Amine induced Z-DNA in poly(dG-dC) · poly(dG-dC): Circular dichroism and gel electrophoresis study

Spermine and [Co(NH₃)₆]³⁺ are known to induce the B → Z transition in poly(dG-dC) · poly(dG-dC) DNA. In this paper gel electrophoresis shift assays using the long DNA poly(dG-dC) · poly(dG-dC) are developed to complement circular dichroism (CD) measurements of the conformation of the DNA in the presence of amines including spermine, spermidine, [Co(NH₃)₆]³⁺, (+)-[Co(en)₃]³⁺ and (-)-[Co(en)₃]³⁺ (en: ethylenediamine). CD is used to monitor the average conformation of the DNA and the gels show when there is a population of a given type of DNA in the solution. Spermidine is found to induce the transition at very low ionic strength, in contrast to literature wisdom. The cobalt amine-induced Z-DNA melts, with a broad transition, at a lower temperature than B-DNA. The electrophoresis data shows the presence of a DNA more flexible than B-DNA at low amine concentrations. This is ascribed to the existence of B/Z junctions. At higher amine concentrations, a DNA (Z-DNA) that is stiffer than B-DNA is observed. The Z-DNA induced by spermine is stiffer (runs more slowly on the gels) than that induced by the other amines. The spermine-DNA interaction is found to be very cooperative, whereas that of the cobalt amines is uncooperative. The kinetics of the B → Z transition with spermine at room temperature is very slow, whereas the cobalt amines induce it within seconds.