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

摘要

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, 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 P^R 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.