Isolation and characterization of a new bacteriochlorophyll-c bearing a neopentyl substituent at the 8-position from the bciD-deletion mutant of the brown-colored green sulfur bacterium Chlorobaculum limnaeum

We recently constructed the mutant of the brown-colored green sulfur bacterium Chlorobaculum limnaeum lacking BciD which was responsible for formation of a formyl group at the 7-position in bacteriochlorophyll(BChl)-e biosynthesis. This mutant exclusively gave BChl-c, but not BChl-e, as the chlorosome pigments (Harada et al. in PLoS One 8(4):e60026, 2013). By the mutation, the homolog and epimer composition of the pigment was drastically altered. The methylation at the 8²-position in the mutant cells proceeded to create BChl-c carrying large alkyl substituents at this position. Correspondingly, the content of BChls-c having the (S)-configuration at the chiral 3¹-position remarkably increased and accounted for 80.6 % of the total BChl-c. Based on the alteration of the pigment composition in the mutant cells, a new BChl-c bearing the bulkiest, triple 8²-methylated neopentyl substituent at the 8-position (N,E]BChl-c) was identified. The molecular structure of N,E]BChl-c was fully determined by its NMR, mass, and circular dichroism spectra. The newly identified N,E]BChl-c was epimerically pure at the chiral 3¹-position and its stereochemistry was determined to be an (S)-configuration by modified Mosher’s method. Further, the effects of the C8²-methylation on the optical absorption properties of monomeric BChls-c were investigated. The Soret but not Qy absorption bands shifted to longer wavelengths by the extra methylation (at most 1.4 nm). The C8²-methylation induced a slight but apparent effect on absorption properties of BChls-c in their monomeric states.