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 (BPhL) 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+BPhL- and probably P+BChlL- as well (BChlL is the L-side bacteriochlorophyll molecule). P+I- decays in ∼150 ps both by electron transfer to give P+QA- (∼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 BPhL is replaced with a bacteriochlorophyll (beta-type RCs) or a pheophytin. However, the native BPhL pigment is retained in the F(L121)D mutant. We propose that the Asp at L121 raises the free energy of P+BPhL-, the...