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At least 1,369 records · Page 76Linked to original sources
PubMed2003

Quantitative rate constants for the reaction of dyes and alkenes with alpha-hydroxyalkyl radicals, measured by laser flash photolysis.

Rate constants are measured for the addition reactions of 1-hydroxy-1-cyclohexyl (1HC) and 2-hydroxy-2-propyl (2HP) radicals to 7 alkenes and for the 1-electron reduction of 16 organic dyes by 1HC, and a subset of 5 of these dyes by 2HP. This was done to determine to what extent the many reported rate constants for reactions of 2-hydroxy-2-propyl radicals (2HP) may be used to predict the rates of reactions of other tertiary alpha-hydroxy-alkyl radicals, and to give a better understanding of the factors that control dye reduction. The dyes were chosen to represent a wide range of dye types (azo, anthraquinone, phthalocyanine, triaryl-methane, indocyanine and azine dyes). Radicals were produced by laser flash photolysis of the corresponding tertiary alpha-hydroxyketone giving carbonyl and tertiary alpha-hydroxy-alkyl radicals. Control experiments with a bis-acylphosphine oxide were carried out which clearly demonstrated that the carbonyl radicals did not interfere with the kinetics. On average the addition and reduction rate constants for 1HC are only 20% lower than for 2HP. Larger decreases are observed for sterically congested alkenes due to the increased steric bulk of 1HC. The rate constants for 1-electron reduction of the dyes are in the range 4 x 10(7) to 6 x 10(9) mol-1 1 s-1 and may be predicted, reasonably well using the Marcus equation with a reorganisation energy, lambda = 182 kJ mol-1.

Journal Article↗
PubMed2003

The genesis of flash photolysis.

A brief account is given of the construction of the first flash photolysis apparatuses at Cambridge and their application to gas phase systems.

Journal Article↗
PubMed2003

Laser flash photolysis of bismaleimides.

Bismaleimides containing between 2 and 12 methylene spacer units have been synthesized and characterized using laser flash photolysis spectroscopy. In the case of the monofunctional N-methylmaleimide, the 1,4-biradical was observed only at maleimide concentrations high enough to effectively quench the triplet state, indicating that the 1,4-biradical is Formed from self-quenching of the triplet excited state. The transient spectral features and the transient lifetimes of bismaleimides containing 2 and 9 methylene spacer units closely resemble those of N-methylmaleimide and are attributed to triplet-state transients. N,N'-Alkylenebismaleimides with 3 and 6 methylene spacer groups exhibit a transient with a very short lifetime attributed to a singlet 1,4-biradical species formed from intramolecular interaction. The absence of triplet spectra in these latter bismaleimides indicate that the 1,4-biradical is formed directly from the excited singlet state. The large difference in the lifetime of the 1,4-biradical formed in monofunctional maleimide solutions and bismaleimide solutions is reflective of differences in the biradical spin-state multiplicity. The bismaleimide with a 12 methylene spacer group exhibits transient spectra corresponding to both a triplet state of the maleimide and a 1,4-biradical formed by exo interaction of two maleimide groups. As a demonstration of the ability of acrylate monomers to react with the short-lived singlet biradicals, it is shown that the photopolymerization of a multifunctional acrylate monomer is readily initiated by the 1,4-biradical formed from the bismaleimides with 3 and 6 methylene spacer groups, presumably by direct reaction of the biradical with the acrylate.

Journal Article↗
PubMed2003

Photochemically generated arylnitrenium ions: substituent effects on reactivity studied by laser flash photolysis.

Laser flash photolysis is used to examine reaction rates of four arylnitrenium ions: N-methyl-N-(4-chlorophenyl)-, -(4-biphenylyl)-, -(4-methylphenyl)-, and -(4-methoxyphenyl)nitrenium ion. These intermediates are generated from photolysis of appropriately substituted N-aminopyridinium ions. Kinetic analysis of trapping rates shows that the nitrenium ions react with weak nucleophiles in the order 4-Cl approximately 4-Me > 4-Ph > 4-OMe. Generally, amines react with the arylnitrenium ions at or near the diffusion-limited rate. Only in cases where the least reactive nitrenium ion (4-OMe) reacts with sterically hindered amines is any deviation from this generalization observed. The addition of a 2-acetyl substituent to these structures produces an interesting effect. In the case of the 4-Cl and 4-Ph analogs, the electron-withdrawing acetyl group decreases the rate of nucleophilic addition, yet with the 4-OMe and 4-Me compounds, the acetyl group increases the rate of addition. These results are discussed in terms of competing electronic and steric effects.

