Interpretation of 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 l have been measured for 25 Mg in dilute solutions of Na‐DNA/NaCl containing varying amounts of added magnesium(II) ions. The 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 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 \documentclass{article}\pagestyle{empty}\begin{document}$ \frac{5}{2} $\end{document} 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 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 μs at room temperature. The temperature variation of T 1 for 25 Mg has been calculated, giving some qualitative agreement with the data. The correlation time for bound 25 Mg has been found to be about 40 ns at room temperature. © 1992 John Wiley & Sons, Inc.