Resonance Raman characterization of Rhodobacter capsulatus reaction centers with lysine mutations near the accessory bacteriochlorophylls

Lysine residues have been introduced into Rhodobacter capsulatus reaction centers at M-polypeptide position 201 and at L-polypeptide position 178. These positions are in the proximity of ring V of the accessory bacterochlorophylls B_A and B_B, respectively. Resonance Raman studies indicate that the introduction of a Lys residue at either position M201 or L178 results in structural perturbations to the BChl cofactors. Lys at L178 directly interacts with B_B, most likely via a hydrogen bond. The hydrogen bonding interaction is consistent with enhanced B branch electron transfer that is observed in RCs from the S(L178)K/G(M201)D/L(M212)H triple mutant versus the G(M201)D/L(M212)H double mutant (C. Kirmaier et al. (1999) Biochemistry 38 11516-11530). In contrast, the introduction of a Lys at M201 does not result in hydrogen bonding to the B_A cofactor, in contrast to the introduction of a His at M201 (L. Chen et al. (2004) J Phys Chem 3 108: 0457-10464). Accordingly, the alkyl ammonium head group of the side chain of the Lys at M201 residue appears to be distant from B_A.