STRUCTURE OF SINGLE AND DOUBLE-STRANDED DNA SOLUTIONS AS STUDIED BY ULTRACENTRIFUGATION

The osmotic pressure of dense solutions of DNA fragments in aqueous 0.2-2 M salt (NaCl, etc.) solutions were easily obtained from equilibrium ultracentrifugation data. Short, helical double stranded DNA fragment solutions showed a well defined ordering transition. The osmotic pressure of these helical solutions could be explained by the scaled particle theory of rigid spherocylinders. On the other hand, dense solutions of (single stranded) etheno-DNA derivatives (of about 100 nucleotides) showed no ordering phase transition. The osmotic pressure as a function of the epsilon-DNA concentration verifies a scaling prediction for the random coils in the semi-dilute regime. Good fits to the reduced osmotic pressure are obtained by using the Kleintjens and Koningsveld modified mean field lattice gas equations of state. As inferred from the Zimm clustering function, essentially all intermolecular interactions at high ionic strength are purely repulsive.