Environmental effects on electron transfer from chlorophyll triplet to quinone: role of dielectric constant, viscosity and quinone structure in cellulose acetate films

The effects of environmental parameters on chlorophyll triplet quenching and electron transfer to quinones have been investigated in a system consisting of donor and acceptor incorporated into a cellulose acetate film which was subsequently exposed to solvent. Triplet quenching by a diffusional mechanism was found to occur in the dry film, with steric effects being a major determinant of quencher effectiveness. No formation of separated radicals was found under these conditions, probably because the high viscosity prevented separation of the initially formed radical-ion pair. When the film was subsequently exposed to water, triplet quenching became more effective and separated radical production occurred. This is attributed to effects of decreased microviscosity and increased dielectric constant. Both steric effects and quinone redox potential were found to influence radical yields. Rate constants for reverse electron transfer were independent of quinone redox potential. When solvents other than pure water were used, radical yields were observed to increase with the dielectric constant. This is ascribed to an increase in the ease of separation of the radical-ion pair

Chlorophyll photochemistry in condensed media: triplet state quenching and electron transfer to quinone in cellulose acetate films

Chlorophyll a was incorporated into cellulose acetate films and the triplet decay and electron transfer to p-benzoquinone in aqueous solution was studied using laser flash photolysis and ESR. The triplet was found to decay by first order kinetics with the same rate constant regardless of chlorophyll concentration, but the yield was concentration dependent. This is due to quenching at the ground and/or singlet state levels. In the presence of quinone, the triplet is quenched and, when the quinone is in an aqueous solution in contact with the film, chlorophyll cation radical (C./sup +/) as well as the semiquinone anion radical (Q./sup -/) can be observed. The C./sup +/ decays by second order kinetics with a rate constant of 1.5 x 10/sup 6/ M/sup -1/ sec/sup -1/. Although triplet conversion to radicals is slightly lower in the films as compared to homogeneous solutions (approx. 3 times), the lifetimes of the radicals are greatly increased (approx. 10/sup 3/ times)

Kinetic Studies on the Electron-Transfer Reaction between Cytochrome c3 and Flavodoxin from Desulfovibrio vulgaris Strain Hildenborough

The kinetic properties of the electron-transfer process between reduced Desulfovibrio vulgaris cytochrome c3 and D. vulgaris flavodoxin have been studied by anaerobic stopped-flow techniques. Anaerobic titrations of reduced cytochrome c3 with oxidized flavodoxin show a stoichiometry of 4 mol of flavodoxin required to oxidize the tetraheme cytochrome. Flavodoxin neutral semiquinone and oxidized cytochrome c3 are the only observable products of the reaction. At pH 7.5, the four-electron-transfer reaction is biphasic. Both the rapid and the slow phases exhibit limiting rates as the flavodoxin concentration is increased with respective ratesof 73.4 and 18.5 s-I and respective &values of 65.9 f 9.4 pM and 54.5 f 13 pM. A biphasic electron-transfer rate is observed when the ionic strength is increased to 100 mM KCl; however, the observed rate is no longer saturable, and relative second-order rate constants of 5.3 X lo5 and 8.5 X lo4 M-l s-1 are calculated. The magnitude of the rapid phase of electron transfer diminishes with the level of heme reduction when varying reduced levels of the cytochrome are mixed with oxidized flavodoxin. No rapid phase is observed when 0.66e--reduced cytochrome c3 reacts with an -25-fold molar excess of flavodoxin. At pH 6.0, the electron-transfer reaction is monophasic with a limiting rate of 42 f 1.4 s-1 and a Kd value of -8 pM. Increasing the ionic strength of the pH 6.0 solution to 100 pM KC1 results in a biphasic reaction with relative second-order rate constants of 5.3 X lo5 and 1.1 X lo4 M-l s-l. Azotobacter vinelandii flavodoxin reacts with reduced D. vulgaris cytochrome c3 in a slow, monophasic manner with limiting rate of electron transfer of 1.2 f 0.06 s-l and a Kd value of 80.9 f 10.7 pM. These results are discussed in terms of two equilibrium conformational states for the cytochrome which are dependent on the pH of the medium and the level of heme reduction [Catarino et al. (1991) Eur. J. Biochem. 207, 1107-1 1131