The role of solvents in the stabilization of helical structure: the low pH ribo A8 and A10 double helices in mixed solvents.

The order-disorder transitions of the double helices formed by the ribo-oligoadenylic acids rA8 and rA10 at pH 4.2 have been investigated in a series of organic/aqueous mixed solvents. Melting temperature data, Tm, derived from the uv melting curves were used to define the stability of the double helices in the different mixed solvent systems. It was found that the extent of helix destabilization depended in a predictable fashion on both the quantity and the nature of the added organic solvents. For the C1 through C4 aliphatic alcohols, the longer, less branched alcohols proved to be more effective destabilizers of the helical structure. Significantly, the amides proved to be more powerful destabilizers than the alcohols. Analysis of the melting curves provided the Van't Hoff enthalpy change for each transition. The data are interpreted in terms of the role of solvent in the stabilization of ribonucleic acid structure.