EFFECT OF SALT ON THE STABILITY OF THE PH-4.2-RA8 DOUBLE HELIX IN A SERIES OF ORGANIC-AQUEOUS MIXED-SOLVENTS - TEST OF OLIGOELECTROLYTE THEORY

Temperature‐dependent uv absorption spectroscopy has been used to investigate the salt dependence of the order–disorder transition for the pH 4.2 rA8 double helix in 100% aqueous buffer and in a series of organic/aqueous mixed solvents. Melting temperature, Tm, data were obtained for the transitions in the different solvents by analysis of the uv melting curves. For the pure aqueous buffer solvent, the melting temperature was found to exhibit a reduced salt dependence (∂tm/∂ log Na+) when compared to the corresponding polymer. This reduction is explained in terms of end effects and is shown to be consistent with the theoretical treatments of oligoelectrolyte transitions developed by Record and Lohman [Biopolymers, 17, 159–166 (1978)]. In the mixed solvents, the salt dependence of the melting temperature (∂tm/∂ log Na+) is shown to exhibit a linear dependence on the bulk dielectric constant of the medium for all of the hydroxyl‐containing solvents studied. Significantly, N,N‐dimethylformamide demonstrated different behavior. Copyright © 1979 John Wiley & Sons, Inc.