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 bacteriochlo rophyll dimer (P) that serves as the electron donor in the photochemic al charge-separation reaction of photosynthetic bacterial reaction cen ters; it also is close to the monomeric bacteriochlorophyll (B-L) that probably accepts an electron in this reaction. Electrostatics calcula tions and molecular-dynamics simulations were performed to explore the preferred orientation and the time-dependent fluctuations of the phen olic OH dipole of the tyrosine and to examine the effects of replacing the tyrosine residue by other amino acids. In resting reaction center s, the OH dipole was found to point toward B-L in a way that would fav or formation of the P+BL- ion pair. The molecular-dynamics simulations indicated that the most probable orientation of the OH dipole does no t change significantly upon charge separation but that the potential w ell 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 re lative to the ground state and to increase the P/P+ reduction potentia l (E(m)) by 37-70 mV. The calculated effects on the E(m) are in accord with experimental observations on mutant reaction centers. The effect s on the free energy of P+BL- appear to be sufficient to raise this st ate above the excited singlet state (P), which would explain why char ge separation slows and becomes temperature dependent in the mutants. We also examined the effects of mutations of Phe (L)181, which occupie s a position homologous to Tyr (M)210 on the opposite side of the reac tion 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 e nergy of P+BL- by about 1 kcal/mol relative to the ground state, in ac cord with the observation that this mutation increases the rate of cha rge separation.