Spectroscopic and calorimetric studies on the binding of the phototoxic and cytotoxic plant alkaloid sanguinarine with double helical poly(A)

The interaction of the phototoxic and cytotoxic DNA binding plant alkaloid sanguinarine with double stranded polyadenylic acid has been investigated from multifaceted spectroscopic and calorimetric techniques. The hypochromic and bathochromic shifts in the absorption spectrum, and large enhancement of the fluorescence intensity of sanguinarine indicated strong binding of the alkaloid to the duplex poly(A). The corresponding intrinsic binding constant for the complexation, obtained from visible absorbance and fluorescence spectroscopic methods, respectively, was found to be in the order of 104 M−1. The binding was further characterized by strong energy transfer from the adenine base pairs to the alkaloid, significant polarization of the fluorescence of the alkaloid, stabilization against thermal strand separation of the duplex, and also resulting in remarkable conformational changes in poly(A) with concomitant generation of optical activity in the bound alkaloid molecules that are otherwise achiral. Isothermal titration calorimetric and differential scanning calorimetric studies revealed the binding of the alkaloid to be exothermic and enthalpy driven. All these results together with fluorescence quenching experiments advance good evidence for the intercalation of the alkaloid sanguinarine into the duplex poly(A).