A spectroscopic study on the interactions of porphyrin with G-quadruplex DNAs

Free-base porphyrin (5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphine) (H(2)TMPyP4) has been shown to be an effective telomerase inhibitor by an in vitro assay. Here, we examined the interactions of the H(2)TMPyP4 with three distinct G-quadruplex DNAs, the parallel-stranded (TG(4)T)4, dimer-hairpin-folded (G(4)T(4)G(4))2, and monomer-folded AG(3)(T(2)AG(3))(3), by ultraviolet resonance Raman spectroscopy (UVRR), UV-vis absorption spectroscopy, fluorescence spectroscopy, and surface-enhanced Raman spectroscopy (SERS). The data obtained by the continuous variation titration method show that the binding stoichiometry of H(2)TMPyP4/G-quadruplex is 2:1 for (TG(4)T)4 and 4:1 for (G(4)T(4)G(4))2 or AG(3)(T(2)AG(3))(3). The results of SERS spectra, UV-vis absorption titration, and fluorescence emission spectra together with the binding stoichiometries reveal that two H(2)TMPyP4 molecules are externally stacked at two ends of the parallel (TG(4)T)4 G-quadruplex, whereas H(2)TMPyP4 molecules can intercalate within their diagonal or lateral loop regions and intervals between two G-tetrads for (G(4)T(4)G(4))2 and AG(3)(T(2)AG(3))(3) G-quadruplexes. The binding of H(2)TMPyP4 to (TG(4)T)4 G-quadruplex results in the hypochromicity of the UV Raman signal of (TG(4)T)4, indicating that the stacking effects between H(2)TMPyP4 and DNA bases are significant. The Raman hyperchromicities and shifts are observed after the binding of H(2)TMPyP4 to both (G(4)T(4)G(4))2 and AG(3)(T(2)AG(3))(3) G-quadruplexes. This indicates that the intercalative H(2)TMPyP4 can lengthen the vertical distance between adjacent G-tetrads of (G(4)T(4)G(4))2 and AG(3)(T(2)AG(3))(3) and change their conformations. The present study provides new insights into the effect of H(2)TMPyP4 binding on the structures of G-quadruplexes and also demonstrates that Raman spectroscopy is an ideal method for examining the interaction between drugs and G-quadruplexes.