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 (Gdm⁺) 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 Gdm⁺-Gdm⁺ 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 Gdm⁺–Gdm⁺ interaction is attractive albeit weak—about −0.5 kJ·mol⁻¹ —which aligns with theoretical estimation from MD simulations. We contrast the behavior of Gdm⁺ with that of NH₄⁺ cations, which exhibit no contact ion pairing in water. DFT calculations predict that the NMR chemical shift of Gdm⁺ 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 R₉, which, unlike nona-lysines K₉, 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.

Keywords

cryo-EM / guanidinium / ion pairing / molecular dynamics / NMR / oligoarginine

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