Specific Interactions of Water with Biopolymers

Abstract

There is a widespread confusion about the terms “hydration” and “bound water” in relation to biopolymers, both in solution and in disperse systems. The amount of “bound water” is very much dependent on the experimental techniques and on the definitions and computational procedures used to determine its magnitude. If such definitions and procedures are not given, the term “bound water” should preferably not be used at all. Among the definitions that have been used, implicitly or explicitly, are the following:

• (a) The water that is bound in equilibrium at a certain relative humidity and temperature, or the amount of water that is retained after a prescribed drying procedure.

• (b) The water that fills the first adsorbed monolayer, in connection with a certain theoretical interpretation of the adsorption isotherm (usually the BET theory).

• (c) The water that is not available for the solvation of solutes, depending on the type of solute.

• (d) The water that does not freeze at a sharp transition temperature, e.g., the normal freezing point. This amount may depend on the technique used to determine unfrozen water.

• (e) The water that gives no orientational contribution to the dielectric constant at high frequencies, and hence is not free to rotate with respect to the macromolecule to which it is attached. Such definitions depend on the frequency under consideration.

• (f) The water that moves with the macromolecule in sedimentation, diffusion, or viscosity experiments, increasing the hydrodynamic size of the macromolecule.

• (g) The water that shows a slower rate of rotation as measured by magnetic resonance techniques.

• (h) The water that shows a slower rate of self-diffusion.

• (i) The water that is shown by X-ray or neutron diffraction to occupy regular molecular positions with respect to a macromolecule in a crystal.

• (k) The water that deviates from the normal liquid in density, as determined by light scattering or small-angle X-ray diffraction of macromolecular solutions.

• (l) The water that can be shown by infrared or raman spectroscopy to be engaged in hydrogen bonding to a macromolecule.