CHLOROPHYLL PHOTOSENSITIZED ELECTRON TRANSFER IN PHOSPHOLIPID BILAYER VESICLE SYSTEMS: SECONDARY OXIDATION OF REDUCED VIOLOGEN ACCEPTORS BY Ru(NH₃)₆³⁺

Abstract

Abstract— The kinetics of the oxidation of a homologous series of 4,4′‐di(n‐alkyl)‐bipyridinium (viologen) radicals by Ru(NH3)63+ in vesicle suspensions was studied using laser flash photolysis. The viologen radicals were produced photochemically in the bilayer membrane phase of the vesicles by electron transfer from the triplet state of chlorophyll‐α. At high concentrations of Ru(NH3)63+, the rate of oxidation of the viologen radicals in the aqueous phase was limited by the rate at which the radicals diffused from the membrane to the aqueous phase. The exit rate constant decreased from 2 × 105 s−1 for the methyl viologen radical to 4 × 103 s−1 for the pentyl viologen radical. Both the exit rate constants and the calculated values for the equilibrium association constants of the viologen radicals were unexpectedly insensitive to the length of their alkyl substituents. This, as well as other data, suggests that the radicals that diffused into the aqueous phase tended to remain associated with the membrane‐water interface.

ELECTRON TRANSFER REACTIONS OCCURRING UPON LASER FLASH PHOTOLYSIS OF PHOSPHOLIPID BILAYER SYSTEMS CONTAINING CHLOROPHYLL, BENZOQUINONE AND CYTOCHROME c‐I. NEGATIVELY‐CHARGED VESICLES*

“…light-induced sequential electron transfer reactions can occur with good efficiency, and that laser flash photolysis methods provide a powerful tool for elucidating the kinetics of such processes. These experiments extend the earlier work of Ford and Tollin (1986b) to a more complex situation. It is worthy of note that the principal reactions which occur constitute a cyclic electron transport sequence [cyt c(red) + Chl + Q + cyt c(ox)], in the sense that the same protein which functions as an electron donor also serves as the electron acceptor.…”

“…The pH of the vesicle suspension after Trizma addition was 7.5. The flash apparatus was the same as that used previously (Ford and Tollin, 1986b) with a I mM rhodamine 6G and 1 mM cresyl violet perchlorate laser dye. The excitation wavelength was 655-660 nm.…”

“…of the suspending medium Tollin, 1983, 1984;Tollin, 1985, 1986). Recently, we have begun to study secondary electron transfer reactions in these systems (Ford and Tollin, 1986b). In these latter experiments, a second electron acceptor, Ru(III)(NH&, was present in the reaction mixture and was able to efficiently remove an electron from the viologen molecule which was reduced during the primary photochemical event.…”

Photoinduced electron transfer across membranes

“…Ford and Tollin [161,162] have studied the influence of hydrophobic nature of the electron acceptor (symmetrical dialkylviologens, CnV 2÷, have been used) on the yield of electron phototransfer and on the rate of recombination of the products formed. It has been shown that the rate constant of 3Chl quenching in the lipid membranes increases with the increase of the length of alkyl substituent in viologens due to an increase of the local concentration of the quencher in the membrane (because of the strengthening of CnV 2 ÷ binding to the membrane with the increase of its hydrophobicity).…”

Photoinduced Electron Transfer Across Lipid Bilayers Containing Magnesium Octaethylporphyrin

“…In many studies with vesicles chlorophyll was used as the chromophore but only transient absorbance changes were measured (Ford and Tollin, 1986a, 1986b. Since they are analysing a complex system consisting of chlorophyll triplets and cations in three different locations, we believe their evidence is insufficient to substantiate their claims of transmembrane charge transfer mediated exclusively by Chi a+.…”