Hydration and ion binding

Counterions can affect the strocture of hydration regions, and conversely hydration regions can affect ion binding. We have already touched on this subject in discussing contact and solvent-separated ion pairs in Section 4.2.8. 

Divalent cations cause a much greater disruption of the hydration 

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The osmotic effect of protein is also affected by hydration and ion binding. When a protein is sedimented in the ultracentrifuge, a certain amount of water and salt moves with the protein. The amount of bound water is referred to as the selective solvation. The selective solvation decreases the osmotic effect of proteins. In contrast, the presence of bound ions in the protein increases the osmotic effect of the protein. 

In the model, the smaller ion gives the stronger ion binding. However, it seems that in most cases other factors, e.g., ion hydration and ion polarizability, dominate over the ion size effect, particularly for anions, (cf. Sect. 4). 

Ions are hydrated and they bind water in their solvation shells, causing less free water to be available to accommodate other solutes. Disregarding any direct ion-solute interactions, the presence of ions in a solution then cause an elevation of the activity coefficient yN of a non-electrolyte solute, marked by subscript N, in a ce molar solution of an electrolyte by a factor 1/[1 — (Vw/1000)/zece] in molar units. Here / e is the sum of the hydration numbers of the ions constituting the electrolyte. 

Ion binding is affected by the size and charge of the counterion, the charge and conformation of the polyion, and states of hydration. We will examine these effects in some detail. 

The extent of ion binding depends on a number of characteristics of the polyion degree of dissodation, acid strength, conformation, distribution of ionizable groups and cooperative action between these groups (Wilson Crisp, 1977 Oosawa, 1971 Harris Rice, 1954, 1957). The hydration state of the macromolecule, which is in turn dependent on conformation, also affects ion binding (Begala, 1971). 

The number of binding sites can be determined in this model by a plot of d Ink /dlnm at constant temperature, pH, and ion valency. To do that, it may be assumed that dlny /dlnm is approximately zero. The actual value is -0.04 for 0.1 to 0.5 M sodium chloride and less at lower concentrations. To a first approximation, the stoichiometry of water molecules released by binding protein could be determined from the slope of the plot of dink /dlnm vs. m. However, especially at low salt concentration and near the isoelectric point, the slope of such plots is nonlinear. The nonlinearity may be due to hydrophobic interaction between stationary phase and protein or a large change of ionic hydration on binding.34... 

Cevc, G. Watts, A. Marsh, D., Titration of the phase transition of phosphatidylserine bilayer membranes. Effects of pH, surface electrostatics, ion binding, and head group hydration. Biochemistry 20, 4955 -965 (1981). 

Figure 3.5 Structure (a), porphyrin (b) and metal ion-binding sites (c) in Bacillus subtilis fer-rochelatase. In (c) the two metal ions are a Zn2+ ion (grey) and a fully hydrated Mg2+ ion. The side chains of Glu272, Asp 268 and Glu264 are aligned along a tr-helix (green). (From Al-Karadaghi et al., 2006. Copyright 2006, with permission from Elsevier.)...

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