title: An electron spin-polarized signal of the P800+A1(Q)- state in the homodimeric reaction center core complex of Heliobacterium modesticaldum.
abstract: The function of menaquinone as electron acceptor A 1 was identified by EPR in the purified type 1 homodimeric reaction center core complex (RC core) of an anoxygenic photosynthetic bacterium, Heliobacterium modesticaldum. After illumination of the RC core 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 RC core 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 (+)F X (-) 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 RC core 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 RC core 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 (+)A 1 (-) radical pair. The kinetics and spectral shape of this ESP signal suggest that menaquinone serves as secondary electron acceptor A 1 with the molecular orientation of its ring being somewhat different from that of phylloquinone in photosystem I.
authors.affiliation: Division of Material Science (Physics), Graduate School of Science, Nagoya University, Furocho, Chikusa, Nagoya 464-8602, Japan.
authors.forename: Ryo
authors.initials: R
authors.lastname: Miyamoto
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Mino
authors.forename: Toru
authors.initials: T
authors.lastname: Kondo
authors.forename: Shigeru
authors.initials: S
authors.lastname: Itoh
authors.forename: Hirozo
authors.initials: H
authors.lastname: Oh-Oka
mesh_terms: D001421:Bacteria, Anaerobic / Q000378:metabolism*; D001429:Bacteriochlorophylls / Q000737:chemistry* / Q000378:metabolism; D019281:Dimerization; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D045331:Photosystem I Protein Complex / Q000737:chemistry* / Q000378:metabolism; D024482:Vitamin K 2 / Q000737:chemistry* / Q000378:metabolism
chemical_list: D001429:Bacteriochlorophylls; D045331:Photosystem I Protein Complex; D024482:Vitamin K 2