Molecular Requirement of Chlorosomal Chlorophylls. Self-Organization of a Chlorophyll Derivative Possessing a Hydroxyl Group at Ring II |
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Authors: | Toru Oba Hitoshi Tamiaki |
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Institution: | Department of Bioscience and Biotechnology, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga, Japan |
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Abstract: | Aggregation of zinc 71-hydroxyl-132-demethoxycarbonyl-pheophytin a (Zn-71-OH-Chl) was examined in relation to the structure and function of the self-aggregates of 31-OH-type chlorophylls (Chi) in chlorosomes of green photosynthetic bacteria. The Zn-71-OH-Chl aggregates yielded a Qy 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 (C71-OH…Zn) and hydrogen bonding (C71-OH … O=C131). A nonlinear alignment of the constituent molecules in the oligomeric structure was assumed. Despite the similar molecular linkages, linearly aligned Qy, moments in the Zn-31-OH-Chl aggregate gave a chlorosome-like broader, more redshifted Qy band (740 nm; 670 cm?1, 2.1-fold larger than the monomer's). Because it is advantageous for efficient light harvesting and energy transfer to have several Qy, spectral components, spread over a wide spectral range, that can act as the energy gradient, it is concluded that not only the intermolecular linkages but the linear locations of OH, C=0 and Mg in the molecule are crucial for photosynthetic antenna of the self-assembled chiorosomal Chl. |
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