Conformational Control of the QA to QB Electron Transfer in Bacterial Reaction Centers: Evidence for a Frozen Conformational Landscape below −25 °C

The competition between the P+QA − → PQA charge recombination (P, bacteriochlorophyll pair acting as primary photochemical electron donor) and the electron transfer to the secondary quinone acceptor QA −QB → QAQB − (QA and QB, primary and secondary electron accepting quinones) was investigated in chromatophores of Rb. capsulatus, varying the temperature down to −65 °C. The analysis of the flash-induced pattern for the formation of P+QAQB − shows that the diminished yield, when lowering the temperature, is not due to a homogeneous slowing of the rate constant k AB of the QA −QB → QAQB − electron transfer but to a distribution of conformations that modulate the electron transfer rate over more than 3 orders of magnitude. This distribution appears “frozen”, as no dynamic redistribution was observed over time ranges > 10 s (below −25 °C). The kinetic pattern was analyzed to estimate the shape of the distribution of k AB, showing a bell-shaped band on the high rate side and a fraction of “blocked” reaction centers (RCs) with very slow k AB. When the temperature is lowered, the high rate band moves to slower rate regions and the fraction of blocked RCs increases at the expense of the high rate band. The RCs that recombine from the P+QAQB − state appear temporarily converted to a state with rapid k AB, indicating that the stabilized state described by Kleinfeld et al. (Biochemistry 1984, 23, 5780−5786) is still accessible at −60 °C.