Revealing the Energetics of Ligand-Quadruplex Interactions Using Isothermal Titration Calorimetry.

The thermodynamic characterization of G4-ligand interactions has shown to be a powerful adjunct to structural information in the rational design and optimization of potent G-quadruplex ligands for use in therapeutics, diagnostics, or other technological applications. Isothermal titration calorimetry (ITC) can resolve energetic contributions to complex formation and constitutes the only available experimental method to directly measure binding enthalpies. A general protocol for using ITC in studies on quadruplex-ligand interactions with details on the experimental setup, data analysis, and potential pitfalls is presented.

Thermodynamic signature of indoloquinolines interacting with G-quadruplexes: Impact of ligand side chain.

Binding of indoloquinolines with different aliphatic side chains to a parallel G-quadruplex derived from the MYC promoter sequence was characterized by optical and calorimetric measurements. ITC experiments performed at different temperatures enabled the determination of molar heat capacity changes upon quadruplex binding and a partitioning of the total binding free enthalpy into contributing terms with hydrophobic effects being major driving forces for all derivatives. Whereas affinities increase for indoloquinolines equipped with a long and positively charged side arm, the highest contribution of specific intermolecular interactions anticipated to impart enhanced specificity is found for a ligand with an uncharged ether aliphatic tail.

Ligand-Induced Dimerization of a Truncated Parallel MYC G-Quadruplex.

Binding of an indoloquinoline derivative with an aminoalkyl side chain to a truncated sequence from the MYC promoter region was studied through isothermal titration calorimetry (ITC). The targeted MYC3 sequence lacks 3'-flanking nucleotides and forms a monomeric parallel quadruplex (G4) with a blunt-ended 3'-outer tetrad under the solution conditions employed. Analysis of ITC isotherms reveals multiple binding equilibria with the initial formation of a 1:2 ligand/quadruplex complex.

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°.

Observation of a Dynamic G-Tetrad Flip in Intramolecular G-Quadruplexes.

A MYC sequence forming an intramolecular G-quadruplex with a parallel topology was modified by the incorporation of 8-bromoguanosine (G) analogues in one of its outer G-tetrads. The propensity of the G analogues to adopt a syn glycosidic torsion angle results in an exceptional monomolecular quadruplex conformation featuring a complete flip of one tetrad while keeping a parallel orientation of all G-tracts as shown by circular dichroism and nuclear magnetic resonance spectroscopic studies. When substituting three of the four G-tetrad residues with G analogues, two coexisting quadruplex conformational isomers with an all-syn and all-anti outer G-quartet are approximately equally populated in solution.

Towards the Development of Structure-Selective G-Quadruplex-Binding Indolo[3,2-b]quinolines.

The interaction of phenyl-substituted indolo[3,2-b]quinolines with DNA G-quadruplexes of different topology were studied by using a combination of spectroscopic and calorimetric methodologies. N5-Methylated indoloquinoline derivatives ((Me) PIQ) with an aminoalkyl side chain exhibit high affinities for the parallel-stranded MYC quadruplex and a (3+1)-hybrid structure combined with an excellent discrimination against the antiparallel thrombin-binding aptamer (TBA) and the human telomeric (HT) quadruplexes. Dissociation constants for the binding of the ligand to the MYC quadruplex are in the submicromolar range, being below the corresponding dissociation constants for the antiparallel-stranded quadruplexes by about one order of magnitude.

Molecular Recognition and Visual Detection of G-Quadruplexes by a Dicarbocyanine Dye.

The interactions of a dicarbocyanine dye 3,3'-diethylthiadicarbocyanine, DiSC2(5), with DNA G-quadruplexes were studied by means of a combination of various spectroscopic techniques. Aggregation of excess dye as a result of its positive charge is promoted by the presence of the polyanionic quadruplex structure. Specific high-affinity binding to the parallel quadruplex of the MYC promoter sequence involves stacking of DiSC2(5) on the external G-tetrads; the 5'-terminal tetrad is the favored binding site.