The molar extinction coefficient of bacteriochlorophyll e and the pigment stoichiometry in Chlorobium phaeobacteroides

We have determined the molar extinction coefficient of bacteriochlorophyll (BChl) e, the main light-harvesting pigment from brown-coloured photosynthe tic sulfur bacteria. The extinction coefficient was determined using pure [ Pr,E]BChl e(F) isolated by reversed-phase HPLC from crude pigment extracts of Chlorobium (Chl.) phaeobacteroides strain CL1401. The extinction coeffic ients at the Soret and Q(y) bands were determined in four organic solvents. The extinction coefficient of BChl e differs from those of other related C hlorobium chlorophylls (BChl c and BChl d) but is similar to that of chloro phyll b. The determined extinction coefficient was used to calculate the st oichiometric BChl e to BChl a and BChl e to carotenoids ratios in whole cel ls and isolated chlorosomes from Chl. phaeobacteroides strain CL1401 using the spectrum-reconstruction method (SRCM) described by Naqvi et al. (1997) (Spectrochim Acta A Mol Biomol Spectrosc 53: 2229-2234). In isolated chloro somes, BChl a content was ca. 1% of the total BChl content and the stoichio metric ratio of BChl e to carotenoids was 6. In whole cells, however, BChl a content was 3-4%, owing to the presence of BChl a-containing elements, i. e. FMO protein and reaction centre. An average of 5 BChl e molecules per ca rotenoid was determined in whole cells.