Light-dependent accumulation of new bacteriochlorophyll-e bearing a vinyl group at the 8-position in the green sulfur bacterium Chlorobaculum limnaeum

Abstract

The green sulfur bacterium Chlorobaculum limnaeum producing bacteriochlorophyll(BChl)-e was anaerobically cultured by illumination using red LED light at around 660nm. When the bacterium was cultured at the intensity of approximately 10,000lux (high-light conditions), a pair of new chlorophyll pigments showing the identical electronic absorption and mass spectra was produced at remarkable amounts. The pigments exhibited the Soret and Qy bands at absorption maxima of 480 and 656nm, respectively, in tetrahydrofuran. These maxima were red-shifted compared with those of the original BChl-e at 467 and 651nm, indicating that the vinyl group was substituted at the 8-position. The pigments were identified as epimers at the 31-position, and the molecular structures, including the stereo-configurations, were determined to be (31R/S)-8-vinyl-12-ethyl-BChl-e esterified with a C15 farnesyl moiety as the 17-propionate residue using mass spectrometry and 1H NMR spectroscopy as well as chemical modification. The accumulation of 8-vinylated BChl-e supports the idea that irradiation with intense red light suppresses the activity of divinyl reductase (BciA), which is responsible for hydrogenation of the 8-vinyl group of precursory divinyl-chlorophyllide-a during the biosynthesis of BChl-e.