Melting behavior and ligand binding of DNA intramolecular secondary structures

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 ∼ 10 ⁶ 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.