Spectroscopic study of the interaction of Ni(II)-5-triethyl ammonium methyl salicylidene ortho-phenylendiiminate with native DNA.

The interaction of native calf thymus DNA with the cationic Ni(II) complex of 5-triethyl ammonium methyl salicylidene ortho-phenylendiimine (NiL(2+)), in 1mM Tris-HCl aqueous solutions at neutral pH, has been monitored as a function of the metal complex-DNA molar ratio by UV absorption spectrophotometry, circular dichroism (CD) and fluorescence spectroscopy. The dramatic modification of the DNA CD spectrum, the appearance of a broad induced CD band in the range 350-400nm, the strong increase of the DNA melting temperature (T(m)) and the fluorescence quenching of ethidium bromide-DNA solutions, in the presence of increasing amounts of the NiL(2+) metal complex, support the existence of a tight intercalative interaction of NiL(2+) with DNA, analogous to that recently reported for both ZnL(2+) and CuL(2+). The intrinsic binding constant (K(b)) and the interaction stoichiometry (s), determined by UV spectrophotometric titration, are equal to 4.3x10(6)M(-1) and 1.0 base pair per metal complex, respectively. Interestingly, the value of K(b) is slightly higher and 10 times higher than that relative to the CuL(2+)-DNA and the ZnL(2+)-DNA systems, respectively. Speculations can be performed to rationalize the observed trend, on the basis of the electronic and geometrical structures of the three complexes of the same ligand. Analogously to what previously observed for CuL(2+), the shape of the CD of the NiL(2+)-DNA system at NiL(2+)-DNA molar ratios higher than 0.5 is indicative of the formation of supramolecular aggregates in solutions, as a possible consequence of the electrostatic interaction between the cationic complex and the negatively charged phosphate groups of DNA.