B-side electron transfer in the HE(M182) reaction center mutant from rhodobacter 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 wildtype 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*.