Effects of OxodG on the binding affinity of TMPyP4 with G-quadruplex DNA

Formation of G-quadruplex DNA structures in G-rich telomeres is a promising approach to inhibit the binding of telomerase in cancer cells and regulate its biological functions. In recent years, many small molecules, known as G4 ligands, have been developed to assist the Gquadruplex formation via noncovalent interactions. It is also known that G-rich telomeric DNA is especially susceptible to oxidative damage due to the lower oxidation potential of guanine relative to the other nucleobases. In general, 8-oxo-dGuanosine (OxodG) is the major DNA lesion of guanines found under physiological conditions. OxodG can significantly destabilize G-quadruplex structures. However, to the best of our knowledge, the effect of OxodG present in Gquadruplexes on the binding of G4 ligands has never been studied. Herein, we study the interactions of 5,10,15,20- tetrakis(1-methyl-4-pyridyl)-21H, 23H-porphine (TMPyP4), a well-known G4 ligand, with 11 different DNA quadruplexes that contain an OxodG lesion in various locations. Fluorescent intercalator displacement (FID) assay is used to study the binding affinity of TMPyP4 towards different G-quadruplex DNA. Our fluorescence titration results showed that TMPyP4 exhibits an almost similar binding affinity towards different G-quadruplex DNA irrespective of the location of OxodG. The binding affinity for all the 11 quadruplexes was 106 orders of magnitude, except for the one with OxodG in the middle G quartet which is in order of 105. Overall, our study suggests that the OxodG modification at various positions in the G-quartet does not impede TMPyP4 binding of telomeric quadruplexes, which could provide useful information for future G-4 ligand design