The structure of aqueous guanidinium chloride solutions.

The combination of neutron diffraction with isotopic substitution (NDIS) experiments and molecular dynamics (MD) simulations to characterize the structuring in an aqueous solution of the denaturant guanidinium chloride is described. The simulations and experiments were carried out at a concentration of 3 m at room temperature, allowing for an examination of any propensity for ion association in a realistic solution environment. The simulations satisfactorily reproduced the principal features of the neutron scattering and indicate a bimodal hydration of the guanidinium ions, with the N-H groups making well-ordered hydrogen bonds in the molecular plane, but with the planar faces relatively deficient in interactions with water. The most striking feature of these solutions is the rich ion-ion ordering observed around the guanidinium ion in the simulations. The marked tendency of the guanidinium ions to stack parallel to their water-deficient surfaces indicates that the efficiency of this ion as a denaturant is due to its ability to simultaneously interact favorably with both water and hydrophobic side chains of proteins.