Synthesis of 32-substituted bacteriochlorophyll-d analogs and their self-aggregation in a nonpolar organic solvent

Zinc complexes of 31-epimerically pure 32,32,32-trifluoro-bacteriopheophorbide-d methyl ester were prepared and their self-aggregation was examined in 1% (v/v) dichloromethane and hexane by visible and circular dichroism spectroscopies. Both the synthetic 31-trifluoromethylated diastereomers gave amorphous self-aggregates in the nonpolar organic solvent, while the corresponding nonfluorinated compounds possessing the 31-methyl group formed well-ordered and large oligomers similar to in vivo aggregates of bacteriochlorophyll-d in a chlorosome, a main light-harvesting antenna of green photosynthetic bacteria. The difference is ascribed to steric factor of the 3-CF3 group, which was supported by optical properties of other synthetic zinc 31-hydroxy-131-oxo-chlorins possessing trifluoromethyl or secondary/tertiary substituents at the 31-position in the nonpolar organic solvent.