Postulated effects on water structure of some salts and protein denaturants as inferred from measurements of viscosity B coefficients: example of HbS polymerization.

Viscosity measurements have been performed on aqueous solutions of some solutes commonly used in biochemical practice, with a view to determine the B coefficients and their temperature dependence. The temperature dependence measured for some anions leads to a ranking of the latter in an order similar to that reported earlier from entropy data for their postulated efficiency in disrupting "water structure". The well known dependence of denaturing power of guanidinium salts on the anions is shown to be related to the temperature dependence of B coefficients. Urea and formamide do not appear from this criterion to be significantly "structure-breaking"; alkyl-substituted derivatives of urea and formamide, on the other hand, possess significant "structure-forming" properties. The results are of interest in relation to the known effects of salts and other solutes on the stability of protein structures in respect of denaturation, subunit dissociation or self assembly. A typical application is illustrated by studying the effect of two anions, namely nitrate and sulphate, on the polymerization of sickle cell hemoglobin (HbS). Nitrate, which is believed to disrupt water structure, acts as an inhibitor of HbS polymerization which, on the contrary, is favoured by sulphate, a postulated structure- former.