Showing papers on "Flash photolysis published in 1977"

The Photoenolization of 2-Methylacetophenone and Related Compounds

Abstract: A reinvestigation of 2-methylacetophenone (1) by ns flash photolysis has provided detailed evidence for the reaction sequence of photoenolization. The triplet reaction proceeds adiabatically from the lowest excited triplet state of the ketone, 3K (1), to the enol excited triplet state, 3E (1), which decays both to enol and ketone ground state. The Z- and E-isomers of the photoenol, Z-E (1) and E-E (1) are formed in about equal yield by the triplet pathway, while direct enolization from the lowest excited singlet state of 1 yields (predominantly) the Z-isomer. Intramolecular reketonization from Z-E (1) to 1 proceeds at a rate of ca. 108s−1 in cyclohexane, but can be retarded to ca. 104s−1 in hydrogen-bond-acceptor solvents. The proposed mechanism is summarized in Scheme 1 and rationalized on the basis of a state correlation diagram, Scheme 2. 3,3,6,8-Tetramethyl-1-tetralone (2) was used as a reference compound with fixed conformational position of the carbonyl group, and some results from a brief investigation of 2,4-dimethylbenzophenone (3) are also reported.

Photolysis mechanism of aqueous tryptophan

Abstract: Flash photolysis experiments indicate that monophotonic photoionization is the principal initial photolysis act in aqueous tryptophan under high intensity, 265-nm laser excitation, leading to the Trp/sup +/ radical cation (lambda/sub max/ 580 nm) with quantum yield 0.12 +- 0.01 and e/sub aq//sup -/ with quantum yield 0.10 +- 0.01 from pH 4 to 8. The neutral Trp radical (lambda/sub max/ 510 nm) is formed by deprotonation with rate constant 1.5 x 10/sup 6/ s/sup -1/. The disappearance of e/sub aq//sup -/ follows a complex decay inconsistent with homogeneous kinetics. The results are in agreement with a recombination model in which the electron diffuses through the medium as a quasi-free entity, during which period back reactions with the original radical, or electrons and radicals generated in nearby pairs, compete with scavenging by tryptophan or added solutes. The well-established result that Trp yield observed after electron decay is not enhanced by oxygen or other e/sub aq//sup -/ scavengers is attributed to the formation of an unstable intermediate product of the electron-radical back reaction which subsequently oxidizes additional tryptophan to Trp.

The effect of chemical structure on the photosensitizing efficiencies of porphyrins

Abstract: — A kinetic investigation was performed on the photooxidation of methionine sensitized by various porphyrins at different oxygen concentrations. The rate of photooxidation was found to be strongly dependent on the nature of the sensitizer. In the case of hematoporphyrin, chelation of Mg2+ and Zn2+ and especially of Cu2+ and Fe2+ caused a significant decrease of the photosensitizing efficiency. Fluorescence and/or flash photolysis studies showed that such a decrease is ascribed to an enhancement of the non-radiative decay of the first excited singlet state as well as to a reduction of the triplet lifetime. The sensitizing efficiency is also dependent on the nature of the porphyrin side chains. A reaction mechanism involving 1O2 as the oxidizing agent is proposed.

Photosensitized splitting of pyrimidine dimers by indole derivatives and by tryptophan‐containing oligopeptides and proteins

Abstract: — Indole derivatives including tryptophan can be used as photosensitizers of the splitting of pyrimidine dimers. The reaction can take place in frozen aqueous solutions as well as in fluid medium. Electron transfer from the indole ring to the dimer appears to be involved in the photosensitized reaction. Solvated electrons produced by flash photolysis in the presence of indoles or by pulse radiolysis are also able to split thymine dimers. The splitting of pyrimidine dimers in DNA can be photosensitized by indole derivatives such as serotonin and by tryptophan-containing oligopeptides. Several methods including fluorescence and nuclear magnetic resonance have been used to show that the indole ring of these oligopeptides is able to stack with bases in nucleic acids. These stacked complexes are involved in the photosensitized reaction. The splitting of pyrimidine dimers in DNA has also been photosensitized by the protein coded by gene 32 of phage T4 which binds strongly and cooperatively to single-stranded DNA. The mechanism of the splitting reaction as well as the possible use of this reaction to investigate the role of tryptophan residues in the binding of proteins to nucleic acids are discussed.

