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

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Absorbance changes reflecting the formation of a transient radical-pair state, PF, 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 PF 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, 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, 3P; 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 that 3P 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 106 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.

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