Comprehensive Thermodynamic Profiling for the Binding of a G-Quadruplex Selective Indoloquinoline

Binding of a positively charged indoloquinoline derivative to a G-quadruplex formed by the G-rich promoter element of the c-MYC oncogene was subjected to a rigorous isothermal calorimetric analysis. Binding of the indoloquinoline is primarily enthalpy-driven but is also promoted by a favorable entropy term. Both binding enthalpy ΔH° and binding entropy ΔS° exhibit a noticeable temperature dependence with almost complete enthalpy-entropy compensation as a result of a negative change in heat capacity ΔC_p°. Salt-dependent polyelectrolyte effects only moderately contribute to the overall free energy of association. More details on the binding process are revealed in an attempt to dissect the total free energy into individual contributory terms. Accordingly, specific intermolecular interactions between the indoloquinoline ligand and G-quadruplex substantially contribute in addition to hydrophobic effects in promoting the association. Comparing thermodynamic profiles for various quadruplex ligands indicates different energetic patterns that may aid in the rational design of more efficient quadruplex binding ligands in the future.