Self-aggregation of synthetic zinc bacteriochlorophyll-d analogs with pinacol boronates on the B-ring

Abstract

Zinc 3-hydroxymethyl-131-oxo-bacteriochlorins possessing a variety of pinacol boronates on the B-ring were prepared from chemical modification of naturally occurring chlorophyll-a. The synthetic bacteriochlorophyll-d analogs were dissolved into an aqueous Triton X-100 solution to exhibit red-shifted and broadened electronic absorption bands with reversed S-shape circular dichroism bands, compared with their monomeric bands. The behaviors indicated that most synthetic models possessing a bacteriochlorin π-system self-aggregated inside the aqueous micelles to give similar oligomers as the J-aggregates of bacteriochlorophyll-d bearing a chlorin π-skeleton in major light-harvesting antennas of green photosynthetic bacteria, chlorosomes. The methylboronate self-aggregated more largely than the phenylboroante, and the self-aggregation was suppressed by ortho-substitution on the phenyl ring: phenyl > 1-naphthyl ≫ 9-anthryl and phenyl > 2,6-dimethylphenyl. Along with an increase in the size of the substituents on the boron atom of their boronates, the formation of the chlorosome-like self-aggregates decreased substantially.