Electron Transfer in Reaction Centers of Blastochloris viridis:  Photosynthetic Reactions Approximating the Adiabatic Regime

Femtosecond time-resolved experiments on native and mutant (L168 H → F) reaction centers of Blastchloris viridis offer a direct approach to study the reaction mechanisms and the optimization stategies for the primary steps in the light reaction of bacterial photosynthesis. The experiments focus on a temperature-dependent analysis of reaction rates and an evaluation of the oscillatory contributions. For wild-type reaction centers the reaction times τ1a (dominant component of the first electron-transfer step) and τ2 (second electron-transfer step) decrease toward lower temperatures. τ1a deceases by a factor of 1.5 between 300 and 30 K, while τ2 decreases by a factor of 4. Interestingly, the L168 H → F mutant, which exhibits a much faster primary charge separation than wild type, shows a similar strong acceleration also for the component τ1a. For both types of reaction centers, pronounced wave-packet-like absorption changes in the range of the stimulated emission are observed at low temperatures. The results are discussed within the scope of electron transfer theory. They suggest that a transition from the nonadiabatic to the adiabatic regime occurs for the fastest reactions at low temperatures and that the first reaction step must stay in the nonadiabatic regime in order to ascertain optimum photosynthetic efficiency.