Journal Article↗
PubMed2004

Formation of radical cations of aziridines generated by laser flash photolysis.

The radical cations of 1-butyl-trans-2,3-diphenyl aziridine (1), 1-butyl-2-phenyl aziridine (2), 1,2-diphenyl aziridine (3) and 1-(p-methoxyphenyl)-2-phenyl aziridine (4) were generated upon laser flash photolysis in aqueous and aqueous acetonitrile solutions by direct photoionisation as indicated by the broad absorption band of the solvated electron above 550 nm as well.

Aziridines↗
PubMed2004

An empirical study of the effect of the variables in a flash vacuum pyrolysis (FVP) experiment.

The effect of the variation of the experimental parameters on the conversion of precursor to products in a typical flash vacuum pyrolysis (FVP) experiment was investigated empirically. Temperature-conversion plots can be used to optimise FVP conditions and their mechanistic significance is exemplified. At a given temperature, the conversion can be increased by an increase in the background pressure, or by packing a section of the furnace tube with inert material (particularly when placed at the trap end of the furnace tube) or by employing a catalyst. Despite the prevailing view that only intramolecular reactions take place by FVP, it has been shown by a 'dual-FVP' cross-over experiment that the dimerisation of benzyl radicals occurs in the gas-phase, before the cold trap, under standard conditions. However, reduction in through-put rate, increase in furnace temperature and reduction in background pressure all reduce the amount of gas-phase coupling.

Journal Article↗
PubMed2005

Flash photolytic release of alcohols from photolabile carbamates or carbonates is rate-limited by decarboxylation of the photoproduct.

Flash photolysis of a 7-nitroindolinyl carbamate derivative in neutral aqueous solution rapidly generated a monoalkyl carbonate salt. The rate constant for subsequent decarboxylation of this salt [mono(2-phosphoryloxyethyl) carbonate], determined by rapid scan IR difference spectroscopy, was 0.4 s(-1) at pH 7.0, 20 degrees C. This rate reflects release of the product alcohol upon photolysis of the parent compound. In general, alcohols protected as photolabile carbamate (or carbonate) derivatives will therefore be released too slowly for studies of the kinetics of millisecond time scale biological processes.

Alcohols↗
PubMed2005

Intermediates formed by laser flash photolysis of [PtCl(6)](2-) in aqueous solutions.

The stationary photolysis of [PtCl(6)](2-) in aqueous solutions (10(-5)-10(-4) M) at the region of 313 nm leads to its photoaquation with a quantum yield of 0.19. Laser flash photolysis experiments (308 nm) provided evidence of the formation of Pt(iii) intermediates, namely [PtCl(4)(OH)(H(2)O)](2-) and [PtCl(4)](-), and Cl(2) (-) radical anions. The Pt(iii) complexes formed as a result of an intrasphere electron transfer from Cl(-) ligands to the excited Pt(iv) ion. However, the main ( approximately 90%) photolysis channel was not accompanied by the transfer of Cl atoms to the solvent bulk. The photoaquation of [PtCl(6)](2-) results from the back electron transfer in the secondary geminate pair, [PtCl(5)(H(2)O)](2-)-Cl. The relative yield of Pt(iii) intermediates, recorded after the completion of all processes in the geminate pair, was less than 10% of the number of disappearing initial [PtCl(6)](2-) complexes.

Chlorine↗
PubMed2006

Studies of site selective hydrogen atom abstractions by Cl atoms from isobutane and propane by laser flash photolysis/IR diode laser spectroscopy.

The kinetics of chlorine atom abstractions from normal and selectively deuterated propane and isobutane have been measured at room temperature and 195 K using a laser flash photolysis system, and following the course of the reaction via IR diode laser absorption measurements of HCl product. In conjunction with the kinetic measurements, a comparison of the HCl signal heights from pairs of measurements on normal and selectively deuterated systems has allowed the determination of the branching fractions of the reactions at the primary, secondary (propane) and tertiary (isobutane) positions. The kinetic data (all in units of cm(3) molecule(-1) s(-1)) for the reaction of Cl atoms with propane ((1.22 +/- 0.02) x10(-10), 195 K; (1.22 +/- 0.03) x10(-10) 298 K) and isobutane ((1.52 +/- 0.02) x10(-10), 195 K; (1.25 +/- 0.04) x10(-10), 298 K) are generally in good agreement with literature data. No data are available for comparison with our measurements for the reactions of Cl atoms with CH(3)CD(2)CH(3) ((1.02 +/- 0.03) x10(-10), 195 K; (1.09 +/- 0.02) x10(-10), 298 K) or (CH(3))(3)CD ((1.32 +/- 0.03) x10(-10), 195 K; (1.12 +/- 0.04) x10(-10), 298 K). Rate coefficients at 195 K for the reactions of Cl atoms with ethane ((5.04 +/- 0.08) x10(-11) and n-butane ((2.19 +/- 0.03) x10(-10)) were also measured. The branching fractions for abstraction at the primary position increased with temperature for both propane ((40 +/- 3)% at 195 K to (48 +/- 3)% at 298 K) and isobutane ((49 +/- 4)% at 195 K to (62 +/- 5)% at 298 K). The direct measurements from this study are in good agreement with most calculations based on structure activity relationships.

