Reaction Center Heterogeneity Probed by Multipulse Photoselection Experiments With Picosecond Time Resolution

Using double pulse excitation of an electron-transfer-impaired reaction center mutant from Rhodobacter sphaeroides, it has been possible to provide direct evidence for reaction center conformational heterogeneity which affects the charge separation yield. A saturating prepulse was used to photoselect a subpopulation of reaction centers which returned rapidly to the ground state instead of forming the long-lived P+Q A - state. Femtosecond transient absorbance measurements were then performed on this photoselected subpopulation. It was found that the yield of electron transfer in this subpopulation was substantially lower than that in the bulk reaction center population. Though these measurements are preliminary, they represent the first direct demonstration of a reaction center conformational heterogeneity that clearly affects the efficiency of the initial electron transfer reaction.