Activity coefficient comparisons of forcefields: High precision chemical potentials of urea

We performed molecular dynamics and free energy simulations of urea solutions at different concentrations with two urea models (OPLS and KBFF) and experiment in order to examine the structures responsible for the thermodynamic solution properties. Our simulation results showed that hydrogen bonding properties such as the average number of H-bonds, and their lifetime distribution were nearly constant at all concentrations between infinite dilution and the solubility limit. This implies that the characterization urea-water solutions in the molarity concentration scale as nearly ideal is a result of facile local hydrogen bonding rather than a global property. We calculated the activity coefficients by thermodynamic integration, perturbation, and the Bennett acceptance ratio method. The physicochemical origin of non-ideality of urea solution was considered. Urea displays almost a constant excess chemical potential values at different concentrations due to the cancellation between the vdW and electrostatic part of the chemical potential. The strong ideality of urea solutions, implying a lack of water perturbation, is related to recent ideas on the mechanism of urea denaturation of proteins.