Probing the binding mode of psoralen to calf thymus DNA

The binding properties between psoralen (PSO) and calf thymus DNA (ctDNA) were predicted by molecular docking, and then determined with the use of UV-vis absorption, fluorescence, circular dichroism (CD) and Fourier transform infrared (FT-IR) spectroscopy, coupled with DNA melting and viscosity measurements. The data matrix obtained from UV-vis spectra was resolved by multivariate curve resolution-alternating least squares (MCR-ALS) approach. The pure spectra and the equilibrium concentration profiles for PSO, ctDNA and PSO-ctDNA complex extracted from the highly overlapping composite response were obtained simultaneously to evaluate the PSO-ctDNA interaction. The intercalation mode of PSO binding to ctDNA was supported by the results from the melting studies, viscosity measurements, iodide quenching and fluorescence polarization experiments, competitive binding investigations and CD analysis. The molecular docking prediction showed that the specific binding most likely occurred between PSO and adenine bases of ctDNA. FT-IR spectra studies further confirmed that PSO preferentially bound to adenine bases, and this binding decreased right-handed helicity of ctDNA and enhanced the degree of base stacking with the preservation of native B-conformation. The calculated thermodynamic parameters indicated that hydrogen bonds and van der Waals forces played a major role in the binding process.