Melting behavior and ligand binding of DNA intramolecular secondary structures

Abstract

We use a variety of biophysical techniques to determine thermodynamic profiles, including hydration, for the unfolding of DNA stem-loop motifs (hairpin, a three-way junction and a pseudoknot) and their interaction with netropsin and random cationic copolymers. The unfolding thermodynamic data show that their helix–coil transition takes place according to their melting domains or sequences of their stems. All hairpins adopted the B-like conformation and their loop(s) contribute with an immobilization of structural water. The thermodynamic data of netropsin binding to the ^5′–AAATT–^3′/TTTAA site of each hairpin show affinities of ~ 10^6–7 M⁻¹, 1:1 stoichiometries, exothermic enthalpies of − 7 to − 12 kcal mol⁻¹ (− 22 kcal mol⁻¹ for the secondary site of the three-way junction), and water releases. Their interaction with random cationic copolymers yielded higher affinities of ~ 10⁶ M⁻¹ with the more hydrophobic hairpins. This information should improve our current picture of how sequence and loops control the stability and melting behavior of nucleic acid molecules.