Abstract
Counterions play a crucial role in biomolecular systems, influencing the structure and function of proteins and nucleic acids. Most counterions are not visible in experimentally determined biomolecular structures because the ions dynamically diffuse even while interacting with biomolecules. Over the past five decades, researchers have utilized (23)Na NMR to study sodium ions and their electrostatic interactions with biomolecules. Other inorganic ions as counterions of biomolecules have also been studied with (25)Mg, (31)P, (35)Cl, and (39)K NMR, for example. However, investigating the dynamic properties of ions around biomolecules using NMR has been challenging. Recently, there have been significant advances in NMR studies on the behavior of various biologically relevant ions around proteins and nucleic acids. Advances in probe hardware capable of generating strong field gradients have enabled NMR-based diffusion measurements of various inorganic ions interacting with biomolecules. The diffusion data have revealed the highly mobile nature of counterions around biomolecules and quantitative information about the release of counterions upon protein-DNA association. Quantitative NMR (qNMR) approaches have been developed to determine the number of counterions accumulated around a biomolecule. Applications of the diffusion and qNMR methods appear promising since the feasibility of ion NMR has already been demonstrated for large biomolecule systems, such as ribosomes, genomic DNA, biomolecular condensates, and living organisms.