Strong field gradients enable NMR-based diffusion measurements for K⁺, Mg²⁺, Cl⁻, and SO₄²⁻ ions in biomolecular solutions

NMR-based diffusion measurements of potassium (K⁺), magnesium (Mg²⁺), chloride (Cl⁻), and sulfate (SO₄²⁻) ions have been challenging even though these ions are biologically important. For these ions, the gyromagnetic ratios of the NMR-active nuclei, ³⁹K, ²⁵Mg, ³⁵Cl, and ³³S, are less than 1/10 of the ¹H gyromagnetic ratio, causing a low sensitivity in NMR detection and a low efficiency in NMR dephasing needed for diffusion measurements. These nuclei also undergo rapid longitudinal and transverse NMR relaxation via the quadrupolar mechanism, severely limiting the effectiveness of NMR-based diffusion measurements. Interactions with biomolecules promote the NMR relaxation of these ions, hindering measurements of the ion diffusion. We demonstrate that, despite these challenges, diffusion of K⁺, Mg²⁺, Cl⁻, and SO₄²⁻ ions in biomolecular solutions can be measured accurately and precisely through use of appropriately designed high-field NMR probe hardware that can generate strong field gradients >1000 G/cm. The NMR-based diffusion coefficients measured at 17.6 T for these ions in the absence of biomolecules agreed well with conductivity-based values in the literature. This consistency supports that ion diffusion along the magnetic field is unaffected by the Lorentz force acting on the ions, as previously predicted. Our data on ion diffusion in solutions of proteins and DNA illuminate the effect of electrostatic interactions on the apparent diffusion coefficients of ions. Thus, high-field NMR probe hardware that can generate strong field gradients opens a new avenue to characterize the dynamic behavior of various ions around biomolecules and their effect on biomolecular electrostatics.