Long wavelength absorption transitions in the D1/D2/cytochrome b-559 complex as revealed by selective pigment photobleaching and circular dichroism measurements

Photobleaching of the isolated D1/D2/cytochrome (cyt) b -559 was performed at room temperature and 80K and the absorption spectra analysed. While at room temperature the permanent bleaching of four red absorbing transitions is observed, at 80K the selectivity is greatly increased with bleaching of transitions at 675 nm and 683 nm. The signal associated with the 683 nm structure is sufficiently strong to permit analysis of the bandwidth and band symmetry characteristics, which in the linear electron-phonon coupling assumption indicates an electron-phonon coupling strength ( S ) of about 0.7–1.0 for coupling to bath phonons of frequency ν m =20–30 cm −1 , and an essentially symmetrical Q y (0,0) band shape. Circular dichroism (CD) spectra of the D1/D2/cyt b -559 complex were measured at room temperature and 150K and analysed in terms of gaussian subbands. A minimal description with three subbands at 668 nm, 674 nm and 682 nm was found, though a more convincing description required four subbands with peak positions at 668 nm, 674–675 nm, 680.5 nm and 683 nm. The 680.5 nm transition is thought to be associated with P680 and has bandwidth characteristics in line with the published electron-phonon coupling strength (D. Tang, R. Jankowiak, M. Seibert, C.F. Yocum, G.J. Small, J. Phys. Chem. 94 (1990) 6519–6522), indicating that this transition has not undergone significant excitonic narrowing. The 683 nm CD subband is probably associated with the same transition which gives rise to the major absorption photobleaching signal on the basis of its wavelength position and band shape characteristics. The probable presence of four transitions in the CD spectrum of the D1/D2/cyt b -559 complex is in line with the suggestion that multiple (weak) excitonic interactions may occur between pigments in this chlorophyll-protein complex.