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 neat the photoactive bacteriopheophytin (BPh(L)) is characterized usin g ultrafast transient absorption spectroscopy. At 285 K, initial charg e separation from P proceeds with essentially unity quantum yield in similar to 6 ps to form a transient denoted P+I-. This transient is pr oposed to involve P(+)BPh(L)(-) and probably P(+)BChl(L)(-) as well (B Chl(L) is the L-side bacteriochlorophyll molecule). P+I- decays in sim ilar to 150 ps both by electron transfer to give P(+)Q(A)(-) (similar to 78% yield) and by charge recombination to the ground state (similar to 22% yield). These results indicate that the F(L121)D mutant is clo sely related, in terms of its electron transfer properties, to previou sly reported RCs in which BPh(L) is replaced with a bacteriochlorophyl l (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 ra ises the free energy of P(+)BPh(L)(-), thereby giving rise to the alte red photochemistry. At 77 K, the P+I- lifetime is shortened slightly t o similar to 120 ps and the yield of P(+)Q(A)(-) is increased to simil ar to 88%. This result is somewhat different from that obtained for be ta-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 t o the values near room temperature. We exploit these differences in de veloping a model for the charge separation process in the F(L121)D mut ant. The effects of introducing an Asp near BPh(L) are compared to tho se obtained previously in two mutants in which an Asp is introduced ne ar BChl(L).