Effects of Asp residues near the L-side pigments in bacterial reaction centers

The primary photochemistry in Rhodobacter capsulatus reaction centers (RCs) containing the Phe to Asp mutation at L polypeptide residue 121 near the photoactive bacteriopheophytin (BPh(L)) is characterized using ultrafast transient absorption spectroscopy. At 285 K, initial charge separation from P* proceeds with essentially unity quantum yield in ~6 ps to form a transient denoted P⁺I^-. This transient is proposed to involve P⁺BPh(L)^- and probably P⁺BChl(L)^- as well (BChl(L) is the L-side bacteriochlorophyll molecule). P⁺I^- decays in ~150 ps both by electron transfer to give P⁺Q(A) (~78% yield) and by charge recombination to the ground state (~22% yield). These results indicate that the F(L121)D mutant is closely related, in terms of its electron transfer properties, to previously reported RCs in which BPh(L) is replaced with a bacteriochlorophyll (beta-type RCs) or a pheophytin. However, the native BPh(L) pigment is retained in the F(L121)D mutant. We propose that the Asp at L121 raises the free energy of P⁺BPh(L)^- , thereby giving rise to the altered photochemistry. At 77 K, the P⁺I^- lifetime is shortened slightly to ~120 ps and the yield of P⁺Q(A)^- is increased to ~88%. This result is somewhat different from that obtained for beta-type RCs at low temperature, where the P⁺I^- lifetime lengthens and the yield of P⁺Q(A) diminishes or stays about the same compared to the values near room temperature. We exploit these differences in developing a model for the charge separation process in the F(L121)D mutant. The effects of introducing an Asp near BPh(L) are compared to those obtained previously in two mutants in which an Asp is introduced near BChl(L).