almost diffusion controlled rates by the nucleotides leading to uracil reduction. Lysyl-tryptophyl-α-lysine (Lys-Trp-Lys) forms stable complexes in solution with normal or mercurated poly(uridylic acid) [poly(U)]. In the Poly(rU)-Lys-Trp-Lys complex the Trp triplet state is completely quenched, whereas the Trp triplet formation quantum yield is enhanced in complexes with mercurated poly(U). In this last case, the 'heavy atom effect' is characterized by a shortening of the Trp triplet lifetime in agreement with low temperature experiments. Our results also show that photoionization of Trp does occur in the complexed state with both polymers. The eaq lifetime is however longer with the complexed than with the free peptide. 相似文献
9.
P. Morlière O. Avice † T. Sa E Melo‡ L. Dubertret M. Giraud R. Santus ‡ 《Photochemistry and photobiology》1982,36(4):395-399
The E ⇄ Z photoisomerization of 4'-methoxycinnamates, used as sunscreens in cosmetics, has been studied by steady state and laser flash photolysis, in aqueous and organic solutions. Photoisomerization quantum yields are found to be fairly high (˜0.5-1), although no intermediate is detected upon laser flash photolysis. Cinnamates are not photodynamic sensitizers but are able to quench the 8-methoxy-psoralen and 5-methoxypsoralen triplets which produces E → Z isomerization. 相似文献
10.
Suppiah Navaratnam Edward J. Land Barry J. Parsons Iqbal Ahmad Glyn O. Phillips 《Photochemistry and photobiology》1983,38(2):153-159
Abstract— Transient absorption spectra produced by laser flash-photolysis of an aqueous solution of ephedrine have been measured under a variety of conditions. Ephedrine was found to photoionise via a biphotonic process. The apparent yield of photoionisation increases with lowering of pH, a value of 8.8 being found for the p K a associated with this change. The cation radical absorption spectrum has been determined using the techniques of both pulse radiolysis and laser flash photolysis. The extinction coefficient of the cation at 295 nm was determined to be 1.37 × 104 dm3 mol-1 cm-1 and 1.2 × 104 dm3 mol-1 cm-1 by the two techniques, respectively, at pH 11. It is also shown that the rate constant for electron abstraction by the azide radical to form the ephedrine cation is controlled by protonation of the amine group in the side chain. The ephedrine anion radical spectrum and its extinction coefficient at 305 nm were also determined. The excited states responsible for photoionisation and photodegradation are discussed. 相似文献
11.
Abstract— The physical and chemical properties of the triplet state of eight ortho-substituted anilides including N -formylkynurenine (FK), the major trp UV-photooxidation product and a remarkable photodynamic agent, have been investigated using both pulse radiolysis and 265 nm laser flash photolysis techniques. The molar extinction coefficient, the intersystem-crossing quantum yield and the oscillator strength of the T 1 → T n absorption band (Λmax ˜ 450 nm) have been determined. It is shown that anilides having n,π* triplets readily react with most solvents whereas those having π ,π* triplets slowly react with alcohols. In both cases, the semi-reduced species are formed. In water, the formation of the semi-reduced. species most probably involves the first excited singlet state. The triplet state properties of the FK derivatives (i.e. ortho-substituted anilides having a side chain bearing charged groups such as carboxylic or amino groups) are strongly modified by the ionization state of the charged side chain. In the case of the FK derivatives possessing an uncharged amino group, quenching of the triplet state occurs via a fast reversible electron transfer reaction from the NH2 to the triplet anilide. 相似文献
12.
