Solvation of the Guanidinium Ion in Pure Aqueous Environments. A Theoretical Study from an "Ab Initio"-based Polarizable Force Field.
JOURNAL OF PHYSICAL CHEMISTRY B(2017)
摘要
We report simulation results regarding the hydration process of the guanidinium cation in water droplets and in bulk liquid water, at a low concentration of 0.03 M, performed using a polarizable approach to model both water/water and ion/water interactions. In line with earlier theoretical studies, our simulations show a preferential orientation of guanidinium at water-vacuum interfaces, i.e., a parallel orientation of the guanidinium plane to the aqueous surface. In an apparent contradiction with earlier simulation studies, we show also that guanidinium has a stronger propensity for the cores of aqueous systems than the ammonium cation. However, our bulk simulation conditions correspond to weaker cation concentrations than in earlier studies, by 2 orders of magnitude, and that the same simulations performed using a standard nonpolarizable force field leads to the same conclusion. From droplet data, we extrapolate the guanidinium single hydration enthalpy value to be -82.9 +/- 2.2 kcal mol(-1). That is about half as large as the sole experimental estimate reported to date, about -144 kcal mol(-1). Our result yields a guanidinium absolute bulk hydration free energy at ambiant conditions to be -78.4 +/- 2.6 kcal mol(-1), a value smaller by 3 kcal mol(-1) compared to ammonium. The relatively large magnitude of our guanidinium hydration free energy estimate suggests the Gdm(+) protein denaturing properties to result from a competition between the cation hydration effects and the cation/protein interactions, a competition that can be modulated by weak differences in the protein or in the cation chemical environment.
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