Orientation of the OH dipole of tyrosine (M)210 and its effect on electrostatic energies in photosynthetic bacterial reaction centers

The side chain of tyrosine (M)210 is located close to the bacteriochlorophyll dimer (P) that serves as the electron donor in the photochemical charge-separation reaction of photosynthetic bacterial reaction centers; it also is close to the monomeric bacteriochlorophyll (B-L) that probably accepts an electron in this reaction. Electrostatics calculations and molecular-dynamics simulations were performed to explore the preferred orientation and the time-dependent fluctuations of the phenolic OH dipole of the tyrosine and to examine the effects of replacing the tyrosine residue by other amino acids. In resting reaction centers, the OH dipole was found to point toward B-L in a way that would favor formation of the P+BL- ion pair. The molecular-dynamics simulations indicated that the most probable orientation of the OH dipole does not change significantly upon charge separation but that the potential well constraining the dipole deepens and the frequency of oscillations about the minimum increases. Replacing Tyr (M)210 by Phe, Ile, or Trp was calculated to increase the free energy of P+BL- by 4-5 kcal/mol relative to the ground state and to increase the P/P+ reduction potential (E(m)) by 37-70 mV. The calculated effects on the E(m) are in accord with experimental observations on mutant reaction centers. The effects on the free energy of P+BL- appear to be sufficient to raise this state above the excited singlet state (P*), which would explain why charge separation slows and becomes temperature dependent in the mutants. We also examined the effects of mutations of Phe (L)181, which occupies a position homologous to Tyr (M)210 on the opposite side of the reaction center's axis of pseudosymmetry. Replacing Phe (L)181 by Tyr was calculated to lower the E(m) of P/P+ by 55 mV and to reduce the free energy of P+BL- by about 1 kcal/mol relative to the ground state, in accord with the observation that this mutation increases the rate of charge separation.