Charge Asymmetries in Hydration of Polar Solutes

David L. Mobley, Alan E. Barber II, Christopher J. Fennell, and Ken A. Dill*
Department of Pharmaceutical Chemistry, and Department of Biopharmaceutical Science, University of California at San Francisco, San Francisco, California 94143
J. Phys. Chem. B, 2008, 112 (8), pp 2405–2414
DOI: 10.1021/jp709958f
Publication Date (Web): February 6, 2008
Copyright © 2008 American Chemical Society

Abstract

We study the solvation of polar molecules in water. The center of water's dipole moment is offset from its steric center. In common water models, the Lennard-Jones center is closer to the negatively charged oxygen than to the positively charged hydrogens. This asymmetry of water's charge sites leads to different hydration free energies of positive versus negative ions of the same size. Here, we explore these hydration effects for some hypothetical neutral solutes, and two real solutes, with molecular dynamics simulations using several different water models. We find that, like ions, polar solutes are solvated differently in water depending on the sign of the partial charges. Solutes having a large negative charge balancing diffuse positive charges are preferentially solvated relative to those having a large positive charge balancing diffuse negative charges. Asymmetries in hydration free energies can be as large as 10 kcal/mol for neutral benzene-sized solutes. These asymmetries are mainly enthalpic, arising primarily from the first solvation shell water structure. Such effects are not readily captured by implicit solvent models, which respond symmetrically with respect to charge.

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History

  • Published In Issue February 28, 2008
  • Received October 12, 2007
    Revised December 3, 2007

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