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Molecular Requirement of Chlorosomal Chlorophylls. Self‐Organization of a Chlorophyll Derivative Possessing a Hydroxyl Group at Ring II

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.