Interpretation of super(25)Mg spin relaxation in Mg-DNA solutions. Temperature variation and chemical exchange effects

The temperature dependencies of line shapes and spin-lattice relaxation times T sub(1) have been measured for super(25)Mg in dilute solutions of Na-DNA/NaCl containing varying amounts of added magnesium(II) ions. The super(25)Mg spectrum is clearly non-Lorentzian, due to the presence of motions modulating the quadrupolar interaction that are slow compared to the inverse of the Larmor frequency. The weakly temperature-dependent line shapes and relaxation rates appear to be influenced by the relatively slow exchange of the Mg super(2+) ions between the DNA surface and the aqueous bulk phase. The observed temperature dependencies depend on the ratio of total magnesium to DNA phosphate, Mg/P. The line shape as well as the temperature dependence of the line width at half height can be qualitatively reproduced with a two-site discrete exchange model for the quadrupolar relaxation of a spin 5/2 nucleus in isotropic solution. The calculations give a value of the lifetime for magnesium bound to DNA of 4 ms at room temperature. Previously reported temperature-dependent super(43)Ca relaxation measurements in DNA solution can be reproduced under the assumption of a mean lifetime of bound calcium that is not larger than 2 ms but not smaller than 50 mu s at room temperature. The temperature variation of T sub(1) for super(25)Mg has been calculated, giving some qualitative agreement with the data. The correlation time for bound super(25)Mg has been found to be about 40 ns at room temperature.