Preferential Adsorption of Lower-Charge Glutamate Ions on Layered Double Hydroxides: An NMR Investigation

13 C MAS NMR spectroscopy of isotopically enriched samples of the layered double hydroxide hydrotalcite (HT) [(Mg 2 Al)(OH) 6 A −, n H 2 O, where A − is a counteranion that may bear different charges] exchanged with glutamate (Glu) shows an unexpected preferential adsorption of the lower-charged species (Glu 1− ) relative to the higher-charged species (Glu 2− ) by a layered double hydroxide (LDH) compound. At pH 11.0, the Glu 1− /Glu 2− ratio is about 0.44, an order of magnitude greater than expected in solution. Previous studies of phosphate and carbonate exchange onto LDH compounds (refs 25 and 26 ) show a strong preference for the higher-charged anion. The preference for Glu 1−, in which the amine site is protonated, may be due in part to −NH 3 + allowing for an energetically more favorable H-bonding network among the anions, the metal hydroxide substrate, and the interlayer and surface water molecules compared to −NH 2 . Changes in the pH and the p K a of Glu near the HT surface and due to nanoconfinement may also play important roles. These results suggest that the interactions dominating the exchange of amino acids and proteins onto LDH compounds may be quite different from those that control the exchange of small inorganic anions.