NEWS OF THE WEEK PHOTOSYNTHESIS
LIGHT-HARVESTING ANTENNA MODEL Hexamer ring resembles ring of chlorophylls in photosynthetic bacteria
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SUPRAMOLECULAR R I N G
of porphyrin dimers synthesized in Japan mimics a circular assembly of light-harvesting bacteriochlorophyll molecules in photosynthetic bacteria. Professor of materials science ^bshiaki Kobuke and Ph.D. student Ryoichi Takahashi at the
DYNAMIC Gable-porphyrins self-assemble into hexameric macroring
c=i>
R = n-C 7 H 1 5 NOTE: Some n-heptyl groups on each porphyrin are omitted for clarity.
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 ofhighly 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 ofthe 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
BUSINESS
Clariant Hits Turbulent Waters
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hortly after announcing its financial results for 2002—a $477 million net loss, following extraordinary charges—Clariant has decided to suspend its dividend and sell noncore businesses. Clariant showed a net loss as a result of $656 million in charges associated with its acquisition of fine chemicals company BTP in 2000. If the write-down is excluded, the company shows a profit of $179 million. Clariant says the write-down
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came as developments in its life sciences units did not meet expectations. Last year, the company divested its European emulsions business, its worldwide emulsion powders business, and its North American hydrosulfite operations. Now, it says, businesses that are not core activities—some 7% of sales—will be sold off. Capacities in Clariant's pharmaceuticals and custom synthesis business units will be substantially scaled back, the com-
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-tocenter 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."-MICHAEL FREEMANTLE
pany says. And those two units and its electronic materials business unit, as well as its masterbatches division, will be transformed into legal entities, opening up the possibility of spinning off these operations. Moreover, as part of a review of the company's capital structure, Clariant management is mulling the issuing of new shares to overcome a decline in equity capital caused by the extraordinary charges and negative currency effects. Any concrete proposals, the company said, would be announced within the month.-PATRICIA SHORT
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