Butanes↗
PubMed2006

Flash-photolytic generation of dienols and dienolates from alpha,beta-unsaturated esters and kinetics of their amine-catalyzed ketonization in nonaqueous media.

Dienols of alpha,beta-unsaturated esters were generated and observed by flash photolysis and their decay rates in acetonitrile and in hexane solutions were measured in the presence of amines or aminoalcohols. In acetonitrile, 1 ratio 1 ammonium dienolate transient intermediates were observed. In hexane, no dienolate could be detected and two moles of the inductor participated in the rate-determining step for tautomerization.

Journal Article↗
PubMed2006

Time-resolved methods in Biophysics. 2. Monitoring haem proteins at work with nanosecond laser flash photolysis.

Haem proteins have long been the most studied proteins in biophysics, and have become paradigms for the characterization of fundamental biomolecular processes as ligand binding and regulatory conformational transitions. The presence of the haem prosthetic group, the absorbance spectrum of which has a ligation sensitive region conveniently located in the UV-visible range, has offered a powerful and sensitive tool for the investigation of molecular functions. The central Fe atom is capable of reversibly binding diatomic ligands, including O(2), CO, and NO. The Fe-ligand bond is photolabile, and a reactive unligated state can be transiently generated with a pulsed laser. The photodissociated ligands quickly rebind to the haem and the process can be monitored by transient absorbance methods. The ligand rebinding kinetics reflects protein dynamics and ligand migration within the protein inner cavities. The characterization of these processes was done in the past mainly by low temperature experiments. The use of silica gels to trap proteins allows the characterization of internal ligand dynamics at room temperature. In order to show the potential of the laser flash photolysis techniques, combined with modern numerical analysis methods, we report experiments conducted on two non-symbiotic haemoglobins from Arabidopsis thaliana. The comparison between time courses recorded on haemoglobins in solution and encapsulated in silica gels allows for the highlighting of different interplays between protein dynamics and ligand migration.

Arabidopsis↗
PubMed1982

Dynamics of carbon monoxide recombination to fully reduced cytochrome c oxidase in plant mitochondria after low-temperature flash photolysis.

Rebinding of CO to reduced cytochrome c oxidase in plant mitochondria has been monitored optically at 590-630 nm after flash photolysis at low temperature from 160 to 200 K. (1) Under 100%-CO saturation, CO rebinding exhibits a four-step mechanism. The thermodynamic parameters of the first phase have been determined; its activation energy, Ea1, is 38.9 kJ.mol-1 and its enthalpy, delta H+/-1, and entropy, delta S+/-1, of activation are respectively 37.5 kJ.mol-1 and -75.8J.mol-1.K-1. (2) When the CO concentration is decreased to 0.2%, rebinding still occurs according to a four-step mechanism. The rate constant of the first phase is CO-concentration-independent. Under non-saturating conditions there is only one CO molecule per occupied site. The rebinding mechanism does not require additional CO molecules to be present in the haem pocket. (3) Dual-wavelength scanning experiments failed to detect optical forms correlated with the resolved phases. (4) Results are discussed with respect to previous work related to CO rebinding to mammalian cytochrome c oxidase and myoglobin.

Binding Sites↗
PubMed1984

The reaction of fully reduced cytochrome c oxidase with oxygen studied by flow-flash spectrophotometry at room temperature. Evidence for new pathways of electron transfer.

