[Peculiarities of DNA winding with bifunctional cations]

Addition of the bifunctional cations, bis(2-aminoethyl) disulphide-cystamine, and bis(2-guanidoethyl) disulphide-GED, into water solution of DNA results in a decrease in magnitude of the positive circular dichroism (CD) band. However, unlike the similar effect due to alcaline ions or ions of ammonium and guanidinium the above effect occurs at much smaller, stoichiometric with phosphates, concentrations of the dications. Another peculiarity of GED is the attaining of a plato region for the curve of the CD change with the rise of GED concentration. Since the decrease in positive band CD is connected with increase in the rotation angle between the base pairs, the observed behavior of the bifunctional cations are supposedly due to peculiarities of their winding action upon DNA helix. Reducing the disulphide bond in the cations gives rise to increase in the DNA positive CD band up to the values inherent to those in the presence of the corresponding monocations. The higher efficiency of the bifunctional cations is thus due to the cationic groups belong to one and the same molecule. Such compounds could thus be considered as a simple model of DNA-protein interaction.