Thermal Denaturation of DNA G-Quadruplexes and Their Complexes with Ligands: Thermodynamic Analysis of the Multiple States Revealed by Mass Spectrometry

Designing ligands targeting G-quadruplex nucleic acid structures and affecting cellular processes is complicated because there are multiple target sequences and some are polymorphic. Further, structure alone does not reveal the driving forces for ligand binding. To know why a ligand binds, the thermodynamics of binding must be characterized. Electrospray mass spectrometry enables one to detect and quantify each specific stoichiometry (number of strands, cations, and ligands) and thus to simultaneously determine the equilibrium constants for each complex. Using a temperature-controlled nanoelectrospray source, we determined the temperature dependence of the equilibrium constants, and thus the enthalpic and entropic contributions to the formation of each stoichiometry. Enthalpy drives the formation of each quartet-K+-quartet unit, whereas entropy drives the formation of quartet-K+-triplet units. Consequently, slip-stranded structures can become more abundant as the temperature increases. In the presence of ligands (Phen-DC3, TrisQ, TMPyP4, Cu-ttpy), we observed that, even when only a 1:1 (ligand/quadruplex) complex is observed at room temperature, new states are populated at intermediate temperatures, including 2:1 complexes. In most cases, ligand-G4-quadruplex binding is entropically driven, and we discuss that this may have resulted from biases when ranking ligand potency using melting experiments. Other thermodynamic profiles could be linked to topology changes in terms of number of G-quartets (reflected in the number of specific K+ ions in the complex). The thermodynamics of ligand binding to each form, one ligand at a time, provides unprecedented detail on the interplay between ligand binding and topology changes in terms of number of G-quartets.