Isolation and structure determination of a complete set of bacteriochlorophyll-d homologs and epimers from a green sulfur bacterium Chlorobium vibrioforme and their aggregation properties in hydrophobic solvents

Abstract

Eight bacteriochlorophyll (BChl)-d homologs and epimers were isolated from a strain of the green sulfur bacterium Chlorobium vibrioforme. By a combination of mass spectrometry and ¹H-NMR spectroscopy using the chemical shifts of meso- and 3¹-protons and ¹H–¹H NOE correlations, the molecular structures were determined as (3¹R)-8-ethyl-12-methyl, (3¹R)-8-ethyl-12-ethyl, (3¹R)-8-propyl-12-methyl, (3¹S)-8-propyl-12-methyl, (3¹R)-8-propyl-12-ethyl, (3¹S)-8-propyl-12-ethyl, (3¹S)-8-isobutyl-12-methyl and (3¹S)-8-isobutyl-12-ethyl. The aggregation behavior of the epimerically pure BChls-d in hydrophobic organic solvents was examined to investigate the absolute configuration of the 3-(1-hydroxyethyl) group as well as the bulkiness of the C8 and C12 side-chains by using electronic-absorption and fluorescence-emission spectroscopies. At high concentration of the BChls-d in CH₂Cl₂, the absolute configuration of the 3-(1-hydroxyethyl) group governed the formation of a subunit as a building block for the subsequent higher assembly. Upon dilution of the resulting subunit with hexane, the bulkiness of the C8 and C12 side-chains were found to affect the association of the subunits differently: the bulkiness of the C8 side-chain acted as a promoter for the association due to a stabilized hydrophobic interaction among the relevant larger side-chain, whereas the bulkiness of the C12 side-chain acted as an inhibitor for that association due to introduction of a particular steric-hindrance around the side-chain in the aggregates.