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.
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