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, P(F), were measured in reaction centers from Rhodopseudomonas sphaeroides under conditions that blocked electron transfer to a later carrier (a quinone, Q). The temperature dependence of the absorbance changes suggests that P(F) is an equilibrium mixture of two states, which appear to be mainly (1)[P([unk])B([unk])] and (1)[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 of (1)[P([unk])B([unk])] is about 0.025 eV above that of (1)[P([unk])H([unk])], (1)[P([unk])H([unk])] can decay to a triplet state, P(R), 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 P(R) indicates that (3)P lies 0.40 eV below the excited singlet state, P(*), which is about 0.05 eV above (1)[P([unk])H([unk])]. The (1,3)[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 10(6) times larger than the splitting between (1)[P([unk])H([unk])] and (3)[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.