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PHOTOSYNTHESIS
A supramolecular ring of porphyrin dimers synthesized in Japan mimics a circular assembly of light-harvesting bacteriochlorophyll molecules in photosynthetic bacteria. Professor of materials science Yoshiaki Kobuke and Ph.D. student Ryoichi Takahashi at the Nara Institute of Science & Technology prepared the porphyrin macroring by self-assembly of six phenylene-bridged porphyrin dimers with a gable structure [J. Am. Chem. Soc., 125, 2372 (2003)]. "This model must be a major milestone for further investigation to elucidate the mechanism of highly efficient light harvesting as well as the evolutional strategy of such ring structures in the natural photosynthetic system," they claim. The porphyrin units, which contain zinc and imidazolyl substituents, are arranged in the ring in the same way that the magnesium-containing bacteriochlorophyll units known as B850 form a ring in LH2--one of the light-harvesting pigment-protein complexes in photosynthetic organisms. "The hexameric macroring is very similar to natural B850 with respect to imidazolyl-to-metal coordination, arrangement of the dimer units in the macroring, and the distance and orientation of the chromophores," Kobuke tells C&EN. "There is a charge-separation center in the bacterial photosynthetic reaction center called a special pair, where two parallel chlorophyll molecules face each other with a slipped center-to-center disposition. Our macroring interlocks slipped-cofacial dimer units, but without any protein matrices." Kobuke and Takahashi used conventional porphyrin synthesis methods to prepare the imidazolyl-substituted gable-porphyrins. Insertion of zinc into the gable-porphyrins resulted in an oligomeric mixture of linear and cyclic structures with a broad distribution of molecular weights. The researchers "reorganized" the mixture into cyclic hexamers using polar and nonpolar solvents that respectively cleave and form coordinate bonds. The porphyrin macroring is easy to access, notes Steven C. Zimmerman, chemistry professor at the University of Illinois, Urbana-Champaign. "Given the current interest in artificial photosynthetic systems, the array is likely to have interesting photophysical properties," he says. "In addition, the idea that the coordinative self-assembly initially produces polymers but can, under appropriate conditions, reorganize to a cyclic structure represents an impressive example of a dynamic assembly process."
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