Dynamics of Cations around DNA and Protein as Revealed by (23)Na Diffusion NMR Spectroscopy

[Image: see text] Counterions are vital for the structure and function of biomolecules. However, the behavior of counterions remains elusive due to the difficulty in characterizing mobile ions. Here, we demonstrate that the dynamics of cations around biological macromolecules can be revealed by (23)Na diffusion nuclear magnetic resonance (NMR) spectroscopy. NMR probe hardware capable of generating strong magnetic field gradients enables (23)Na NMR-based diffusion measurements for Na(+) ions in solutions of biological macromolecules and their complexes. The dynamic properties of Na(+) ions interacting with the macromolecules can be investigated using apparent (23)Na diffusion coefficients measured under various conditions. Our diffusion data clearly show that Na(+) ions retain high mobility within the ion atmosphere around DNA. The (23)Na diffusion NMR method also permits direct observation of the release of Na(+) ions from nucleic acids upon protein–nucleic acid association. The entropy change due to the ion release can be estimated from the diffusion data.