Cooperative Aspects of Small Ion, Small Molecule Interactions with Nucleic Acids: Potassium-Polyriboadenylic Acid, Polyribouridylic Acid

Ionic conductivity measurements vs. temperature were made on KCl solutions of K-polyriboadenylic acid + K-polyribouridylic acid (K-poly-A + K-poly-U) in regions of temperature and composition where the helix-coil transition occurred. It was possible to relate the measurements to a differential helix-to-coil binding of K⁺ to nucleotide. The results were, within experimental error, the same as those obtained from a limited number of differential KCl activity coefficient measurements and from a theoretical interpretation of polymer free-boundary electrophoretic mobilities. It was concluded that the alkali ion-phosphate interaction in polynucleotides must be regarded as cooperative in nature and several criteria for recognition of such phenomena were given. A brief outline for a proposed statistical mechanical model for binding was presented.