Melting thermodynamics of reversible DNA/ligand complexes at interfaces.

A variety of solution methods exist for analysis of interactions between small molecule ligands and nucleic acids; however, accomplishing this task economically at the scale of hundreds to thousands of sequences remains challenging. Surface assays offer a prospective solution through array-based multiplexing, capable of mapping out the full sequence context of a DNA/ligand interaction in a single experiment. However, relative to solution assays, accurate quantification of DNA/ligand interactions in a surface format must contend with limited understanding of molecular activities and interactions at a solid-liquid interface. We report a surface adaptation of a solution method in which shifts in duplex stability, induced by ligand binding and quantified from melting transitions, are used for thermodynamic analysis of DNA/ligand interactions. The results are benchmarked against solution calorimetric data. Equilibrium operation is confirmed through superposition of denaturation/hybridization transitions triggered by heating and cooling. The antibiotic compound netropsin, which undergoes electrostatic and sequence-specific minor groove interactions with DNA, is used as a prototypical small molecule. DNA/netropsin interactions are investigated as a function of ionic strength and drug concentration through electrochemical tracing of surface melt transitions. Comparison with solution values finds excellent agreement in free energy, though reliable separation into enthalpic and entropic contributions proves more difficult. The results establish key guidelines for analysis of DNA-ligand interactions via reversible melting denaturation at surfaces.