Absorption changes during the O2 reaction of reduced bovine cytochrome c oxidase were investigated by the rapid-reaction technique of flow-flash spectrophotometry in the Soret, visible and near-i.r. spectral regions. New features in the time courses of absorption change were observed relative to the earlier findings reported by Greenwood & Gibson [(1967) J. Biol. Chem. 242, 1782-1787]. These new features arise in the Soret and near-i.r. regions and allow the reaction to be described at all wavelengths as a composite of three exponential processes. There is a rapid O2-sensitive phase detectable in the Soret and visible region. The second phase has a rate that is somewhat less dependent on O2 concentration than is the fastest phase rate and is detectable in all three spectral regions. The rate of the third phase is almost independent of the O2 concentration and is also detectable in all spectral regions. Analysis of the three phases gives their rates and absorption amplitudes. The fast phase reaches a rate of 2.5 X 10(4) s-1 at the highest O2 concentration available at 20 degrees C, whereas the phase of intermediate rate is limited at a value of 7 X 10(3) s-1 and the slow phase rate is limited at 700 s-1. The ratios of the kinetic difference spectra for the fast phase and the slow phase do not correspond to the spectra of the individual haem centres. A branched mechanism is advanced that is able to reconcile the kinetic and static difference spectra. This mechanism suggests that some of the cytochrome a is oxidized along with cytochrome a3 in the initial O2-sensitive phase. In addition, the model requires that CuA is oxidized heterogeneously. This fits with the complex time course of oxidation observed at 830 nm while retaining CuA as virtually the sole contributor to absorbance at this wavelength.

Electron Transport↗
PubMed1989

Photoaffinity labelling of isopenicillin N synthetase by laser-flash photolysis.

Isopenicillin N synthetase (IPNS) from Acremonium chrysogenum was photolabelled by laser-flash photolysis in the presence of a diazirinyl-containing substrate, 2-[3-(3-trifluoromethyl-3H-diazirin-3-yl)-phenoxy]acetyl-S- methyloxycarbonylsulphenyl-L-cysteinyl-D-valine (DCV). Labelling of IPNS by DCV is partially inhibited in the presence of an excess of L-alpha-aminoadipoyl-L-cysteinyl-D-valine (ACV), the natural substrate. In the absence of light, DCV is converted into the corresponding penicillin with comparable Km but significantly depressed Vmax relative to ACV. Selective incorporation of [14C]DCV into IPNS has been demonstrated by fluorography of IPNS analysed by SDS/polyacrylamide-gel electrophoresis. Scintillation counting of labelled IPNS purified on an ion-exchange f.p.l.c. column confirms this result. This methodology may be applicable for studies aimed at investigating the binding of substrates to IPNS.

Acremonium↗
PubMed1979

An 11-year assessment of 93 flash-frozen homograft valves in the aortic position.

Flash-frozen homograft aortic valves were used for isolated aortic valve replacement at the National Heart Hospital between 1968 and 1971. This study of 93 patients with an average follow-up of 8.5 years complements a previous report on 30 patients but shows a higher incidence of degeneration. There was an early mortality of 4.8% and a late mortality of 15.9% over 11 years. Only five of the 15 late deaths were directly attributable to valve malfunction. There were no episodes of thromboembolism during the 11 year follow-up and only three cases of infective endocarditis. Despite the late onset of valve degeneration there is a 77% probability of patient survival at 11 years. The use of frozen valves delayed the time of onset of degenerative changes when compared over an 11-year period with the earlier use of freeze-dried valves. The evaluation of replacement cardiac valves is thus seen to be time-dependent and needs a long follow-up and an adequate number of patients in the sample for a meaningful assessment. There was no significant difference in the long-term performance between the frozen valves sterilized in ethylene oxide or sterilized with gamma irradiation.

Adult↗
PubMed1993

[Functional imaging of the brain using low dose gadolinium-DTPA and Turbo-FLASH sequences].

A method for measuring regional blood volume and flow is described based on the reduced relaxation time following intravenous injection of Gd-DTPA. The optimal Gd dose and other parameters were first determined using turbo-flash sequences and a dilution phantom. A single maximum was found for a Gd-DTPA concentration of 4 mmol/l. Taking account of normal heart/time volumes and in order to stay below this arterial value Gd-DTPA was injected as a bolus at the rate of 1 mmol/sec. From the phantom measurements there appears to be a functional connection between measured signal intensity and Gd concentration. The method was applied to 15 patients with cerebral tumours, using altogether 26 perfusion examinations. The results from 4 patients are given as examples; in these, blood volume and flow were calculated for each pixel in a 128 x 128 matrix. It was found that the Gd-DTPA injected is sufficient to be shown with certainty in the grey and white matter and that this is able to demonstrate pathological changes in a functional manner. Comparison with Gd-DTPA doses such as are used for morphological imaging (typically, 10 mmol corresponding to 0.1 mmol/kg body weight) has shown that doses above 1.6 mmol may lead to distortion of the concentration curves.

Aged↗