Prospect of Bioflavonoid Fisetin as a Quadruplex DNA Ligand: A Biophysical Approach

Quadruplex (G4) forming sequences in telomeric DNA and c-myc promoter regions of human DNA are associated with tumorogenesis. Ligands that can facilitate or stabilize the formation and increase the stabilization of G4 can prevent tumor cell proliferation and have been regarded as potential anti-cancer drugs. In the present study, steady state and time-resolved fluorescence measurements provide important structural and dynamical insights into the free and bound states of the therapeutically potent plant flavonoid fisetin (3,3',4',7-tetrahydroxyflavone) in a G4 DNA matrix. The excited state intra-molecular proton transfer (ESPT) of fisetin plays an important role in observing and understanding the binding of fisetin with the G4 DNA. Differential absorption spectra, thermal melting, and circular dichroism spectroscopic studies provide evidences for the formation of G4 DNA and size exclusion chromatography (SEC) proves the binding and 1∶1 stoichiometry of fisetin in the DNA matrix. Comparative analysis of binding in the presence of EtBr proves that fisetin favors binding at the face of the G-quartet, mostly along the diagonal loop. Time resolved fluorescence anisotropy decay analysis indicates the increase in the restrictions in motion from the free to bound fisetin. We have also investigated the fingerprints of the binding of fisetin in the antiparallel quadruplex using Raman spectroscopy. Preliminary results indicate fisetin to be a prospective candidate as a G4 ligand.