Estimating Hydration

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It has become common practice to specify a value of 0.25 or 0.3 for δ1, with little justification. For unconjugated proteins, typical values of "hydrodynamic hydration" range between 0.2-0.6 g-water/g-protein (Ref. 55). It would seem that glycosylation should tend to increase this value, although there is little evidence in the literature on it. For collagen, a fibrous protein, δ1 has been estimated to be 0.48 g-water/g-protein (Ref. 53).

One estimate of the degree of hydration can be made based on the amino acid composition and using the method of Kuntz (Ref. 51). This method relies on the amount of "unfreezable" water being an accurate reflector of hydration as it relates to hydrodynamics. The calculation is simply:


Where Hi is the hydration assigned to each amino acid in the phyconst database and Ni is the number of residues of type i. There are two sets of values for , one set is used at pH 4.0 and below and the second is used at all other pH values and was measured at pH 7.0. Estimates of δ1 calculated using equation 26 are not otherwise adjusted for the effects of pH, but are reported to be valid to within 10% above pH 6. Values for Ji at pH 4 reflect the lower hydration expected of the carboxyl groups when they are protonated. The quantity 18/Mrconverts the value from the relative molar scale to the relative mass scale.

Use of equation 26 assumes that all of the amino acids are exposed to solvent, which for globular proteins clearly cannot be the case. The expectation is that δ1 would tend to be overestimated. Although there have been studies correlating predicted hydration using equation 26 with "non-freezable" water (Ref. 51 54), there has been minimal systematic exploration of whether these values correlate with hydrodynamic properties. However, empirical evidence suggest that calculated with this method is "better" than anticipated, and the values of are useful in determining Φ2 from equation 9 (Ref. 42).