Kinetics of electron transfer in duroquinone-reconstituted reaction centers from photosynthetic bacteria

Abstract

Reaction centers isolated from photosynthetic bacteria are of interest both as a model system for biochemical electron transfer and as a simple system for study of the primary photochemistry of photosynthesis. The light-induced electron-transfer processes in purple photosynthetic bacteria begin with excitation of a bacteriochlorophyll (Bchl) dimer with maximum absorbance at 870 nm (P870). A relatively stable charge-separated state, consisting of oxidized P870 and reduced ubiquinone (UQ), is reached in < 1 ns [ 1,2]. An earlier unstable state almost certainly consists of P870+ and reduced bacteriopheophytin (BPh) [3]. Ubiquinone extraction leads to an apparent loss of photochemical activity than can be recovered by addition of UQ or a variety of other quinones [4-71. Here, we report experiments on reaction centers reconstituted with duroquinone (DQ, 2,3,5,6tetramethylbenzoquinone). We have found that the quantum yield of formation of the state P870+DQis essentially 1.0. The temperature dependence of the recombination kinetics of this state are anomalous, with temperature-independent regions at both low and high temperatures.