Properties of urea–water solvation calculated from a new a b i n i t i o polarizable intermolecular potential

Abstract

In this work, we present a quantum chemical-based urea–water potential which includes many-body effects by using explicit polarizabilities, and mimics the dipole and quadrupole moments of the individual molecules. We compare this potential in detail with explicit ab initio calculations and with other potentials used in molecular simulations of urea–water systems. Several deficiencies in earlier work are pointed out, such as the quality of the basis set, the accuracy of the electrostatics, and polarization effects. In the minimum energy configuration of the urea–water dimer, the water molecule is engaged in two hydrogen bonds with urea, forming a cyclic structure, with an energy of −10.9 kcal/mol. In order to test the reliability of using polarizable molecular models in simulations of dipolar solutes in liquid water, a molecular dynamics simulation with one urea molecule and 210 water molecules was carried out. The hydration of urea is characterized by the ability of urea to fit into the water structure as a ‘‘waterlike’’ molecule. The dynamics of the system was investigated by studying diffusion and relaxation processes. The relative values of the diffusion coefficients for urea and bulk water are preserved, but as in a previous polarizable water simulation, their magnitudes are too small. The shell waters are more tightly bound to urea than bulk waters are to each other. Supported by previous simulations, we conclude that urea does not act as a water structure breaker and that this effect is unimportant in biological systems.