Modified reaction centers from Rhodobacter sphaeroides R26 : Exchange of monomeric bacteriochlorophyll with 13 2-hydroxy-bacteriochlorophyll

Abstract

Incubation of photosynthetic reaction centers from Rhodobacter sphaeroides R26 with exogenous 13 2-OH-bacteriochlorophyll a p or a GG according to Scheer et al. (1987) results in the exchange of endogenous bacteriochlorophyll a p. The exchange amounts to ⩽ 50% according to HPLC analysis, corresponding to a complete replacement of the ‘monomeric’ bacteriochlorophylls, b m and b l, by exogenous pigment. The absorption spectra show small, but distinct changes in the Q x-region of the bacteriochlorophylls, and bleaching of the modified reaction centers is retained. The corresponding binding sites must be accessible from the exterior, and allow for the introduction of a polar residue at C-13 2. This is supported by the observation of side reactions of the endogenous ‘monomeric’ bacteriochlorophylls within the reaction center pigments, e.g. epimerization and hydroxylation at C-13 2.