The energetics of the primary electron donor (the special pair P) in reaction centers from Rhodopseudomonas viridis were modified by site-directed mutagenesis of histidine L168 to phenylalanine. This leads to the loss of a hydrogen bond between the amino acid side chain and the ring 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-reduction potential occur. Femtosecond spectroscopy revealed art acceleration 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 monomer (B) band of the mutant reaction centers showed strong bleaching. This is direct evidence that bacteriochlorophyll b is a real intermediate in electron transfer. The changes in redox potential and time constants allow one to estimate the energetics in the wildtype and mutated reaction centers according to the Marcus electron transfer theory.