Do planar alkaloids from Strychnos usambarensis intercalate into the DNA helix?

The mode of binding of melinonine F and normelinonine F to DNA has been investigated through physico-chemical methods which proved most valuable in the study of dye-DNA interactions, namely absorption spectrophotometry, circular and linear dichroism, fluorescence polarization, viscosity increase of linear DNA and circular DNA unwinding. The spectroscopic properties resembled those observed with planar heterocyclic compounds intercalating between the DNA base pairs: bathochromic shift and hypochromism in the ligand absorption bands; presence of a strong binding process with an association constant of the order of 10(6)M-1 and a number of binding sites of 0.1 to 0.15; appearance of extrinsic optical activity in the ligand absorption bands with a dependence on the binding ratio typical for an excitonic origin; negative reduced electric dichrosim of the ligand chromophore of the same order of magnitude as that of the DNA heterocyclic bases. However the increase of the length of sonicated DNA determined from viscosity changes was much smaller than for most intercalating compounds and no unwinding of PM2 circular DNA was detected.