The purpose of this review is to examine the various effects of low- molecular-weight electrolytes on the associations and interactions of proteins and nucleic acids. Our primary interest is in general electrostatic effects, rather than chemical effects (specific interactions) of particular ions (e.g. transition metals, protons). We consider those interactions in which a variation in salt concentration has a significant effect on the macromolecular equilibrium, and analyse the effects of salt in these situations in terms of (i) direct participation of ions in the biopolymer reaction, (ii) Debye—Hückel screening by salt ions of the charge interactions on the biopolymers, and (iii) the reduction in water activity brought about at high salt concentrations.

Although the importance of the polyelectrolyte character of DNA has been recognized for some time (Felsenfeld & Miles 1967), few of the implications have been explored, primarily because of a lag in translating the breakthroughs in polyelectrolyte theory of the last decade into a form that is well adapted to the analysis of the specialized problems of biophysical chemistry. Perhaps an analogous situation existed in the field of protein chemistry during the period after the formulation and confirmation of the Debye—Hückel theory of ionic solutions but before Scatchard's incorporation of the theory into his analysis of the binding properties of proteins. An achievement for polynucleotide solutions parallel to Scatchard's was recently presented by Record, Lohman, & de Haseth (1976) and further developed and reviewed by Record, Anderson & Lohman (1978).