Singlet → triplet intersystem crossing quantum yields of photosynthetic and related polyenes

Abstract: The extinction coefficients for the triplet–triplet maxima of all-trans-β-carotene, 15,15′-cis-β-carotene, all-trans-lycopene, 7,7′-dihydro-β-carotene, all-trans-retinol, all-trans-retinal, 13-cis-retinal and all-trans-retinylidene-N-butylamine have been determined via pulse-radiolysis using a refinement of the energy transfer technique introduced previously. Together with Wolff and Witt's data, the coefficient found for β-carotene suggests that ∼10 % of the total carotene present in photosynthesising spinach chloroplasts is available as a protective valve against overillumination.These extinction coefficients were employed to estimate, via laser flash photolysis, the corresponding singlet → triplet intersystem crossing quantum yields using 265 and 353 nm excitation. The very low yields ( < 0.001) found for β-carotene and derivatives mean that carotenoid triplets formed in chloroplasts during the course of the valve function could not have been formed directly, but only via triplet–triplet energy transfer sensitation via another triplet (chlorophyllT?), or via1O2 quenching. The low cross-over efficiencies also mean that in their accessory pigment role no carotenoid excited singlets are wasted in crossing over to the triplet state.

Flash photolysis of tryptophan and n‐acetyl‐l‐tryptophanamide; the effect of bromide on transient yields

Abstract: — The initial yields of the cation and neutral radicals and the triplet are greatly enhanced when high concentrations of Br- are present during the flash photolysis of aqueous solutions containing either tryptophan or N-acetyl-L-trytophanamide. The present study is an attempt to elucidate the mechanism by which Br- induces these effects. The results obtained indicate that the initial event involves an interaction between the fluorescent state and Br- to promote the formation of a long-lived radical precursor that may be the triplet. It is shown that all of the Br--induced neutral and cation radical formation originate from this long-lived state. Furthermore, it was found that the mechanism of radical production from the Br--induced long-lived precursor does not involve hydrated electron formation.

Photoreduction of 1,10-phenanthroline

Abstract: The effects of solvent polarity on the photophysical properties of 1,10-phenanthroline indicate the close proximity of (n, π*) and (π, π*) excited singlet states, whilst the lowest triplet state is (π, π*) in all solvents. In hydrocarbon solvent, the first excited singlet state is of (n, π*) character but in water a (π, π*) is situated ∼ 700 cm–1 below this state. Even in water, the (n, π*) state is capable of abstracting a hydrogen atom from the solvent forming an intermediate semidiaza radical. The radical can be detected by flash photolysis and e.s.r. techniques and, in organic solvents, it decays with second order kinetics due to a disproportionation reaction. The major product of reaction is dihydrophenanthroline which, in hydrocarbon solvents, is formed with a quantum efficiency of ∼ 25 %. In aqueous solvent, the yield and rate of decay of the semidiaza radical are pH dependent, and the pK for protonation of the radical is 7.3. Thus, irradiation in water at pH 5 results in formation of the stable diprotonated radical cation.

