Right-handed and left-handed helices of poly(dA-dC).(dG-dT)

The secondary structures of poly(dA-dC;. were studied using CD and IR spectroscopies. We give spectroscopie evidence of secondary structure transitions of poly .(dG-dT> frgra a B to a Z-like helix, induced by transition metal ions (Ni~*> in presence of high concentrations of Cs and Na . In the pre- sence of Na+, the B^Z transition occurs at any temperature, whereas premelting conditions are required in presence of Cs . For, these two alkali ions the Z-like form is only induced by Ni~* ions through their specific interactions at N7 of purines, under conditions of low water activity due to the high alkali salt concentration^ we also show that the CD spectrum obtained in presence of Cs* ions and characterized by a negative band at 275 nra, cannot be interpreted in terms of Z-like left-handed helix but reflects a modified B right-handed helix.