Resolution of in situ Interaction Energies between Molecules and the Reaction Center Q_A Site: Implications for Electron Transfer Function

Abstract

We are interested in identifying the features of the cofactor-protein interaction which allow quinones and other molecules to perform intraprotein electron transfer reactions when bound at the primary quinone, or Q_A, site of the photosynthetic reaction center protein (RC) of Rhodobacter sphaeroides R26. This requires quantitation of the energetic contributions of specific, non-bonded contacts between cofactor and protein, such as hydrogen bonds and other strong electrostatic interactions. The strengths of these interactions are highly sensitive to the charge state of the conjugate oxidized and reduced forms (see 1), and thus influence the free energy gap for the electron transfer reaction through modulation of the quinone in situ midpoint potential (E_m).