Dynamics of Chloride Anion Binding to Slowly Tumbling Molecules Probed by 35Cl Quadrupole-Central-Transition NMR...

We report a comprehensive 35Cl nuclear magnetic resonance (NMR) study of chloride anion binding to slowly tumbling molecules in viscous solvents. The chemical systems examined in this work include (i) small inorganic and organic chloride salts such as RbCl, l-cysteine hydrochloride, and l-glutamic acid hydrochloride dissolved in glycerol, and (ii) choline chloride (ChCl)-based deep eutectic solvents (DES) such as ChCl:glycerol (1:2), ChCl:urea (1:2), and ChCl:urea:l-arginine (1:2:0.2). We obtained high-quality 35Cl NMR signals from these systems under a wide range of molecular tumbling conditions by controlling solvent viscosity, sample temperature, and applied magnetic field. We demonstrate that, for slowly tumbling molecules, 35Cl quadrupole-central-transition (QCT) NMR signals are sensitive not only to 35Cl NMR parameters but also to the rapid chemical exchange between free and bound states of the Cl- anion under observation. Using a two-site chemical exchange model, we were able to analyze the experimental variable-temperature 35Cl QCT NMR linewidth data collected at multiple magnetic fields. From such analyses, we obtained information about the chloride binding environment through 35Cl isotropic chemical shifts quadrupole parameters, and anion exchange dynamics. Our results suggest that 35Cl QCT NMR is potentially useful for studying chloride anions tightly bound to biological macromolecules such as chloride-dependent enzymes.