Flash photolysis of ketene. Photolysis mechanism and rate constants for singlet and triplet methylene

Abstract: The photochemistry of ketene has been studied, using flash photolysis coupled with end product analysis, for wavelengths above 160 nm. The data were analysed numerically, enabling the proposed reaction mechanisms to be assessed quantitatively. The major novel feature is the observation of products normally derived from CH21A1 at high pressures where deactivation to CH23B1 is expected to be complete. It is suggested that these products derive from CH21B1 is expected to be complete. It is suggested that these products derive from CH21B1. The data require that this state is comparatively stable with respect to collisional deactivation, but reacts with H2(k= 2 × 10–11 cm3 molecule–1 s–1) and CH2CO (k= 3 × 10–11 cm3 molecule–1 s–1). These rate constants depend on an assumed radiative lifetime (1/e) for CH21B1 of 10–6 s. An anomalously high yield of acetylene is found at low pressures in the presence of hydrogen. Rate constants for CH21A1 with CH2CO and CH23B1 with C2H2, NO, O2 and CO are reported.

A quantum yield determination of O(1D) production from ozone via laser flash photolysis

Abstract: The quantum yield of electronically excited atomic oxygen produced from ozone photolysis was measured at 298 K from wavelengths of 293.0 to 316.5 nm. The reaction of the atomic oxygen with N2O to form excited NO2 was used to monitor the O production; a frequency-doubled flashlamp-pumped dye laser which provided tunable ultraviolet in the desired spectral region with 0.1-nm linewidth served as the photolysis source. The atomic oxygen quantum yield was found to be constant below 300 nm, with a sharp decrease centered at 308 nm and a diminution to less than one tenth of the constant value by 313.5 nm.

Role of biradical intermediates in the photochemistry of o-methylacetophenone

Abstract: A laser flash photolysis technique was used to examine the behavior of the biradical intermediate in the photoenolization of o-methylacetophenone. The results showed that the transient produced by intramolecular hydrogen abstraction has a lifetime of 300 nsec. It reacts in the electron transfer to paraquat dications in the same manner as the biradicals produced in the Norrish type II reactions. The same biradical is apparently produced from both triplet states as required by Wagner's mechanism. The biradical interacts with oxygen at nearly diffusion controlled rates. (JSR)

Photoredox reactions of thionine

Abstract: The photoredox reaction between thionine and iron (II) has been studied by flash photolysis techniques. The principal reaction intermediate is the semithionine radical anion which decays via a dismutation process forming ground state thionine and the fully reduced leucothionine. Subsequently, leucothionine is reoxidised slowly by iron (III) which is formed in the primary electron transfer reaction. Rate constants for all the reaction steps have been determined.The reaction has been extended to include reduction with cobalt (II) and manganese (II). Here, the ground state quenchers do not possess the necessary reducing power to form the semithionine radical anion, and reaction leads to enhanced deactivation of triplet thionine. However, when the transition metal ion is excited directly, full electron transfer takes place with concomitant formation of the radical ions.

Binding of carbon monoxide to hemoglobin Zürich. Proposal for a kinetic model.

Abstract: When the spectroscopic properties and kinetics of combination with carbon monoxide of hemoglobin Zurich (HbZh) and its isolated abnormal chains with two sulphydryl groups blocked by mercuribenzoate (βZhSHgR) were studied by flash photolysis and rapid-mixing techniques, large differences compared to human hemoglobin (HbA) and normal isolated chains were observed as follows. 1 The position of the absorption maximum and its coefficient in the Soret region is very different for the CO derivatives of βZhSHgR and tetrameric abnormal HbZh from that of normal chains and hemoglobin. 2 Flash photolysis on βZhSHgR chains show they react with a single rate constant of 2.7 × 107 M−1 s−1 compared to 4.5 × 106 M−1 s−1 for isolated α chains; photochemical efficiency of CO dissociation is also less. 3 Kinetic analysis of CO binding to HbZh by the stopped-flow technique gave a second-order rate constant for βZh chains within the tetramer in the deoxy (T) quaternary conformation of 13.4 × 106 M−1 s−1 compared to 4.7 × 106 M−1 s−1 for α chain in the tetramer in its liganded (R) state. 4 Flash photolysis on solutions of fully saturated HbZh reveal a very fast initial phase during recombination with CO with a second-order rate constant comparable to that of βZhSHgR and attributed to βZh in the dimer. At very low levels of saturation there is a single phase only with a rate constant similar to that of βZh chains in the tetramer. Reduction of the intensity of the photodissociating light causes a decrease in the relative amplitude of the intial fast phase. The proposed kinetic scheme, based on a modified two-state (R and T) allosteric model, implies that βZh chains in the tetramer with a deoxy T configuration have a higher affinity for CO than α chains and a very high combination rate constant. However, when the tetramer flops to the liganded R configuration these chains increase their combination velocity to that of βZh chains in the dimer (R state).

