Guanidinium Like‐Charge Ion Pairing and Oligoarginine Aggregation in Water by Nuclear Magnetic Resonance, Cryo‐Electron Microscopy, and Molecular Dynamics

ABSTRACT Like‐charge pairing is a physical manifestation of the unique solvation properties of certain ion pairs in water. Water's high dielectric constant and related charge screening capability significantly influence the interaction between like‐charged ions, with the possibility to transform it—in exceptional cases when noncovalent interactions are involved—from repulsion to attraction. Guanidinium cations () represent a quintessential example of such like‐charge pairing due to their specific geometry and electronic structure. In this work, we present experimental validation and quantification of – contact ion pairing in water utilizing nuclear magnetic resonance (NMR) spectroscopy complemented by molecular dynamics (MD) simulations and density functional theory (DFT) calculations. The observed – interaction is attractive albeit weak—about —which aligns with theoretical estimation from MD simulations. We contrast the behavior of with that of cations, which exhibit no contact ion pairing in water. DFT calculations predict that the NMR chemical shift of dimers is different than that of monomers, in agreement with NMR titration curves that display a nonlinear Langmuir‐like behavior. Additionally, we conducted cryo‐electron microscopy—to our knowledge, for the first time—on concentrated oligoarginines , which, unlike nona‐lysines , exhibit aggregation in water. These results point to like charge pairing of the guanidinium side chain groups, as corroborated also by MD simulations and free energy calculations.