Circular dichroism of sanguinarine-DNA complexes: effect of base composition, pH and ionic strength

The interaction of sanguinarine with various naturally occurring and synthetic deoxyribonucleic acids of different base composition and sequence has been studied from the measurement of circular dichroism spectroscopy in buffer of various ionic strengths and pH values where physico-chemical properties of DNA remain unchanged. Binding of sanguinarine to DNA causes a change in the circular dichroism spectrum of DNA itself by showing the increase of both positive and negative bands and appearance of a broad positive band with peak at 340 nm. All systems studied exhibit extrinsic positive band that is independent of DNA base composition and sequence. Calf thymus DNA-sanguinarine complexes in different salt concentrations show that, at saturation, the magnitude of molar ellipticity at 340 nm increases with decreasing sodium ions concentration. Sanguinarine-DNA complexes in buffer of various pH values also show that the molar ellipticity at 340 nm is greater in acidic pH and lower in alkaline pH. It is concluded that the alterations of secondary structure of DNA upon binding of sanguinarine is maximum in buffer of low ionic strength and acidic pH and the number of bound alkaloid molecules per base pair, at saturation, is more in G-C rich DNA than in A-T rich DNA.