25Mg-nmr investigations of the magnesium ion–DNA interaction

²⁵Mg-nmr data are reported that address the nature of the magnesium ion–DNA interaction. It is found that competitor ions such as calcium, mercury, zinc, and cobalt ions are not effective in competing for all of the magnesium ion–DNA interaction that is reported by the ²⁵Mg-nmr spectrum. The temperature dependence of the ²⁵Mg-nmr spectrum in DNA solution studied at high concentrations of competitor ion indicates that the chemical-exchange lifetime of the magnesium ions at DNA binding sites makes a major contribution to the ²⁵Mg-nmr line width. However, the activation parameters are not consistent with the temperature dependence of either transport properties or chemical exchange with phosphate groups alone, but are consistent with a sum of at least two processes that provide opposing contributions to the ²⁵Mg-nmr relaxation. It is also shown that the non-Lorentzian character of the ²⁵Mg-nmr line previously reported is consistent with the effect of an incompletely averaged static nuclear electric quadrupole interaction and/or an exchange process that is slow with respect to the magnitude of this interaction. Because the concentrations employed in these experiments are high, the present data do not provide a direct or critical test of the electrostatic theories of ion–polyelectrolyte interaction. The present data do demonstrate, however, that such theories alone are insufficient as a basis for understanding the ²⁵Mg-nmr data.