ELECTROSTATIC CALCULATIONS OF AMINO-ACID TITRATION AND ELECTRON-TRANSFER, Q(A)(-)Q(B)-]Q(A)Q(B)(-), IN THE REACTION-CENTER

The titration of amino acids and the energetics of electron transfer f rom the primary electron acceptor (Q(A)) to the secondary electron acc eptor (Q(B)) in the photosynthetic reaction center of Rhodobacter spha eroides are calculated using a continuum electrostatic model. Strong e lectrostatic interactions between titrating sites give rise to complex titration curves. Glu L212 is calculated to have an anomalously broad titration curve, which explains the seemingly contradictory experimen tal results concerning its pK(a). The electrostatic field following el ectron transfer shifts the average protonation of amino acids near the quinones. The pH dependence of the free energy between Q(A)(-) Q(B) a nd Q(A)Q(B)(-) calculated from these shifts is in good agreement with experiment. However, the calculated absolute free energy difference is in severe disagreement (by similar to 230 meV) with the observed expe rimental value, i.e., electron transfer from Q(A)(-) to Q(B) calculate d to be unfavorable. The large stabilization energy of the Q(A)(-) sta te arises from the predominantly positively charged residues in the vi cinity of Q(A) in contrast to the predominantly negatively charged res idues near Q(B). The discrepancy between calculated and experimental v alues for Delta G(Q(A)(-)Q(B) --> Q(A)Q(B)(-)) points to limitations o f the continuum electrostatic model. Inclusion of other contributions to the energetics (e.g., protein motion following quinone reduction) t hat may improve the agreement between theory and experiment are discus sed.