Conformations of DNA in the presence of nanomole concentrations of Co+2 ions and mezo-tetra(4-N-oxyethylpiridil) porphyrin.

Conformation changes of calf thymus DNA induced by nanomole concentrations of Co(+2) and TOEPyP4 and influence of this ion on porphyrin binding mode with DNA duplex were studied at two NaCl concentrations (10 and 100 mM) using circular dichroism spectroscopy and absorption data. It was shown that addition of 85 nM Co(+2) into TOEPyP4/DNAb.p. = .015 solution caused an immediate change of the intercalative binding mode of porphyrin to external binding modes at low ionic strengths (10 mM NaCl). Increase of Co(+2) concentration from 165 to 650 nM caused a cooperative transition of DNA from B form to C-like conformation. Similar changes in binding mode and DNA conformation were observed at 100 mM NaCl, but they took place at higher concentrations of Co(+2) ions. Incubation of TOEPyP4/DNAb.p. = .015 in the presence of nanomole concentrations of Co(+2) ions at 36.6 °C showed a transition of DNA-B form into Z-like conformation. Maximal percent of Z-like DNA was observed when the incubation time was from 6 to 8 h. Longer incubation times of [DNA-TOEPyP4]+Co(+2) complexes caused a considerable recovery of CD spectra in UV region and a strong decrease in absorption at 260 nm. Here, we discuss some mechanisms of transition from B-DNA to C- and Z-like conformations.