Spectroscopic study of the interaction of NiII-5-triethyl ammonium methyl salicylidene ortho-phenylendiiminate with native DNA

Abstract

The interaction of native calf thymus DNA with the cationic Ni(II) complex of 5-triethyl ammonium methyl salicylidene ortho-phenylendiimine (NiL2+), in 1 mM 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 350–400 nm, the strong increase of the DNA melting temperature (Tm) and the fluorescence quenching of ethidium bromide–DNA solutions, in the presence of increasing amounts of the NiL2+ metal complex, support the existence of a tight intercalative interaction of NiL2+ with DNA, analogous to that recently reported for both ZnL2+ and CuL2+. The intrinsic binding constant (Kb) and the interaction stoichiometry (s), determined by UV spectrophotometric titration, are equal to 4.3 106 M 1 and 1.0 base pair per metal complex, respectively. Interestingly, the value of Kb is slightly higher and 10 times higher than that relative to the CuL2+–DNA and the ZnL2+–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 CuL2+, the shape of the CD of the NiL2+–DNA system at NiL2+–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.