Conditions for the stability of the alternative structures of duplex poly(dA‐dT)

The alternative structures of the lithium, sodium, potassium, and rubidium salts of poly(dA‐dT) were studied using ir spectra of hydrated, nonoriented gels. Poly (dA‐dT) · Na existed as the B structure at high hydration and as the A structure at moderate hydration. A disordered state occurred at low hydration. No hint of the C, D, or other structures was observed at any hydration for nonoriented gels containing NaCl up to 0.56 moles NaCl/mole of nucleotide residue. Poly (dA‐dT) · Li existed as the B structure at high hydration, made a gradual transition to the C structure at moderate hydration, and became disordered at low hydration. Poly (dA‐dT) · K and poly (dA‐dT) · Rb existed as the B structure at high hydration and as the presumptive D structure at lower hydration. The addition of Zn(II) to poly (dA‐dT) · Na produced a gradual transition from the B to the C structure upon dehydration. The addition of NaNO 3 to poly (dA‐dT) · Na gave only the A and B structures, and not the C or Z structures that are promoted by NO 3 in other base sequences. The DNA secondary structure stabilized by a given ion is determined by a specific base sequence and not simply by the properties of the ion.