Thermodynamics of Hydrogen Bonding in Hydrophilic and Hydrophobic Media

The thermodynamics of hydrogen bond breaking and formation was studied in solutions of alcohol (methanol, ethanol, 1-propanol) molecules. An extensive series of over 400 molecular dynamics simulations with an aggregate length of over 900 ns was analyzed using an analysis technique in which hydrogen bond (HB) breaking is interpreted as an Eyring process, for which the Gibbs energy of activation ΔG⧧ can be determined from the HB lifetime. By performing simulations at different temperatures, we were able to determine the enthalpy of activation ΔH ⧧ and the entropy of activation TΔS ⧧ for this process from the Van't Hoff relation. The equilibrium thermodynamics was determined separately, based on the number of donor hydrogens that are involved in hydrogen bonds. Results (ΔH) are compared to experimental data from Raman spectroscopy and found to be in good agreement for pure water and methanol. The ΔG as well as the ΔG ⧧ are smooth functions of the composition of the mixtures. The main result of the calculations is that ΔG is essentially independent of the environment (around 5 kJ/mol), suggesting that buried hydrogen bonds (e.g., in proteins) do not contribute significantly to protein stability. Enthalpically HB formation is a downhill process in all substances; however, for the alcohols there is an entropic barrier of 6−7 kJ/mol, at 298.15 K, which cannot be detected in pure water.