B-Side Electron Transfer in the HE(M182) Reaction Center Mutant fromRhodobacter sphaeroides

The histidine (H) ligand of the bacteriochlorophyll monomer molecule on the B-side of the photosynthetic reaction center (RC) from Rhodobacter (Rb.) sphaeroides was replaced with a glutamic acid residue (E) (mutant HE(M182)). The photochemical properties of this mutant are markedly different from those of wild-type RCs. The excited state of the initial electron donor (P*) decays with a lifetime of 2.8 ± 0.1 ps, which is about 10% faster than in wild-type RCs. The faster decay of the excited state is due to an additional electron-transfer pathway in the mutant from P to the monomer bacteriochlorophyll on the B-side (B B ) of the RC, forming the state P + B B - . The initial yield of the B-side electron transfer is estimated at about 35%, whereas the remaining 65% of P* leads to electron transfer along the A-side pigments forming the charge-separated state P + H A - . The P + B B - state formed during initial charge separation decays with a lifetime of 45 ps. Of the 35% P + B B - initially formed, 10% decays to form P + H A - via back electron transfer to P* and subsequent A-side charge separation. The other 25% of the state P + B B - recombines to the ground state. There is no observable further electron transfer from P + B B - to the B-side bacteriopheophytin molecule, H B . Apparently, P + H B - is at least as high in free energy as is P + B B - in this mutant, preventing further B-side electron transfer. From analysis of the long-lived fluorescence kinetics and transient absorbance data, the standard free energy of the state P + B B - in the HE(M182) mutant is estimated to be 70 meV below P*. Thus, the standard free energy of the state P + H B -, which should be similar in the mutant and the wild-type RCs, is apparently less than or equal to 70 meV below P*.