Solubility, crystallization and chromatographic properties of macromolecules strongly depend on substances that reduce the ionic strength of the solution.

The differential solubility of macromolecules governs their crystallization and is thus of great importance to crystallography (Blundell and Johnson, 1976; McPherson, 1982, 1985a,b, 1990; Feher and Kam, 1985; Giegé and Mokol, 1989; Ollis and White, 1990; Giegé and Ducruix, 1992; Giegé et al., 1994). In addition, precipitation is widely applied as a fractionation method for the purification of macromolecules (Ingham, 1984, 1990; Scopes, 1987; Harris, 1989; Roe, 1989; Englard and Seifter, 1990; Kennedy, 1990; Rossomando, 1990). The literature offers several observations of factors which influence macromolecular solubility, among which organic solvents and polyethylene glycol (PEG) play a major role: it is known that salts have a ‘poisoning effect’ in PEG crystallization experiments (McPherson, 1982, 1985b, 1990). Dioxane (McPherson, 1982), PEG 400 (Ray, 1992) and glycerol (Sousa and Lafer, 1990) may enhance salting-out crystallization with ammonium sulfate. Additionally, after ethanol precipitation, more salt is needed for salting-out precipitation (Harris, 1989). In this paper, we attempt to rationalize these empirical observations and to derive some basic principles that determine macromolecular solubility in aqueous electrolytic solutions that contain organic solvents or PEG. Understanding these principles may help in the development of more rational strategies in macromolecular crystallization and in the use of ion-exchange and hydrophobic interaction chromatography. For conciseness, organic solvents or PEG will be termed ‘ionic strength reducers’ or simply ‘reducers’. The solubility of macromolecules in electrolytic aqueous solutions depends on the ionic strength of the solute as expressed by Green’s general equation (Arakawa and Timasheff, 1985; Ries-Kautt and Ducruix, 1992):