The Origins of Arginine “Magic”: Guanidinium Like-Charge Ion Pairing and Oligoarginine Aggregation in Water by NMR, Cryoelectron Microscopy, and Molecular Dynamics Simulations

The phenomenon of like-charge pairing of hydrated ions is a physical manifestation of the unique solvation properties of certain ion pairs in water. Water’s high dielectric constant and related ion screening capability significantly influence the interaction between like-charged ions, with the possibility to transform it – in some cases – from repulsion to attraction. Guanidinium cations (Gdm ⁺ ) represent a quintessential example of such like-charge pairing due to their specific geometry and charge distribution. In this work, we present experimental quantification of Gdm ⁺ –Gdm ⁺ contact ion pairing in water utilizing nuclear magnetic resonance (NMR) spectroscopy experiments complemented by molecular dynamics (MD) simulations and density functional theory (DFT) calculations. The observed interaction is very weak — about –0.5 kJ · mol ^{− 1} — which aligns with theoretical estimation from MD simulations. We also contrast the behavior of Gdm ⁺ with NH ₄ ⁺ cations, which do no exhibit contact ion pairing in water. DFT calculations predict that the NMR chemical shift of Gdm ⁺ dimers is smaller than that of monomers, in agreement with NMR titration curves that display a non-linear Langmuir-like behavior. Additionally, we conducted cryo-electron microscopy experiments on oligoarginines R ₉ , which (unlike nona-lysines K ₉ ) exhibit aggregation in water. This points again to like charge pairing of the guanidinium side chain groups, as corroborated also by molecular dynamics simulations of these peptides in water.