Collisional energy transfer in the reactions I + NO + M -> INO + M and I + NO2 + M -> INO2 + M

Abstract: The low-pressure recombination rate consts. of the reactions I + NO + M -> INO + M (with 14 different M) and I + NO2 + M -> INO2 + M (with 26 different M) were measured at 330 K by laser flash photolysis. The collision efficiencies bc are analyzed and compared with other thermal activation systems. Whereas bc increases in 1 reaction with an increasing no. of atoms in M, practically no such effect is found when, for the same M, different reactions with varying complexities of the reacting mols. are considered.

THE EFFECT OF 8α-SUBSTITUTION ON FLAVIN TRIPLET STATE AND SEMIQUINONE PROPERTIES AS INVESTIGATED BY FLASH PHOTOLYSIS*

Abstract: — Flash photolysis techniques have been used to study the effect of 8α-substitution on flavin triplet state formation and decay and on the properties of neutral and anionic serniquinones Compared with riboflavin, the N(1) and N(3) isomers of 8α-histidylriboflavin show a lower triplet yield (˜10%) and a faster rate of decay (˜ 4-Cfold) Acetylation of the histidyl a-amino groups and of the flavin ribityl side chain results in a 2-fold increase in triplet yield and a 2-fold slower rate of decay The yield of neutral 8α-substituted flavin semiquinones upon flash photolysis in the presence of EDTA was approximately 50% that given by riboflavin These substituted flavin neutral semiquinones dismutated at a rate 2–3 times slower than the corresponding unsubstituted form, although the anionic semiquinones dismutated at approximately the same rate In the presence of oxygen, the kinetics of semiquinone decay changed from second order to pseudo-first order upon raising the pH, thus showing anionic semiquinone oxidation as seen previously with unsnbstituted flavins The pK values for the ionization of the neutral 8α-substituted Aavin semiquinones are 1–15 units lower than the unsubstituted form The anionic 8α-substituted flavin semiquinones react with oxygen at a rate 2–10 times more slowly than does the riboflavin form Such alterations in properties probably reflect the electron-withdrawing effect of the 8α-substituents on the flavin ring system

Indirect Demonstration of the Coulomb Cage for Prototropic Dissociation

Abstract: The existence of a Coulomb cage in the dissociation of sodium 8-hydroxypyrene-1,3,6-trisulphonate into the ions RO− and H3O+ was indirectly demonstrated by laser flash photolysis experiments. In order to reduce the dissociation of the ion pair into free ions, the RO−*-fluorescence was quenched with Hg(CN)2. The lifetime of the Coulomb cage pair was found to be about 2 nsec and, by means of the Bjerrum theory, the radius of the Coulomb cage was calculated to be 33 A. Die Existenz eine Coulomb-Cages bei der Dissoziation des 8-Hydroxypyren-1,3,6-Trisulfonat-Natriumsalzes in die Ionen RO− und H3O+ wurde indirekt mit der Laserblitzphotolyse nachgewiesen. Um die Dissoziation des Ionenpaares in freie Ionen zu verhindern, wurde die RO−*-Fluoreszenz mit Hg(CN)2 geloscht. Die Lebensdauer eines Coulomb-Cagepaares ergab sich zu 2 nsec. Nach der Theorie von Bjerrum wurde der Radius des Coulomb-Cages zu 33 A bestimmt.