Craig Pepmiller Eleanor Bedwell Rlobert R. Kuntz Camillo A. Ghiron† 《Photochemistry and photobiology》1983,38(3):273-280
Abstract— The conventional flash photolysis of 1-methylindole in aqueous media was studied at Λexcitation ≥290 nm. The transients observed 20 μs after excitation consisted mainly of the radical cation (R+ ). the hydrated electron (e- aq ) and the triplet state (T). Electron counting experiments indicate that photoionization is the only source of R+ with e- aq /R+ = 1.07±0.09 in neutral media. Quenching of the R+ yield with H+ indicates that the fluorescent state is the precursor to 80% of the photoionization events with the remainder probably arising from a prefluorescent state. The triplet decays with a lifetime of 29 μs in deaerated neutral media. This decay is unchanged by N2 O saturation, but T reacts with acrylamide with k ≥2.8 × 109 M -1 . In 2 M Br- , R+ and T yields are increased by factors of 2–3. Consideration of fluorescence quenching and T enhancement by Br-permits an estimate of φIsc between 0.33 and 0.49. The increased R+ yield at high Br-concentrations cannot be accounted for by induced photoionization or triplet state reactions. 相似文献
13.
PHOTOSENSITIZATION OF PYRIMIDINES BY 2-METHYLNAPHTHOQUINONE IN WATER: A LASER FLASH PHOTOLYSIS STUDY 总被引:3,自引:0,他引:3
2-Methylnaphthoquinone (MQ) has been excited in water with a 20 ns laser flash at 353 nm and the resultant transient species have been observed optically. Triplet-state MQ (3 MQ) decays on a sub-microsecond time scale. It has been characterized in terms of its absorption spectrum and quantum yield. Rate constants have been measured for the decay of 3 MQ in infinitely dilute solution, for self-quenching by ground-state MQ, and for reactions of 3 MQ with oxygen, thymine, uracil, 6-methyluracil, and orotic acid. The interaction of 3 MQ with pyrimidines involves charge transfer to give the pyrimidine cation radical and the MQ : anion radical. These reactions are discussed in relation to the mechanism of pyrimidine photooxidation sensitized by MQ. 相似文献
14.
Abstract— Employing nanosecond laser flash photolysis, β-ionone (BI) has been examined as an acceptor and as a donor of triplet excitation. In the limit of diffusion-control as well as below it, the rate constants for the quenching of a series of sensitizer triplets by BI are2–3 times smaller than those by 2,4-hexadienal (HD), although the triplet energies (spectroscopic) of the two carbonyl-containing dienes are estimated to be the same (∼55 kcal mol-1 ). We attribute the difference to a steric effect arising from ground-state geometric distortion and heavy alkyl-substitution in BI. In spite of possible exothermic energy transfer, BI triplet is nearly nonquenchable by azulene and ferrocene; this is explainable by torsional relaxation to an equilibrium geometry at which the vertical energy gap is smaller than 40 kcal mol-1 . The singlet oxygen yield from the interaction of BI triplet with oxygen in benzene is estimated to be 0.5, suggesting that spin-exchange and energy-transfer may be involved to the same extent in the oxygen quenching process. 相似文献
15.
Abstract— The microsecond flash photolysis of 5-methoxyindole in aqueous solutions has been studied at γexc ≥ 290 nm. Transients identified in this time realm in neutral solutions are: eaq - , the 5-methoxyindole radical cation (γmax ≅ 440 nm), the neutral transient with γmax ≅ 530 nm) and an unidentified oxygen sensitive transient with γmax ≅ 435 nm. Radical cations and e- aq are shown to be produced in equal amounts consistent with a photoionization process as the only source of both transients. H+ quenching of fluorescence and radical cation production gives equivalent Stern-Volmer constants indicating that photoionization occurs from the fluorescent state. The unidentified oxygen sensitive transient exhibits a pK a of2–2.5 and is quenched at lower pH values indicating that it also has a fluorescent state precursor. 相似文献
16.
C. M. Previtali 《Photochemistry and photobiology》1984,40(6):689-692
The laser flash photolysis of indole at 265 nm in the presence of glycine, proline and hydroxy proline was studied. The relative yields of c aq , triplet state, and indole cation radical were determined in the absence and in the presence of the amino acids. The yields were determined as a function of laser intensity and the values at very low intensity were compared with the fluorescence quenching results. It was concluded that in these conditions the photoionization of indole occurs via the fluorescent state. From the curves of triplet yield vs laser intensity, the triplet quantum yield extrapolated at low laser intensity was obtained, φr = 0.55 φ 0.05, relative to the literature value of 0.15 for φeag . This gives φF +φeaq = 1.0 ± 0.1 at room temperature. When proline and hydroxy proline were used as singlet quenchers, the yield of In was greater than the yield of caq . This was considered as evidence that a fraction of the quenching processes leads to complete electron transfer from indole to the amino acids. 相似文献
17.
