Strong Effects of an Individual Water Molecule on the Rate of Light-driven Charge Separation in the Rhodobacter sphaeroides Reaction Center*

The role of a water molecule (water A) located be-tween the primary electron donor (P) and first electron acceptor bacteriochlorophyll (BA) in the purple bacte-rial reaction center was investigated by mutation of glycine M203 to leucine (GM203L). The x-ray crystal structure of the GM203L reaction center shows that the new leucine residue packs in such a way that water A is sterically excluded from the complex, but the structure of the protein-cofactor system around the mutation site is largely undisturbed. The results of absorbance and resonance Raman spectroscopy were consistent with ei-ther the removal of a hydrogen bond interaction be-tween water A and the keto carbonyl group of BA or a change in the local electrostatic environment of this carbonyl group. Similarities in the spectroscopic prop