ABSTRACT
The reaction center of bacterial photosynthesis drives a light-induced open-chain intraprotein electron transfer that is part of a larger-scale cyclic electron transfer connecting the cyt bc 1 complex via mobile electron carriers of cyt c and quinones (Q). The electron transfer can be tracked by kinetic changes of the bacteriochlorophyll (BChl) fluorescence originating either from the reaction center or from the antenna. Here, we will concentrate on induction and relaxation of fluorescence and on delayed fluorescence in the millisecond-to-minute time range. The fluorescence induction is decomposed into three (photochemical, triplet quenching, and slow) phases. The light intensity and redox reactions at the donor and acceptor sides as possible bottlenecks of the turnover of the reaction center are reviewed and analyzed critically. We demonstrate that (1) the induction and relaxation of the yield of the BChl fluorescence are fast and convenient tools to monitor the rate-limiting steps in view of closing and reopening the bacterial reaction center upon (and after) continuous illumination, and (2) under physio logical conditions in Rhodobacter sphaeroides, mainly, the donor-side reactions limit the turnover time to some tens of microseconds, (3) which progressively increases with longer illumination in accordance with finite pool sizes and structural confinements of reduced cyt c 2 and oxidized quinones in the periplasmic and membrane phases of the photosynthetic unit, respectively. Fluorescence techniques recently have gained a broad spectrum of applications in (1) easy and inexpensive determination of the photosynthetic capacity of bacteria under field conditions, (2) ontogenesis of the photosynthetic apparatus, and (3) production of novel systems and (nano)materials mimicking the natural processes of the bacterial photosynthesis.
