The reaction centres of most bacterial species contain three polypeptides (MW 20,000, 24,000 and 30,000). The BChl, BPh and carotenoids are all located on the first two polypeptides, which only are essential for photochemical activity. The reaction centre of Rhodopseudomonas gelatinosa has only two polypeptides -(MW 24,000 and 34,000).
In the primary photochemical reaction light quanta absorbed by the light harvesting pigments migrate to the reaction centre BChl (P810). The energy of the absorbed photons raises chlorophyll to an excited state and an electron is expelled from the molecule. P870 thus requires a positive charge and becomes P870+.BChl (P870)-->P870+ (oxidized BChl complex)
The excited P870+ transfers an electron to an intermediate electron acceptor (I) which is reduced to (I-). According to one interpretation, I consist of interacting BChl and bacteriopheophytin (BPh) molecules. It appears that one of the two BPhs of the reaction centre is a major component of I.
The reduced intermediate acceptor (I-) than transfers the electron to another acceptor (X) which appears to be a quinone (X-->X-). In Chromatium venosum and some other species menaquinone is the primary acceptor instead of ubiquinone.
X- releases the electron to another quinone, which is reduced. The secondary quinone rapidly transfers a pair of electrons to an external pool of quinones.
