Proton transfer in the photosynthetic reaction center of Blastochloris viridis

Photosynthetic reaction centers of Blastochloris viridis require two quanta of light to catalyse a two-step reduction of their secondary ubiquinone QB to ubiquinol. We employed capacitive potentiometry to follow the voltage changes that were caused by the accompanying transmembrane proton displacements. At pH 7.5 and 20 °C, the QB-related voltage generation after the first flash was contributed by a fast, temperature-independent component with a time constant of ∼30 μs and a slower component of ∼200 μs with activation energy (Ea) of 50 kJ/mol. The kinetics after the second flash featured temperature-independent components of 5 μs and 200 μs followed by a component of 600 μs with Ea ∼ 60 kJ/mol.