Mn2+ and other transition metals at low concentration induce the right‐to‐left helical transformation of poly[d(G‐C)].

The effects of the first‐row transition metal ions on the right(B)‐ to left(Z)‐handed helical transition of poly[d(G‐C)] have been determined. The Z conformation is induced by MnCl2 at submillimolar concentrations. The forward reaction has a very large activation energy (440 kJ/mol) so that a facile conversion occurs only at temperatures above 45 degrees C. However, the left‐handed form remains stable upon cooling. The addition of ethanol (20% v/v) eliminates the requirement for elevated temperature. The transition is highly co‐operative and is accompanied by spectral changes (absorption, circular dichroism) characteristic for the B–‐Z conformational transition. NiCl2 and CoCl2 also induce the B–‐Z transition in poly[d(G‐C)] but the activation energies and thus the temperature requirements for the forward reaction are lower than those observed with MnCl2. The left‐handed DNA formed in the presence of Mn2+ is similar to ‘Z DNA’ previously described in Mg2+‐EtOH (van de Sande and Jovin, 1982): (a) it readily sediments out of solution at low speed as a consequence of intermolecular association which, however, is not accompanied by turbidity; and (b) it supports the binding of ethidium bromide although this drug interacts preferentially with the B form of DNA. With Ni2+, the B–‐Z isomerization step can be separated from the subsequent specific Z–‐Z* association. Mn2+, Ni2+, and Co2+ also promote the B–‐Z transition of poly[d(G‐m5C)] at substoichiometric concentrations with respect to DNA nucleotide.