MULTISTATE GAUSSIAN MODEL FOR ELECTROSTATIC SOLVATION FREE-ENERGIES

We develop and test a multistate Gaussian model for the distribution o f electrostatic solvation energies of a solute in liquid water. The mu ltistate Gaussian model depends on the discovery of simple indicators of structural substates that individually display Gaussian fluctuation s of electrostatic interactions. The probability distribution of elect rostatic interactions is then modeled as a superposition of Gaussian d istributions of electrostatic interactions of the substates. We find t hat the number of hydrogen bonds to the solute is a suitable substate indicator that eliminates the chief failures of single Gaussian models for the distribution of electrostatic interactions and of quadratic m odels of the electrostatic contribution to the excess chemical potenti al. These results should improve calculations of ionic chemical proces ses in water, i.e., acid-base chemistry, in particular those involving organic acids such as proteins and nucleic acids. The multistate Gaus sian approach provides a specific and effective alternative to commonl y discussed electrostriction and dielectric saturation modifications o f dielectric continuum models. Moreover, the representation of complex energy distributions by a sum of simpler distributions based on struc tural substates is general and should be applicable in a variety of th ermodynamic problems of solution chemistry.