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 perfor med at room temperature and 80K and the absorption spectra analysed. W hile 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 associa ted with the 683 nm structure is sufficiently strong to permit analysi s of the bandwidth and band symmetry characteristics, which in the lin ear electron-phonon coupling assumption indicates an electron-phonon c oupling strength (S) of about 0.7-1.0 for coupling to bath phonons of frequency v(m) = 20-30 cm(-1), and an essentially symmetrical Q(y)(0,0 ) band shape. Circular dichroism (CD) spectra of the D1/D2/cytb-559 co mplex were measured at room temperature and 150K and analysed in terms of gaussian subbands. A minimal description with three subbands at 66 8 nm, 674 nm and 682 nm was found, though a more convincing descriptio n required four subbands with peak positions at 668 nm, 674-675 nm, 68 0.5 nm and 683 nm. The 680.5 nm transition is thought to be associated with P680 and has bandwidth characteristics in line with the publishe d electron-phonon coupling strength (D. Tang, R. Jankowiak, M. Seibert , C.F. Yocum, G.J. Small, J. Phys. Chem. 94 (1990) 6519-6522), indicat ing that this transition has not undergone significant excitonic narro wing. The 683 nm CD subband is probably associated with the same trans ition which gives rise to the major absorption photobleaching signal o n the basis of its wavelength position and band shape characteristics. The probable presence of four transitions in the CD spectrum of the D 1/D2/cytb-559 complex is in line with the suggestion that multiple (we ak) excitonic interactions may occur between pigments in this chloroph yll-protein complex. (C) 1998 Elsevier Science B.V. All rights reserve d.