A photosynthetic reaction-centre complex is found in certain green sulphur bacteria such as Chlorobium vibrioforme which are anaerobic photo-auto-trophic organisms. The primary electron donor is P840, a probable B-Chl a dimer, and the primary electron acceptor is a B-Chl monomer. Also on the donor side c-type cytochromes are known to function as electron donors to photo-oxidised P840. This family is thus the secondary endogenous donor of the photosynthetic reaction-centre complex and is a membrane-bound cytochrome containing a single haem group.

1: A membrane-bound monoheme cytochrome c551 of a novel type is the immediate electron donor to P840 of the Chlorobium vibrioforme photosynthetic reaction center complex. Okkels JS, Kjaer B, Hansson O, Svendsen I, Moller BL, Scheller HV; J Biol Chem. 1992;267:21139-21145. PMID:1383218