Theory and Modeling of Specific Ion Effects in Chemistry and Biology

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Thomas Beck of the University of Cincinnati is supported by the Theory, Models and Computational Methods program in the Chemistry Division to carry out computational studies of specific ion or Hofmeister effects for anions. This work employs a novel theoretical approach for the statistical mechanics of liquids, the quasi-classical theory (QCT). The QCT partitions the excess chemical potential into three contributions by enacting a spatial partitioning with a hard-sphere constraint: 1) the cavity free energy for digging a hole in the solvent 2) the free energy for placing the solute into the cavity, and 3) the free energy to release the hard-sphere constraint, allowing direct contact between the solute and solvent. For ion solvation problems, step 2) above yields the largest contribution to the hydration free energy. It was found that a mean-field estimate of this term is quite accurate. This method permits computations of free energies at the quantum mechanical level, and in binding sites in proteins.Specific ion or Hofmeister effects are very important in a wide array of chemical, physical, and biological systems, for example the swelling of biological membranes, solvation free energies and activities, surface tension increments, bubble interactions, ion channel transport, colloid interactions, pH measurements, buffer behavior and protein folding and stability. Professor Beck is involved in organizing a workshop on ions in chemistry and biology to be held in Telluride CO. He is also heavily involved in undergraduate and graduate curriculum development.