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

Abstract

13 C MAS NMR spectroscopy of isotopically enriched samples of the layered double hydroxide hydrotalcite (HT) [(Mg 2 Al)(OH) 6 A -,nH 2 O, where Ais 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 2ratio 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 highercharged 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 pK 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.

A solid state NMR study of layered double hydroxides intercalated with para-amino salicylate, a tuberculosis drug

“…To fully understand the composition of LDH, bulk characterization techniques such as powered X-ray diffraction (PXRD), elemental analysis and microscopy must be combined with probes of the local environment such as vibrational spectroscopy, e.g., Raman and Infra-Red (IR) spectroscopy, as well as solid state NMR spectroscopy (SSNMR) 10 . SSNMR especially has provided detailed insight into both the local structure of LDHs revealing cation ordering in the cation layer [10][11][12][13][14] , the dynamics of organic and inorganic anions in the interlayer [15][16][17] . Moreover, SSNMR has only been used in a few studies of drug molecules such as benzoxaboraolates, salicylic acid, ibuprofen, and L-dopa intercalated in LDHs 8,[18][19][20][21] .…”

Intercalative Ion‐Exchange Route to Amino Acid Layered Double Hydroxide Nanohybrids and Their Sorption Properties

“…In the case of Glu-LDH, the basal spacing was determined to be 1.22 nm, as shown in Figure 1D, which is similar to that previously reported for Glu-LDH. [21,22] Consequently, the gallery height could be estimated as 0.74 nm (1.22-0.48 nm), which is even smaller than the molecular length of Glu (0.90 nm). It is, therefore, expected that the Glu molecules are oriented perpendicularly to the basal plane with a monolayer structure resulting in a tilting angle of about 54°with respect to the LDH ab plane, as shown in Figure 1D.…”

“…On the other hand, the acid dissociation constants of Glu are pK a1 = 2.16, pK a2 = 4.15, and pK a3 = 9.58 at 298 K in aqueous solution. [21,23] Since the intercalation reaction of Glu into LDH by the ion-exchange method was performed at a pH of 11.0, the Glu molecules were preferentially intercalated into the LDH interlayer in their dianionic form, as shown in Figure S2B(d), which is different from the intercalation by coprecipitation. [21] …”

“…This is probably due to the lack of understanding of the equilibrium between the intercalation reaction of the amino acid into the LDH and its deintercalation out of the LDH. [20][21][22] In this respect, we intended to solve the hitherto known problems on AALDHs in order not to mislead the readers.…”

“…Secondly, since amino acids are amphiprotic, it is quite clear that the intercalative ion-exchange reaction with LDHs should be influenced by the solution pH. [21][22][23] Taking into account the pK a values of two different amino acids, the pH of the intercalation reaction has been carefully controlled. AA-LDHs prepared by this method were characterized by powder X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy in order to rationalize the heterostructure models for the two hybrid materials.…”