Thermodynamics of aqueous solvation: Solution properties of alcohols and alkanes

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Abstract
Molecular dynamics is used, with thermodynamic-cycle perturbation theory, to calculate relative Helmholtz free energies of hydration, ΔΔAsolv, for ethane, propane, methanol, and ethanol. The thermodynamic derivative properties ΔΔEsolv and ΔΔSsolv are also calculated from direct finite difference methods. The calculated values of ΔΔAsolv for hydrophobic→hydrophilic ‘‘mutations’’ (and the reverse) are in excellent agreement with the experimental values of ΔΔGsolv. This is believed to be the case because this transformation is dominated by the change in electrostatic interactions between solute and solvent. Similar agreement is not found for transformations between hydrophobic moieties where solute–solvent electrostatic interactions play no role. Changes to the existing model are suggested, namely the inclusion of explicit hydrogens containing partial atomic charges, and their evaluation through the use of ‘‘free energy’’ simulations is outlined.