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.