Role of Neutral Salts on the Denaturation Temperature of Biopolymers

Abstract

IN recent years, a great deal of attention has been devoted to the study of the effect of salts on the denaturation of biopolymers, both in the bulk state (for example, shrinkage of fibrous collagen) and in dilute solutions (for example, helix-coil transitions)^1,2. Recent work in this laboratory³ has demonstrated that two main factors must be considered in describing the effects of both salt type and salt concentration on the shrinkage of collagen at neutral pH (when polyelectrolyte effects are absent). These are: the diluent and the specific effect. The former effect is predominant for salting-out agents, such as potassium chloride and magnesium sulphate, whereas the latter occurs with salting-in agents, such as potassium thiocyanate and lithium bromide. The diluent effect is satisfactorily described by the conventional melting point depression theory developed for binary polymer-diluent systems where T_m is the melting temperature of the polymer in the presence of the diluent, T_m° the melting temperature of the undiluted polymer, ΔH_u is the heat of fusion per mole of repeating unit, V_u the molar volume of the repeating unit, V₁ the molar volume of the diluent, ζ₁ the volume fraction of diluent, and χ₁ represents a polymer–diluent interaction parameter which can be interpreted³ in terms of conventional salting-out theories⁴.