Electrostatics in Macromolecular Solutions - Food Colloids: Self-Assembly and Material Science - page 135

Figure 4 Binding constant shifts as a function of protein net charge: ——, DH theory; O,
MC simulations. The protein is modelled as a sphere of radius 14 A ˚ with two
binding sites close to the surface. The shift refers to a change in salt concen-
tration from 1 to 500 mM. The protein concentration is 20 m M and the binding
process involves two divalent ions
The electrostatic model in colloid chemistry has always been one with
a uniform dielectric permittivity for the whole system, typically chosen to be
equal to that of water. In the calculations reported above we have followed this
tradition. Obviously, the dielectric permittivity of a protein is different from
that of bulk water; but we do not know its exact value; and, to be more formal,
it is not a well-defined quantity. We also note that charged species prefer the
high dielectric region - ions dissolve in water and not in oil! Another way to
express it is to say that the electric field lines remain in the aqueous phase.
Hence a small body of low dielectric material has only a marginal effect on the
electrostatic interactions. These conclusions are supported by a wealth of
experimental results on colloidal systems.
In biophysics, the opposite paradigm prevails: the low dielectric interior of a
protein is usually assumed to be the clue to many properties of biochemical
interest. The electrostatic approach is based on the PB or DH equation.
A technical feature with the 'low dielectric' assumption is that the calculations
contain a divergence, which can cause numerical problems. Or it can be used as
a 'fitting parameter'. The divergence in electrostatic calculations invoking the
low dielectric region is apparent in many applications. One such clear example
relates to the apparent pK a values in the protein calbindin (Table 3), which have
been determined experimentally by Kesvatera et al. 12 and theoretically by
Spassov and Bashford 13 assuming a low dielectric response for the protein.
Juffer and Vogel 14 have extended the DH calculations of Spassov and Bashford
and allowed for a high dielectric response from the protein. The paper by