Interaction of DNA with water soluble complex of Nickle and formation of DNA cross-links

Interaction of double stranded DNA with bulky and hydrophobic Salen type Schiff base complex: [N, N' Bis [3- tert-butyl-5-[triphenyl-phosphonium - methyl] - salicylidene] 1,2 ethylene-diamine nickel(III) acetate (refer to Ni Salen complex) was extensively investigated using the spectroscopic techniques and gel electrophoresis. Absorption titration experiment showed the hypochromic effect and the significant red shift of the complex absorption. In competition experiments with ethidium bromide (EB), Ni Salen complex exhibited non-competitive binding at high concentrations. UV-vis absorption and fluorescence emission data agreed on a binding constant of (1.64 ± 0.01) μM^-1, thereby showing the strong interaction of the complex with DNA; also, a binding site size of 2.33 ± 0.01 base pairs per complex was achieved. Thermal denaturation experiment showed that T_m of calf thymus-DNA was increased by approximately 10 °C at a molar ratio of the dye/base of 0.2. The CD spectra of DNA exhibited an increase in both positive and negative peaks without any shift in the position of bands upon addition of the complex. The amplitude of the LD spectra of DNA was decreased in the presence of the complex. Reduced linear dichroism (LD^red) revealed that the transition moment of complex was parallel to the DNA helix axis. Gel electrophoresis experiments confirmed that Ni Salen complex had no nuclease/DNA cleaving activity; also, DNA-DNA cross links were formed at high concentrations of complex, leading to the aggregation of DNA.