Metal‐Induced sequential transitions among DNA conformations

The action of [Co(NH 3 ) 6 ]Cl 3 on poly(dGdC) · poly(dGdC) can lead to a series of consecutive reactions, in which B‐DNA is first converted to Z‐DNA [M. Behe and G. Felsenfeld (1981) Proc. Natl. Acad. Sci USA 78 , 1619–1623], which in turn is transformed into an unidentified conformer that we tentatively call “U”, and finally the highly associated anisotropic ψ‐DNA is produced. Conditions are given under which the sequence B ⇄ Z ⇄ “U” ψ(+) can be stopped at any point in the direction from left to right. The reverse processes, from right to left, occur when ψ(+) or “U”‐DNA are treated with various amounts of salt concentrations and lowering temperature. Thus it is demonstrated that four conformers of poly(dGdC) · poly(dGdC) are readily interconvertible, and that Z‐DNA and “U” conformers are intermediates in the reversible transformations of B‐ and ψ‐DNA.