Affinity of Anticancer Drug Daunomycin toward Tetrahymena Telomeric G-Quadruplex DNA D-[GGGG(TTGGGG)3]

The anthracycline drug daunomycin exhibits complex mechanisms of anticancer action, which are not well understood. It plays a role in telomere dysfunction and binds to G-quadruplex DNA besides duplex DNA. Using the surface plasmon resonance technique, we demonstrate that it binds to a 22-mer D-[GGGG(TTGGGG)(3)] telomeric DNA sequence from Tetrahymena thermophilia in K+-rich aqueous solution. Changes in absorption/circular dichroism spectra and efficient quenching of fluorescence accompanied by a minor change in wavelength establish external binding of daunomycin with no scope of classical intercalation as observed on its binding to duplex DNA. Daunomycin-DNA complexes with stoichiometries of 1:1 and 2:1 coexist in solution. The daunomycin dimers in free solution are disrupted on binding. Proton nuclear magnetic resonance (NMR) spectra show significant shifts in aromatic protons of ring B/D, daunosamine sugar protons, and 14 short intermolecular contacts, exhibiting specificity of interaction. Large downfield shifts in phosphorus-31 NMR spectra, expected on account of classical intercalation, are absent. Molecular docking confirms external binding by the formation of a daunomycin-DNA complex with negative binding energy. Differential scanning calorimetry experiments show binding profiles with melting temperature T-m increasing with the daunomycin to DNA ratio and total thermal stabilization, Delta T-m = 10 degrees C, which is expected to interfere with telomerase access to its functional site at telomeres, causing telomere dysfunction. The findings have implication in the design of analogues with different chemical modifications that could produce de novo anthracycline that acts as a potent telomerase functional inhibitor with enhanced selectivity toward G-quadruplex and hence result in reduced cell toxicity.