Energies and kinetics of radical pairs involving bacteriochlorophyll and bacteriopheophytin in bacterial reaction centers

Absorbance changes reflecting the formation of a transient radical-pair state, PF , were measured in reaction centers fromRhodopseudomonas sphaeroides under conditions that blocked electron transfer to a later carrier (a quinone, Q). The temperature dependence of the absorbance changes suggests that PF is an equilibrium mixture of two states, which appear to be mainly1 [P[unk] B[unk] ] and1 [P[unk] H[unk] ]. P is a bacteriochlorophyll dimer, B is a bacteriochlorophyll absorbing at 800 nm, and H is a bacteriopheophytin. In the presence of Q[unk] , the energy of1 [P[unk] B[unk] ] is about 0.025 eV above that of1 [P[unk] H[unk] ],1 [P[unk] H[unk] ] can decay to a triplet state, PR , which also is an equilibrium mixture of two states, separated by about 0.03 eV. The lower of these appears to be mainly a locally excited triplet state of P,3 P; the upper state contains a major contribution from a triplet charge-transfer state,3 [P[unk] B[unk] ]. The temperature dependence of delayed fluorescence from PR indicates that3 P lies 0.40 eV below the excited singlet state, P* , which is about 0.05 eV above1 [P[unk] H[unk] ]. The1,3 [P[unk] B[unk] ] charge-transfer states thus appear to interact with the locally excited states of P and B to give singlet and triplet states that are separated in energy by about 0.35 eV. This is 106 times larger than the splitting between1 [P[unk] H[unk] ] and3 [P[unk] H[unk] ] and implies strong orbital overlap between P[unk] and B[unk] . This is consistent with recent picosecond studies which suggest that electron transfer from P* to B occurs within 1 ps and is followed in 4 to 10 ps by electron transfer from B[unk] to H.