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

Abstract

Absorbance changes reflecting the formation of a transient radical-pair state, PF, were measured in reaction centers fform 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 2 states, which appear to be mainly .**GRAPHIC**. and .**GRAPHIC**. P is a bacteriochlorophyll dimer, B is a bacteriochlorophyll absorbing at 800 nm and H is a bacteriopheophytin. In the presence of .**GRAPHIC**. the energy of .**GRAPHIC**. is about 0.025 eV above that of .**GRAPHIC**. .**GRAPHIC**. can decay to a triplet state, PR, which also is an equilibrium mixture of 2 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, .**GRAPHIC**. 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 .**GRAPHIC**. The .**GRAPHIC**. 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 .**GRAPHIC**. and .**GRAPHIC**. and implies strong orbital overlap between .**GRAPHIC**. and .**GRAPHIC**. This is consistent with recent picosecond studies which suggest that electron transfer form P* to B occurs within 1 ps and is followed in 4-10 ps by electron transfer from .**GRAPHIC**. to H.