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ION–SOLVENT INTERACTIONS 193

Fig. 2.78. A schematic drawing of protein
and water. The regions of the protein and
the excluded volume of it are represented
as A and A + B, respectively. The regions
of solute (low dielectric media) and solvent
(high dielectric media) are represented as
A and B + C, respectively. (Reprinted from
M. Irisa, T. Takahashi, F. Hirata, and T.
Yanagida, J. Mol. Liquids 65: 381, 1995.)

As with normal electrolytes (Section 2.12.1), the dielectric constant depends
linearly on concentration:

where:

Here c is the concentration in gram percent, is the partial specific volume of the
protein, is the high-frequency value of the dielectric constant of the protein (= 2)
or of water (=6) and is that. The unknown hereis the number of irrotationally
bound waters and this can then be calculated. It is usual to assume that the water is
adjacent to charged groups.

2.24.1. A Model for Hydration and Diffusion of Polyions

For polyelectrolytes containing discrete charged groups, the usual configuration
is cylindrical. Thus, the field midway along the chain is described by Conway (1982)
as