Bacteriochlorophyll g epimer as a possible reaction center component of heliobacteria

Abstract

Normal-phase HPLC analysis of acetone extracts of cells, membranes and antenna-reaction center complexes of Heliobacterium chlorum and Heliobacillus mobilis showed the presence of bacteriochlorophyll (BChl) g′, the 132-epimer of BChl g. The molar ratio of BChl g:BChl g′ in these preparations was approx. 18 in cells and membranes of both species. This value, when combined with a molar ratio of BChl g to the primary electron donor, P-798, of 35–40, yields a BChl g′:P-798 ratio of 2, suggesting that P-798 may be a dimer of BChl g′. The amount of BChl g′ in the antenna-reaction center complex of H. chlorum was slightly higher, suggesting that some epimerization may have occurred during the isolation of the complex. In contrast, bacteriopheophytins (BPhe) g and g′ were present in too small amouts to be essential components of the photosynthetic apparatus. This confirms the idea that the reaction center of heliobacteria is basically different from those of purple bacteria and Photosystem II. A detailed interpretation, based on Correlated Spectroscopy and Double Resonance experiments is given of the 1H-NMR spectra of BChl g and BChl g′, confirming the structure and identity of both pigments. Absorption and circular dichroism spectra of BChl g and BChl g′ are also presented.