Direct Observation of the Accessory Bacteriochlorophyll in the Primary Electron Transfer in Bacterial Reaction Centers

Abstract

Early femtosecond experiments on bacterial reaction centers (RC) performed nearly one decade ago did not exhibit transient absorption features related to the reduction of the accessory bacteriochlorophyll (B) located between the primary electron donor (the special pair P) and the electron accepting bacteriopheophytin H. The absence of such a component led to the conclusion that B is not a real electron carrying intermediate. However in later experiments |1–3| we found a subpicosecond component which had relatively weak amplitudes. The spectral dependence of this component and additional experiments on mutated and modified RC suggested that B is a real electron carrier. This interpretation was not generally accepted in the literature. One main objection came from the fact that the subpicosecond kinetic component was only observed in "congested" spectral region where other intermediates caused strong absorption changes. It is the purpose of this contribution to present new experimental results proving unambiguously that the accessory bacteriochlorophyll is a real electron carrier.