Preferential hydration model

Extensive literature has developed related to the preferential interaction of different solvents with proteins or peptides in bulk solution.156-5X1 Similar concepts can be incorporated into descriptions of the RPC behavior of peptides and employed as part of the selection criteria for optimizing the separation of a particular peptide mixture. As noted previously, the dependency of the equilibrium association constant, /CassoCji, of a peptide and the concentration of the solvent required for desorption in RPC can be empirically described1441 in terms of nonmechanistic, stoichiometric solvent displacement or preferential hydration models, whereby the mass distribution of a peptide P, with n nonpolar ligands, each of which is solvated with solvent molecules Da is given by the following ... 

The greater than unit value of Kf in pure solvents is the result expected by the field effect model (33) on a distance basis. The lower than unit value ofKj in the mixed aqueous organic solvents appears to be related to preferential hydration of the reaction center, which results in an increased effective dielectric constant from the m- compared to the p- position. The fact that values fall into two separate categories for pure organic and mixed aqueous organic solvents does not support the treatments of Exner (20) or Yukawa and Tsuno (16). 

In the preferential interaction model, the equilibrium association constant, K.ISVIC, for a peptide docked to a RPC sorbent can be related via eq 6 to the changes in the preferential interaction of the solvent and water with the peptide P- the ligands L, and the peptide-ligand complex PtL as a function of solvent concentration, [3]m. The effect of preferential hydration/solvation of a peptide on k can be represented in terms of the following expression t85l... 

The effect of salts on the biopolymers is clearly shown by the results of v. Hippel and Wong218 (Fig. 32). Salts with possibility of giving salt-in effects reduce Tm of ribonuclease. Salt-out effects giving ions like sulfate increase 7, they reduce the interactions of biopolymere bulk water. Using models, there are opinions that proteins are preferentially hydrated in the presence of SO4 , less with Cl- > Br- > CNS- > J-218. If this is true, weaker interaction with the bulk water should strengthen the stability of fixed hydrates on the polymers. 

Support for a concerted model for the yeast enzyme has come from X-ray small angle scattering experiments (162) as well as from hydro-dynamic and optical rotation studies (163, 164). A. volume contraction of about 5% occurs on binding of NAD to the apoenzyme, presumably related to tightening of the tetramer and expulsion of water mojecules. The relation between NAD bound (R) and change of volume (Y) was hyperbolic, in accord with the concerted model. It was lator shown (166) from buoyant density and preferential hydration studies that water is indeed excluded from the yeast enzyme on binding to NAD, such that a volume contraction of about 6% occurs. Furthermore, fluorimetric and calorimetric titrations over the range 6°-40° showed independence of... 

It has been also indicated experimentally that ectoine enhances the thermodynamic stabilities of their folded (native) structures [24c]. This observation has been explained by the preferential exclusion model, which states that CS molecules are expelled from the protein surface [28,29] and the growth of the preferential exclusion corresponds with the increase of excess chemical potential of the protein [28,29]. In fact, onr previous MD simulation also indicated numerically that ectoine molecnles are preferentially excluded near the CI2 surface [39]. Thus, to understand how CS molecnles interact microscopically with proteins, and whether the addition of CS might indirectly stabilize them irrespective of their molecular properties, the hydration strnctnres have been studied not only for CI2 but also for a smaller... 

The Ion-Dipole Model. In this model ion-dipole forces are the principal forces in the ion-water interaction. The result of these forces is orientation of water molecules in the immediate vicinity of an ion (Fig. 2.11). One end of the water dipole is attached electrostatically to the oppositely charged ion. The result of this orienting force is that a certain number of water molecules in the immediate vicinity of the ion are preferentially oriented, forming a primary hydration shell of oriented water molecules. These water molecules do not move independently in the solution. Rather, the ion and its primary water sheath is a single entity that... 

The Preferential Sorption/Capillary Flow Model (Sourirajan and Matsuufa (1985)) is based on the assumption that a layer of water sorbs at the membrane surface, creating a deficit of solute at the surface. The membrane is viewed as a microporous medium, and transport is controlled by the surface chemistry of the membrane and water transport through the membrane. Ions with large hydrated radii are retained better, since they also have to overcome more energy to strip off the water. Ions diffuse through the laj et of strucmred water at the membrane surface and through water cluster channels in the membrane (Staude (1992)), where B is the pure water permeability of the membrane. 

It was shown previously [11] that the positions of preferentially bound water molecules in lysozyme crystals and in solution are similar, as may be tested from the few water molecules fixed in crystaUographic work, on the one hand, and our hydration algorithms, on the other. In this study, the comparative analysis of a variety of proteins in terms of volume, surface and hydration parameters and the respective models shows that the advanced surface and hydration calculations are able to obtain biologically realistic models for the hydrated proteins, irrespective of their nature, size and composition. As first approximations, even simple estimations based on the AA composition may supply reasonable results concerning volume and hydration. 

RGENSEN - 1) Simulation of the step from t-BuCl to the contact ion pair is difficult because it corresponds to an electronic curve crossing in the gas phase. The ground state in the gas phase dissociates to t-Bu + CF, while a highly excited state correlates with the ion pair. The latter surface is, of course, preferentially stabilized by hydration. To model the initial step would consequently require results for both gas-phase surfaces and potential functions for water interacting with the substrate at all points along those surfaces. 

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