STRONG ACCELERATION OF PRIMARY PHOTOSYNTHETIC ELECTRON-TRANSFER IN A MUTATED REACTION-CENTER OF RHODOPSEUDOMONAS-VIRIDIS

The energetics of the primary electron donor (the special pair P) in r eaction centers from Rhodopseudomonas viridis were modified by site-di rected mutagenesis of histidine L168 to phenylalanine. This leads to t he loss of a hydrogen bond between the amino acid side chain and the r ing I acetyl carbonyl oxygen of The bacteriochlorophyll molecule BChl( LP). As a result of the mutation, a 35 nm blue shift of the Q(y) band of the special pair and a decrease of 80 mV in the P/P+ oxidation-redu ction potential occur. Femtosecond spectroscopy revealed art accelerat ion of the first electron transfer step from 3.5 ps in wild type to 1. 1 ps in the mutant. Analysis of changes in the bacteriochlorophyll mon omer (B) band of the mutant reaction centers showed strong bleaching. This is direct evidence that bacteriochlorophyll b is a real intermedi ate in electron transfer. The changes in redox potential and time cons tants allow one to estimate the energetics in the wildtype and mutated reaction centers according to the Marcus electron transfer theory.