Stereoselectivity of pigment exchange with 13(2)-hydroxylated tetrapyrroles in reaction centers of Rhodobacter sphaeroides R26.

Bacteriochlorophyll a and bacteriopheophytin a carry a stereochemically labile asymmetric carbon at position C13(2). The steric requirements of photosynthetic reaction centers from Rhodobacter sphaeroides R26 have been probed by exchange experiments with the respective epimeric 13(2)-hydroxylated pigments, in which epimerisation is blocked. (13(2)S)-13(2)-Hydroxy-bacteriochlorophyll a is accepted at both monomeric binding sites, BA,B, (13(2)S)-13(2)-hydroxy-bacteriopheophytin a exclusively at the inactive site HB. The orientation of the 13(2)-COOCH3 substituents in these pigments is the same as in the native (13(2)R)-bacteriochlorophylls and (13(2)R)-bacteriopheophytins. In no cases are the unnaturally configured 13(2)-hydroxylated (13(2)R)-epimers accepted, even if a large excess (> 95%) is offered. It is concluded that the three binding sites always require the 13(2)-COOCH3 group on the opposite side of the macrocycle (down) than the 17-propionic ester side chain (up).