Self-aggregation of zinc bacteriochlorophyll-d analog bearing B-ring reduced chlorin and 17-acrylate residue

Abstract

Abstract As a model of bacteriochlorophyll(BChl)-d specifically found in major light-harvesting antennae (chlorosomes) of photosynthetic green bacteria, zinc methyl 7,8-dihydro-17,18,171,172-tetradehydro-bacteriopheophorbide-d was prepared from naturally occurring BChl-a through the double dehydration of the cis-17,18-diol. The synthetic B-ring reduced Zn-BChl-d analog bearing the 17-acrylate residue in tetrahydrofuran exhibited a more bathochromically shifted Soret band in the monomeric state than its counterpart with the 17-propionate residue, while the former's Qy band slightly moved to a longer wavelength and was somewhat broadened in comparison with the latter's. Compared with the monomeric bands, the acrylated pigment self-aggregated in 1% tetrahydrofuran and hexane to give more red-shifted and broadened Soret and Qy bands at blue-to-green and far-red regions, respectively, which were similar to the J-aggregates of BChl-d in chlorosomes. By contrast, the propionated pigment remained monomeric under the same conditions. These spectral differences were owing to the 17-substitution effect including the π-conjugating and electron-withdrawing 17-acrylate residue.