MOLECULAR REQUIREMENT OF CHLOROSOMAL CHLOROPHYLLS - SELF-ORGANIZATIONOF A CHLOROPHYLL DERIVATIVE POSSESSING A HYDROXYL GROUP AT RING-II

Aggregation of zinc 7(1)-hydroxyl-13(2)-demethoxycarbonylpheophytin a (Zn-7(1)-OH-Chl) was examined in relation to the structure and functio n of the self-aggregates of 3(1)-OH-type chlorophylls (Chl) in chloros omes of green photosynthetic bacteria, The Zn-7(1)-OH-Chl aggregates y ielded a Q(y) absorption band at 712 nm with a 1.2-fold larger width ( full width at half maximum, 500 cm(-1)) than the monomer's (420 cm(-1) ). Infrared and NMR spectroscopies revealed that each molecule in the aggregate links together with simultaneous coordination (C7(1)-OH ... Zn) and hydrogen bonding (C7(1)-OH ... O=C13(1)), A nonlinear alignmen t of the constituent molecules in the oligomeric structure was assumed , Despite the similar molecular linkages, linearly aligned Q(y) moment s in the Zn-3(1)-OH-Chl aggregate gave a chlorosome-like broader, more redshifted Q(y) band (740 nm; 670 cm(-1), 2.1-fold larger than the mo nomer's), Because it is advantageous for efficient light harvesting an d energy transfer to have several Q(y) spectral components, spread ove r a wide spectral range, that can act as the energy gradient, it is co ncluded that not only the intermolecular linkages but the linear locat ions of OH, C=O and Mg in the molecule are crucial for photosynthetic antenna of the self-assembled chlorosomal Chl.