Solvent Effects on Processes in Aqueous Solutions

In this chapter we shall rederive some of the theories dealt with in the previous chapters and see where the solvent might modify the theory. We shall also discuss some new processes where the solvent plays a crucial role the association and folding of proteins, the formation of micelles, or the aggregation of biological molecules. 

There are essentially two classes of effects that the solvent can have on a given process. These can be demonstrated by the following example. Consider an equilibrium system where p, is the equilibrium concentration of a species i. This can be a Hgand, a polymer with a specific number of ligands, or a one-dimensional system with a specific configuration. The equilibrium concentration (p,)eq is related to the chemical potential of that species, in the ideal gas phase by 

In the liquid phase, the relation between the chemical potential and the equilibrium density of i can be modified in two ways. First, there is the coupling work of i against the entire solvent, which we denote by W i l). Second, the solvent molecules can perturb the internal degrees of freedom of the solvated molecule. The latter is essentially a quantum-mechanical problem. We shall always assume that the effect of the solvent on the 

if we measure the equilibrium density (p/)eq with respect to the same reference state, say a pure solid then the equilibrium densities (pOfq and (p,)iq will be different only because of the coupling work W i l). The internal PF qi is assumed to be the same in (8.1.1) and (8.1.2). 

If the molecule i has internal rotational degrees of freedom, then each conformation of i will be treated as a different species having different coupling work. This will effectively account for the effect of the solvent on the equilibrium distribution of the confor-mers. Proteins are examples of such molecules. These are treated in the subsequent sections of this chapter. 

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