Abstract— Phytochrome control of nitrate reductase activity has been studied in cotyledons and hypocotyls of light-grown Sinapis alba. Under polychromatic irradiation, an increase in the fluence rate of far-red light added to a constant source of photosynthetically active radiation causes a decrease in the phytochrome photoequilibrium and, in the hypocotyl, this results in decreased nitrate reductase activity. However, in the cotyledons this difference is only observed transiently. In both organs, enzyme activity is correlated with the level of the far-red light absorbing form of phytochrome, Pfr . These correlations are not altered when the fluence rate (with respect to phytochrome) is increased, suggesting that the responses are not fluence rate dependent. The results obtained are consistent with the notion that in fully de-etiolated seedlings, Pft alone controls nitrate reductase activity. 相似文献
18.
Abstract— Triplet-and singlet-related photoprocesses of pyrene-1-aldehyde (PA) in various solvents have been investigated in detail using 337.1 and 355 nm laser flash photolysis in conjunction with time-correlated determination of fluorescence lifetimes (τF ) and steady-state photochemical and absorption-emission spectral measurements. In benzene, the lowest triplet of PA (43 < ET < 46 kcal/mol) has a lifetime of about 50 µs (τT ) and displays the absorption maximum at 443 nm with a maximum extinction coefficient (εmax ) of 21000 M -1 cm-1 ; the corresponding ketyl radical has a sharp absorption maximum at 428 nm (εmax ≥ 25000 M -1 cm-1 ). The quantum yields (φT ) of lowest triplet occupation are high in nonprotic solvents (0.6–0.8), decrease in protic solvents (alcohols) as the polarity of the latter is increased, and maintain a complementary relationship with the quantum yields (φF ) of fluorescence. Quantum yields (φPC ) of loss of PA due to photoreactions in some solvents have also been determined under conditions of steady irradiation at 366 nm; φPC is in the range 0.1–0.2 in electron-rich olefinic solvents such as cyclohexene and tetramethylethylene. These results concerning τF , τT , φF . φT and φPC as well as the effects of 1,2,4-trimethoxybenzene and 2,5-dimethyl-2,4-hexadiene as quenchers for fluorescence, triplet yield, and photochemistry are discussed in the light of possible state orders for PA in polar and nonpolar environments. 相似文献
19.
Abstract— Sensitization and quenching experiments demonstrate that laser flash photolysis ofpyridoxal–5'-phosphate populates a triplet state about 244 kJ/mol. Excitation by the 337.1-nm photons from a nitrogen laser generated transient absorption in the 400-nm region which decayed with a fast first order component on a much slower one. Xanthone photosensitized the same absorption; oxygen, nitric oxide, biacetyl, and 1,3-cyclo-hexadiene quenched it. Spectra and lifetimes suggest that the triplet state leads to radical formation. 相似文献
20.
Abstract— Flash photolysis was used to study the reduction of the triplet state of methylene blue by both alkyl- and aryl-amines. The extent of the formation of the semireduced form of the dye yielded rate constants of interaction between the triplet state and the amine ( k 5 ). A correlation between log k 5 and ionization potentials for alkylamines (slope = -1.75 eV-1 ) was interpreted as evidence for the formation of a partial charge-transfer intermediate. The rate constants ( k 5 ) calculated for aryl-amines approached the rate of diffusion in many cases. A Hammett plot for a series of N, N-dimethyl-anilines yielded a moderately large p value (– 3.28) consistent with the formation of a charge-transfer intermediate. It was concluded that reaction of amines with triplet methylene blue leads to the formation of a partial charge-transfer intermediate which may undergo complete electron transfer to yield radicals, or undergo spin inversion and return to the ground state. 相似文献