Effect of urea on the structural dynamics of water

Abstract

We use polarization-resolved mid-infrared pump-probe spectroscopy to study the effect of urea on the structure and dynamics of water. Surprisingly, we find that, even at high concentrations of urea (8 M), the orientational dynamics of most water molecules are the same as in pure liquid water, showing that urea has a negligible effect on the hydrogen-bond dynamics of these molecules. However, a small fraction of the water molecules (approximately one water molecule per urea molecule) turns out to be strongly immobilized by urea, displaying orientational dynamics that are more than six times slower than in bulk water. A likely explanation is that these water molecules are tightly associated with urea, forming specific urea-water complexes. We discuss these results in light of the protein denaturing ability of aqueous urea.hydrogen bonding ͉ infrared pump-probe spectroscopy ͉ orientational dynamics ͉ protein denaturation S olutions of urea in water display a number of interesting properties: hydrocarbons dissolve more readily in them than in pure water, and concentrated solutions can be used to denature proteins in a reversible way. The desire to understand these properties has triggered a great deal of research regarding the structure of aqueous solutions of urea (1-13). An important question that is encountered throughout the literature is to what extent the hydrogen-bond network of water is perturbed by the incorporation of a urea molecule, as one of the models explaining protein denaturation by urea is built on the assumption that urea strongly alters the hydrogen-bond structure of water (14). The urea-water system has been studied by using a variety of experimental and theoretical techniques, all of which shed light on a different aspect of the system. Linear infrared and Raman spectroscopies can provide structural information about the hydrogen-bond network, but they depend on simulations to unravel the effect of intermolecular interactions on spectral band shapes (15). Neutron diffraction experiments produce atom-atom radial distribution functions, and, as such, form more direct methods to obtain structural information (7). A different class of experimental techniques probes the hydrogen-bond network by observing the dynamics of water molecules. A stiffening of the network is revealed by the slowing down of the water dynamics, whereas faster dynamics are indicative of the weakening of the network. Among these methods are dielectric relaxation (5), NMR (6), and optical Kerr effect (OKE) spectroscopy (8). Dielectric relaxation and OKE provide dynamical information down to the picosecond time scale but probe the response of the solution as a whole, making it difficult to separate the water response from the urea response. NMR experiments selectively probe the dynamics of water, but determine a timeaveraged response, so that water molecules in the urea solvation shell cannot be distinguished from molecules in the bulk. As a result, it is not yet clear to what extent the hydrogen-bond structure of water is change...

Toward an atomistic description of the urea-denatured state of proteins

“…The physical nature of the direct interactions between proteins and urea is also subject of debate, with some authors suggesting that it is mostly electrostatic and related to the formation of direct hydrogen bonds (5,7,12,13), and others suggest that dispersion interactions are the main factor (14,15). Some authors supported the idea the denaturing role of urea is not related to the formation of direct urea-protein interactions, but to its ability to "dry" the protein, weakening the hydrophobic effect responsible for stabilizing protein structures (16)(17)(18)(19). However, recent consensus is that this indirect mechanism is not the main explanation of the effect of urea (18)(19)(20), pointing instead to the direct mechanism.…”

Entropic and Enthalpic Contributions to Annexin V-Membrane Binding

“…The EC 50 and calcium stoichiometry (n) values were determined by non-linear least-squares fitting of the calcium titration curves to the following function,…”

“…Consistent with earlier work, the EC 50 values were substantially lower for cardiolipin compared with PS or phosphatidylglycerol. However, the lower EC 50 value was offset by a compensating decrease in slope, leading to a calculated pK d value (cf. Equation 4) that was the same as for PS.…”

“…The ⌬G value was thus calculated from a model developed by Bazzi and Nelsestuen (24), in which one estimates a stepwise EC 50 value at various levels of membrane occupancy for titrations carried out from 0 to 100% occupancy and then calculates the stepwise ⌬G by assuming a constant calcium stoichiometry at all occupancies (Fig. 2, A and B).…”

Rattling the cage: Micro- to mesoscopic structure in liquids as simple as argon and as complicated as water

“…[85] However, infrared pump-probe experiments have indicated that ions have a negligible effect on the structure of water. [20,56,86,87] The ability to measure rotational and translational motions independently in liquid water by using DR and OKE spectroscopy, has been exploited in the study of aqueous salt solutions. [70] The salts NaCl and MgCl 2 were chosen since the monatomic ions do not introduce additional dipole moments or anisotropic polarizabilities, thus ensuring that the measured dynamics is entirely due to the water with no contribution from the rotation of the ions.…”