Interaction of metal ions with polynucleotides and related compounds. XX. Control of the conformation of polyriboadenylic acid by divalent metal ions

Abstract

Divalent metal ions have profound effects on the conformation of poly A, as indicated by changes in optical rotatory dispersion and ultraviolet spectra, with different metal ions having distinctly different effects. At pH 6 and in the presence of 5 × 10 −2 M NaCl, two moles Ni(II) or Co(II) per mole of poly A residue stabilize the single helix; the same amount of Mg(II), Ca(II), Zn(II) or Mn(II) produce a mixture of double and single helix, while Cu(II) and Cd(II) induce the formation of a random coil structure. Under other conditions, these same metals have different effects, with the amount of added NaCl an important factor that can be used to control the way in which a particular divalent metal ion will induce the transition from a double helix to single helix or from helix to coil. The single helix is stabilized by metals binding to phosphate; the double helix is destabilized by metal ions binding to either phosphate or base, the former producing single helix and the latter random coil. The melting temperature of double helical poly A is diminished when metal ions bind to either type of site.