Reaction centers of Rhodobacter sphaeroides R26 containing C-3 acetyl and vinyl (bacterio)pheophytins at sites HA,B.

The native bacteriopheophytin a in reaction centers of Rb. sphaeroides R26 has been exchanged with modified bacteriopheophytins (bacteriochlorins), as well as with plant-type pheophytins (chlorins). Emphasis is on four pigments, which differ by their C-3 substituents (vinyl or acetyl) or their state of oxidation (chlorin or bacteriochlorin). The native BPhe a, which is a member of this group, can be replaced by the other three at both binding sites, HA and HB. However, exchange at HB proceeds more readily. Optical spectra (absorption, cd) show characteristic shifts, and the cd spectra indicate induced interactions between HA,B and BA,B and possibly also with P. Upon flash illumination, all modified reaction centers show reversible electron transfer to QB with recombination times comparable to native reaction centers. Forward rates and electron-transfer yields are also reported for some of the pigments.