The function of menaquinone as electron acceptor A1 was identified by EPR in the purified type 1 homodimeric reaction center core complex (RCcore) of an anoxygenic photosynthetic bacterium, Heliobacterium modesticaldum. After illumination of the RCcore at 210 K in the absence and presence of dithionite, we detected the radical of a special pair of bacteriochlorophyll g molecules (P800+) at g = 2.0033 and a quinone-type radical at g = 2.0062, respectively, at 14 K. Flash excitation of the dark-frozen RCcore at 14 K induced two types of transient EPR signals, i.e., the P800+ radical that decayed with a time constant of 3.7 ms and a much faster decay component that showed the electron spin polarization (ESP) pattern of E/A (E, emission; A, absorption). The latter one was assigned to the P800+FX- radical pair state. A new ESP signal that had an apparent A/E/A/E pattern extended to the lower-magnetic-field side was transiently induced by the flash excitation in the RCcore that was preilluminated at 210 K in the presence of ascorbate and subsequently cooled to 14 K in the light. The 210 K preillumination of the RCcore in the presence of dithionite led to accumulation of the dark stable semiquinone-type signal at g = 2.0062 and increased the intensity of the light-induced P800 triplet signal. Flash excitation at 14 K induced the smaller A/E/A/E-type signal that had the greater contribution of the lower-magnetic-field envelope. This ESP signal could thus be ascribed to the P800+A1- radical pair. The kinetics and spectral shape of this ESP signal suggest that menaquinone serves as secondary electron acceptor A1 with the molecular orientation of its ring being somewhat different from that of phylloquinone in photosystem I.
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