Existence of the C structure in poly(dA-dC).poly(dG-dT).

We show that the lithium salt of calf-thymus DNA can assume the C structure in nonoriented, hydrated gels. The transitions between the B and C structures showed little hysteresis and none of the metastable structural states which occur in oriented gels. Therefore crystal-lattice forces are not needed to stabilize the C structure. The occurrence of the alternative structures of the Li, Na and K salts of poly(dA-dC).poly(dG-dT) was measured as a function of hydration for nonoriented gels. Poly(dA-dC).poly(dG-dT).Li exists in the B structure at high hydrations and in the C structure at moderate hydrations with no A or Z structure at any hydration tested. The Na salt of poly(dA-dC).poly(dG-dT) exists in the B structure at high hydration, as mixtures of B and C at moderate hydrations and in the A structure at lower hydrations. The potassium salt behaves similarly except that mixtures of the C and A structures exist at lower hydrations. ZnCl2 and NaNO3, which promote the Z structure in duplex poly(dG-dC), promote the C structure in poly(dA-dC).poly(dG-dT). Information contained in the sequence of base pairs and not specific ionic interactions appear to determine the stability of the alternative structures of polynucleotides as hydration is changed.