Binding of nucleosides with the cytotoxic plant alkaloid sanguinarine: Spectroscopic and thermodynamic studies

Abstract

In spite of a large number of studies of the interaction of the cytotoxic plant alkaloid sanguinarine (SAN) with nucleic acids, the anticancer mechanism of SAN is still not clear. In contrast to the large number of studies of the interaction mechanism of SAN with DNA, there have been relatively few studies of the interaction of SAN with nucleosides. In this work, the interaction of SAN with three nucleosides—thymidine (T), uridine (U), and adenosine (A)—was investigated using a combination of conventional fluorescence and UV-vis spectroscopic techniques; thermodynamic calculations were also carried out at physiological pH 7.2. The binding processes of SAN with the different nucleosides were characterized by hypochromic and bathochromic effects in the absorption spectra of SAN and by the quenching of the fluorescence intensity of SAN. The measurements of fluorescence lifetime, the variations of the absorption spectra of the fluorophore, and the dependence of the quenching on the temperature indicated that the fluorescence quenching is static. The Stern-Volmer plot is nonlinear and approximately quadratic showing that, in this process, one SAN molecule can bind with two nucleoside molecules. These studies, together with our earlier studies of the binding of SAN with cytidine (C) and guanosine (G), showed that the binding constants of SAN with the five nucleosides at T = 308.15, 318.15, and 328.15 K decreased in the order C > G > T > U > A and at T = 298.15 K decreased in the order G > C > T > U > A, and that the binding of SAN with the various nucleosides is not only slightly exothermic but also entropy-driven. All these results together with fluorescence quenching experiments advance good evidence concerning the interaction of SAN with various nucleosides. Such studies of the interaction mechanism of alkaloids with DNA may promote the development of new drugs.