Energy transfer fluctuation observed by single-molecule spectroscopy of red-shifted bacteriochlorophyll in the homodimeric photosynthetic reaction center

Abstract

The photosynthetic reaction center of heliobacteria (hRC) is a homodimeric chromoprotein responsible for light harvesting and photoelectric conversion. The fluorescence of the hRC is radiated from a bacteriochlorophyll (Bchl) g having the lowest energy level, called red-Bchl g. The homodimeric architecture of the hRC indicates that it includes two red-Bchls g arranged symmetrically in pairs. Red-Bchl g is a fluorescent probe useful for monitoring the energy transfer network in the RC. Here, we show the fluorescence polarization dependences of two red-Bchls g, individually measured with selective excitation of chlorophyll a serving as the primary electron acceptor. The two red-Bchls g exhibit almost the same polarization dependences. Based on the polarization dependence and structural data of the hRC, we propose a candidate molecule for red-Bchl g. The fluorescence spectra of single hRCs represent the spectral heterogeneity reflecting the local conformational inhomogeneity. A time series of the fluorescence spectra indicates occasional peak shifts between blue- and red-shifted states without significant changes in the fluorescence intensity. The spectral fluctuation is interpreted to be due to the local conformational dynamics around a Bchl g mediating the energy transfer, switching the terminal energy acceptor between two red-Bchls g. In conclusion, while the energy transfer network in the RC can be perturbed by microscopic dynamics, the total energy transfer efficiency, i.e., the light-harvesting function, is rather robust. The functional robustness may be due to multiple energy transfer pathways composed of many antenna pigments in the RC.