CAROTENOID TRIPLET-STATE FORMATION IN RHODOBACTER-SPHAEROIDES R-26 REACTION CENTERS EXCHANGED WITH MODIFIED BACTERIOCHLOROPHYLL PIGMENTS AND RECONSTITUTED WITH SPHEROIDENE

Triplet state electron paramagnetic resonance (EPR) experiments have b een carried out at X-band on Rb. sphaeroides R-26 reaction centers tha t have been reconstituted with the carotenoid, spheroidene, and exchan ged with 13(2)-OH-Zn-bacteriochlorophyll a and [3-vinyl]-13(2)-OH-bact eriochlorophyll a at the monomeric, 'accessory' bacteriochlorophyll si tes B(A,B) or with pheophytin a at the bacteriopheophytin sites H(A,B) . The primary donor and carotenoid triplet state EPR signals in the te mperature range 95 - 150 K are compared and contrasted with those from native Rb. sphaeroides wild type and Rb. sphaeroides R-26 reaction ce nters reconstituted with spheroidene. The temperature dependencies of the EPR signals are strikingly different for the various samples. The data prove that triplet energy transfer from the primary donor to the carotenoid is mediated by the monomeric, BChl(B) molecule. Furthermore , the data show that triplet energy transfer from the primary donor to the carotenoid is an activated process, the efficiency of which corre lates with the estimated triplet state energies of the modified pigmen ts.