DNA polymorphism: spectroscopic and electro-optic characterizations of Z-DNA and other types of left-handed helical structures induced by Ni2+.

Abstract

AbstractCD and uv spectroscopy reveal that the synthetic polynucleotides poly(dG–dC) · poly(dG–dC) and poly(dG–m5dC) · poly(dG–m5dC) undergo a transition induced by small amounts of Ni++ cation from a right‐handed B‐form to left‐handed Z‐type conformations. We describe the application of steady‐state and transient electric birefringence to the characterization of the transition observed at very low ionic strength (10 mM Tris HCl, pH 7.4). Dialysis experiments show that the changes in spectroscopic and electro‐optic properties upon addition of Ni++ are completely reversible. The differences in shape of the inverted CD spectra suggest the existence of a family of left‐handed conformations, depending on the polymer used and on the amounts of cation added. The stoichiometry required for inducing the Z‐conformation of poly(dG–m5dC) is 1 cation/4 nucleotide phosphates. The transition is accompanied by a decrease in birefringence, an increase in length, and the more important contribution of a permanent or slowly induced dipole moment to the orientation mechanism. High concentrations of Ni++ promote the Z → Z* transition. Upon increasing the Ni++ concentration, poly(dG–dC) undergoes a biphasic transition, first to one intermediate conformation that is neither B‐ nor Z‐like and then to a left‐handed form that is probably different from Z*. These conversions are accompanied by regular decreases both in birefringence and in chain length, but no transient appears in the field‐reversal experiments.