Stereoselective self-aggregation of zinc bacteriochlorophyll-d analogs possessing an O-substituted oxime moiety at the 13-position

Abstract

Zinc methyl 3-hydroxymethyl-131-deoxo-pyropheophorbides-a possessing a variety of O-substituted oxime moieties at the 13-position were prepared as models of bacteriochlorophyll-d, which could be found in the main light-harvesting system of green photosynthetic bacteria by chemical modification of naturally occurring chlorophyll-a. The E/Z-stereoisomers of the synthetic O-substituted oximes were successfully separated by silica gel column chromatography. Most of these zinc bacteriochlorophyll-d analogs formed J-type self-aggregates in an aqueous Triton X-100 micelle solution, affording large oligomers whose Soret and Qy bands were red-shifted, compared with those of their monomers in tetrahydrofuran. The self-aggregation was dependent on their E/Z-stereochemistry. Thus, the E-stereoisomers showed the red-shifted electronic absorption bands, which were similar to those of the original bacteriochlorophyll-d bearing the 131-oxo group. On the other hand, the Z-stereoisomers exhibited the less red-shifted absorption bands than did the E-stereoisomers, to give different self-aggregates. In addition, the self-aggregation could be controlled by the size of the O-substituents in the oxime moiety.