Partial Pair Correlation Functions of Highly Concentrated Aqueous Urea Solutions Determined by Neutron Diffraction with 14N/15N and H/D Isotopic Substitution Methods

Abstract

Abstract Neutron diffraction measurements were carried out on 14N/15N and H/D isotopically substituted aqueous 25 mol % urea (xurea = 0.25) solutions in order to obtain direct experimental information on the short-range structure on the urea–urea, urea–water, and water–water interactions in highly concentrated aqueous urea solutions near saturation. Scattering cross sections observed for isotopically distinct solutions were successfully combined to derive intermolecular partial structure factors, aNNinter(Q), aNHinter(Q), aNO(NC)inter(Q), aHHinter(Q), aXHinter(Q), and aXXinter(Q) (X: O, N, and C), and corresponding intermolecular partial pair correlation functions. The nearest neighbor intermolecular N···O and N···H distances have been obtained as 3.14(1) and 3.42(1) Å, respectively, indicating that the amino group of the urea molecule forms hydrogen bonds with neighboring water molecules in such a highly concentrated solution. The nearest neighbor intermolecular N···N interaction was found at intermolecular distance of 4.71(5) Å with a coordination number of 3.1(5), which evidences that the hydrogen-bonded urea oligomers exist in the present solution. The nearest neighbor intermolecular O···H distance has been found to be 1.90(1) Å, which indicates that hydrogen bonds among the solvent water molecules are still preserved in the present solutions. On the other hand, corresponding O···H coordination number 0.83(1) is much smaller than that reported for room temperature liquid water. The nearest neighbor intermolecular H···H and O···O distances were determined to be 2.37(5) and 2.86(2) Å, respectively. The corresponding H···H and O···O coordination numbers were obtained as 2.0(5) and 1.4(1), respectively. These values are also much smaller than those observed for the pure water. The present results suggest that the hydrogen-bonded network among water molecules is considerably modified due to the nearest neighbor urea–water and urea–urea interactions, which can be strongly related with protein denaturation ability of the concentrated aqueous urea solutions.