Preferential Hydration of DNA: The Magnitude and Distance Dependence of Alcohol and Polyol Interactions

The physical forces that underlie the exclusion of solutes from macromolecular surfaces can be probed in a similar way as the measurement of forces between macromolecules in condensed arrays using the osmotic stress technique and x-ray scattering. We report here the dependence of alcohol exclusion or, equivalently, the preferential hydration of DNA on the spacing between helices in condensed arrays. The actual forces describing exclusion are quite different from the commonly assumed steric crowding coupled with weak binding. For a set of 12 nonpolar alcohols, exclusion is due to repulsive hydration interactions with the charged DNA surface. Exclusion amplitudes do not depend simply on size, but rather on the balance between alkyl carbons and hydroxyl oxygens. Polyols are included at very close spacings. The distance dependence of polyol inclusion, however, is quite different from nonpolar alcohol exclusion, suggesting the underlying mechanism of interaction is different.