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.

Coulombic interactions of salt ions with polymeric and oligomeric nucleic acids in solution have large and distinctive effects on ion distributions, on thermodynamic coefficients, and hence on equilibrium processes involving nucleic acids, such as their conformational transitions and binding interactions. In experimental or theoretical studies where an oligonucleotide is taken to represent the corresponding polynucleotide, the impact of coulombic end effects on molecular and thermodynamic properties must be taken into account. Observable consequences of coulombic interactions in nucleic acid solutions have been calculated by using models with varying degrees of detail and methods formulated at varying levels of rigor. From comparisons of experimental results with predictions of the prevalent theoretical approaches, this review concludes that the more rigorous methods have proved capable of accounting for thermodynamic (and some molecular) consequences of coulombic interactions with a minimal number of preaveraged parameters that represent the most important structural features of the nucleic acid solution.

Effects of salt concentration on the stabilities of oligonucleotide helices are analyzed directly in terms of delta gamma N----yN identical to gamma denyN - gamma natN, the difference in the salt-nucleotide phosphate preferential interaction coefficients for the denatured state, having yN phosphate charges, and for the native state, having N phosphate charges (y = 1 for hairpin denaturation and y = 0.5 for dimer denaturation). Previous experimental studies of the denaturation of hairpin oligonucleotides (having 18 less than N less than 44) indicate significant differences between delta gamma N----N and delta gamma infinity, the value determined for the denaturation of the corresponding polynucleotide. These differences are thermodynamic manifestations of the oligoelectrolyte end effect. In contrast, the available data on the denaturation of oligonucleotide dimer helices (N less than or equal to 22) imply that differences between delta gamma infinity and delta gamma N----0.5N, and hence oligoelectrolyte end effects, are small or negligible. To determine the origin of these apparently conflicting implications concerning the importance of oligoelectrolyte end effects, we have calculated the N dependence of gamma N from grand canonical Monte Carlo simulations for an idealized model of the structure and charge distribution of each oligomer conformation. Our calculations are in quantitative agreement with the experimental finding for d(TA) hairpin oligomers that -delta gamma N----N decreases linearly as N-1 increases, and with the extant experimental determinations of delta gamma N----0.5N. These results provide an illustration of how the large electrostatic end effects exhibited by the hairpin denaturation data are masked when delta gamma infinity is compared with values of delta gamma N----0.5N for short dimer helices (N less than or equal to 22). For 0.5N greater than 24, -delta gamma N----0.5N is predicted to be a linear function of N-1 whose slope has the opposite sign from, and is more salt-concentration dependent than, the corresponding slope of -delta gamma N----N as a function of N-1. Our calculations also yield predictions about the N dependences of the individual values of gamma N that can be tested by determining Donnan coefficients from membrane dialysis equilibrium experiments. For long enough hairpin and dimer oligonucleotides (yN greater than or equal to 24), in either native or denatured forms, we predict that the (positive) difference gamma infinity - gamma N increases linearly with increasing N-1. For smaller values of N the difference gamma infinity - gamma N continues to increase with increasing N-1.

The thermodynamic consequences of interactions of native bovine serum albumin (BSA) with two smaller solutes (glycine betaine or urea) in aqueous solution are characterized by a novel application of vapor pressure osmometry (VPO), which demonstrates the utility of this method of investigating preferential interactions involving solutes that are either accumulated or excluded near the surface of a protein. From VPO measurements of osmolality (water activity) as a function of the solute concentration in the presence and absence of BSA, we determine the dependence of the solute molarity (C3) on that of BSA (C2) at fixed temperature (37 degrees C), pressure (approximately 1 atm), and osmolality (over the range 0-1.6 molal). After some thermodynamic transformations, these results yield values of [formula: see text] which characterizes the interdependence of solute molalities when temperature, pressure, and the chemical potential of solute 3 are fixed. This form of the preferential interaction coefficient can be interpreted directly in terms of the molecular exclusion or accumulation of the solute (relative to water) near the protein surface. Within experimental uncertainty, [formula: see text] is proportional to m3 both for glycine betaine (0-0.9 m) and for urea (0-1.6 m). For glycine betaine [formula: see text] = -49 +/- 4, a value consistent with the interpretation that this solute is completely excluded from the hydrated surface of BSA, whereas for urea [formula: see text] = 6 +/- 1, which indicates a moderate extent of accumulation at the surface of native BSA. The preferential accumulation of solutes (e.g., urea) that have some binding affinity for a protein can be quantified and interpreted using the two-domain model if the extent of hydration of the protein has been determined using a completely excluded solute (e.g., glycine betaine). Complete exclusion from the local hydration domain surrounding proteins, if general, justifies the use of glycine betaine as a thermodynamic probe of the changes in hydration that accompany protein folding, protein association, and protein-ligand binding interactions.

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