AbstractThe binding of Mg++ to poly A and poly U has been measured quantitatively by using the metallochromic indicator calmagite. The method is described in detail. It is shown that there is electrostatic interaction between the binding sites, viz., the phosphate groups, and the intrinsic association constant, for the specific binding can be determined. After extrapolation to zero ionic strength we find that, for the binding of Mg++ to poly A, kint = 4 × 104 and for that, to poly U, kint = 3 × 104. The intrinsic enthalpy of association is negative. The effect of Mg++ on the secondary structure of poly A and poly U has been studied by measuring the ultraviolet absorbance, optical rotatory dispersion and viscosity as a function of the amount of added Mg++ ions. It was found that Mg++ promotes the formation of a more ordered secondary structure by neutralizing or screening the negative charges. It is concluded from the absorbance measurements that for poly A at pH ⩾ 7 and for poly U at pH >xs 9 this ordering involves stacking of the bases. Likewise, in solutions of UDP with a pH around 10, base stacking occurs on addition of Mg++.