Cryogenic Single-Molecule Spectroscopy of the Primary Electron Acceptor in the Photosynthetic Reaction Center

The photosynthetic reaction center (RC) converts light energy into electrochemical energy. The RC of heliobacteria (hRC) is a primitive homodimeric RC containing 58 bacteriochlorophylls and 2 chlorophyll a s. The chlorophyll serves as the primary electron acceptor (Chl a -A 0 ) responsible for light harvesting and charge separation. The single-molecule spectroscopy of Chl a -A 0 can be used to investigate heterogeneities of the RC photochemical function, though the low fluorescence quantum yield (0.1%) makes it difficult. Here, we show the fluorescence excitation spectroscopy of individual Chl a -A 0 s in single hRCs at 6 K. The fluorescence quantum yield and absorption cross section of Chl a -A 0 increase 2- and 4-fold, respectively, compared to those at room temperature. The two Chl a -A 0 s in single hRCs are identified as two distinct peaks in the fluorescence excitation spectrum, exhibiting different excitation polarization dependences. The spectral changes caused by photobleaching indicate the energy transfer across subunits in the hRC.