Stereoselectivity of Pigment Exchange with 13²-Hydroxylated Tetrapyrroles in Reaction Centers of Rhodobacter Sphaeroides R26

Bacteriochlorophyll a and bacteriopheophytin a carry a stereochemically labile asymmetric carbon at position C13². The steric requirements of photosynthetic reaction centers from Rhodobacter sphaeroides R26 have been probed by exchange experiments with the respective epimeric 13²-hydroxylated pigments, in which epimerisation is blocked. (13²S)-13²-Hydroxy-bacteriochlorophyll a is accepted at both monomeric binding sites, B_{A, B}, (13²S)-l3²-hydroxy-bacteriopheophytin a exclusively at the inactive site H_B. The orientation of the 13²-COOCH₃ substituents in these pigments is the same as in the native (13²R)-bacteriochlorophylls and (13²R)-bacteriopheophytins. In no cases are the unnaturally configured 13²-hydroxylated (13²R)-epimers accepted, even if a large excess (>95%) is offered. It is concluded that the three binding sites always require the 13²-COOCH₃ group on the opposite side of the macrocycle (down) than the 17-propionic ester side chain (up).