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2009, 113, 3378–3381 Published on Web 02/11/2009
Preferential Adsorption of Lower-Charge Glutamate Ions on Layered Double Hydroxides: An NMR Investigation Marc X. Reinholdt,*,†,§ Panakkattu K. Babu,†,‡,| and R. James Kirkpatrick†,⊥ Department of Geology, UniVersity of Illinois at Urbana-Champaign, 1301 West Green Street, Urbana, Illinois 61801, Department of Chemistry, UniVersity of Illinois at Urbana-Champaign, 600 South Mathews AVenue, Urbana, Illinois 61801 ReceiVed: December 12, 2008; ReVised Manuscript ReceiVed: February 2, 2009
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C MAS NMR spectroscopy of isotopically enriched samples of the layered double hydroxide hydrotalcite (HT) [(Mg2Al)(OH)6A-,nH2O, where A- is a counteranion that may bear different charges] exchanged with glutamate (Glu) shows an unexpected preferential adsorption of the lower-charged species (Glu1-) relative to the higher-charged species (Glu2-) by a layered double hydroxide (LDH) compound. At pH 11.0, the Glu1-/ Glu2- 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 highercharged anion. The preference for Glu1-, in which the amine site is protonated, may be due in part to -NH3+ 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 -NH2. Changes in the pH and the pKa 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. Introduction The interaction of biomolecules with solid surfaces is of significant interest in environmental remediation, catalysis, drug delivery, and geochemical transport but is poorly understood on the molecular scale.1-5 Many of these species, such as amino acids, carboxylic species, peptides, proteins, and natural organic matter molecules, are negatively charged at neutral to basic pH values. Thus, they interact strongly with solids having positive structural or pH-dependent charges. Layered double hydroxides (LDHs), also known as anionic clays, have a permanent positive structural charge and are known to exchange many organic species effectively; they are receiving increasing amounts of attention.6-19 We present here an experimental NMR study of the behavior of 13C-enriched glutamate (Glu) adsorbed on the prototypical LDH, hydrotalcite [HT; nominally (Mg2Al)(OH)6A-,nH2O, where A- is a counteranion that may bear different charges] synthesized at pH values from 9.1 to 11.0. 13 C enrichment was used to allow for the collection of MAS NMR measurements under non-crossed-polarization (CP) conditions. In our previous NMR, XRD, and TGA-DSC study of HT containing glutamate, we showed that much of the adsorbed Glu is not intercalated in the HT interlayer but occurs on external * Corresponding author. E-mail:
[email protected]. Phone: 418656-2131 ext.4318. Fax: 418-656-5993. † Department of Geology. ‡ Department of Chemistry. § Present address: De´partement de Ge´nie Chimique, Universite´ Laval, 1065 ave. de la Me´decine, Que´bec, QC G1V 0A6, Canada. | Present address: Department of Physics, Western Illinois University, 1 University Circle, Macomb, Illinois 61455. ⊥ Present address: Office of the Dean, College of Natural Science, Michigan State University, East Lansing, Michigan 48824-1115.
10.1021/jp8109786 CCC: $40.75
SCHEME 1: Structure of Glutamic Acid, pKa ) 2.16 (-COOH/-COO-), pKb ) 9.58 (-NH3+/-NH2), pKc ) 4.15 (δ-COOH/δ-COO-), pI ) 3.22 (isoelectric point).
particle surfaces.19 1H-13C CP-MAS NMR also shows that the relative abundance of Glu2- increases with increasing pH.19 The results for 13C-enriched samples presented here show that the observed Glu1-/Glu2- ratios are much greater than in the solution and that the difference increases with increasing pH. To our knowledge, this is the first NMR-based quantitative analysis of the abundances of amino acids with different charges adsorbed on a solid.20-22 Experimental Methods The samples were synthesized using a coprecipitation method involving the hydrolysis of Mg2+ and Al3+ ions in the presence of glutamic acid (Scheme 1) by a procedure similar to that of Aisawa et al.,8 which we have described previously in detail.19 The Mg2Al-Glu samples (Mg/Al ratio ) 2) containing 13Cenriched glutamic acid were synthesized at 65 ( 1 °C and pH 9.1, 10.0, and 11.0 using a mixture of 1/3 98% enriched and 2/3 unenriched Glu. The values of the final pH are 8.9, 10.3, and 11.3 for the initial pH values of 9.1, 10.0, and 11.0, respectively. The pH differences are close to the error range that we estimate to be 0.2 pH units, and thus we consider that these differences are negligible. To control the hydration state of our samples 2009 American Chemical Society
Letters
Figure 1. 13C MAS NMR spectra of glutamate-hydrotalcite samples synthesized with 33% 13C-enriched glutamic acid at pH 9.1, 10.0, and 11.0 and 65 ( 1 °C.
after synthesis, they were stored over a saturated NH4Cl solution (RH ) 79%). Single-pulse 1H-decoupled 13C NMR spectra were acquired at a frequency of 150.832 MHz on a Varian Infinity-Plus widebore spectrometer using a 4 mm triple-resonance MAS probe (Varian) with zirconia rotors. The 90° pulse durations and the relaxation delays for the experiments were 4 µs and 20 s, respectively. The MAS spectra were obtained by averaging 1800 transients at a spinning speed of 13 kHz, and the chemical shifts are referenced to the methylene resonance of external adamantane at 38.2 ppm with respect to tetramethylsilane. After their acquisition, the free induction decay signals were processed following standard procedures using NMR Utility Transform Software (NUTS, Acorn NMR Software). Background subtraction and baseline correction were applied when needed, and the spectra were then deconvoluted from fits to Gaussian peaks using NUTS. Results and Discussion The 13C NMR spectra of the Glu-HT samples synthesized at pH 9.1, 10.0, and 11.0 demonstrate the presence of both Glu1and Glu2- and show that the Glu1-/Glu2- ratio is much lower at pH 11.0 than at pH 9.1 or 10.0 but is significantly higher than expected in the bulk solution (Figure 1). All three spectra contain resonances readily assigned to the C0-COO-, R-CHNH2/ NH3+-, β-CH2-, γ-CH2-, and δ-COO- sites of Glu (Table
J. Phys. Chem. C, Vol. 113, No. 9, 2009 3379 1),23 though the chemical shifts of the LDH samples seem to be about 2 ppm more shielded compared to glutamate species in aqueous solution. Both the C0-COO- and β-CH2- resonances are titrating in this pH range, and resonances corresponding to these sites in Glu1- and Glu2- are present. On the basis of the fitted intensities for these two types of sites, the Glu1-/Glu2- ratios are 1.7 (C0-COO- site) and 2.0 (β-CH2site) at pH 9.1, 2.3 (C0-COO- site) and 2.0 (β-CH2- site) at pH 10.0, and 0.46 (C0-COO- site) and 0.42 (β-CH2- site) at pH 11.0. In addition to the Glu1-/Glu2- ratios obtained here for three different pH values, samples synthesized at ∼pH 10 in our previous work give ratios of ca. 1.19 The values obtained by NMR are very different from those expected in bulk solution as calculated from the measured pH and from the literature pKa values.24 These values are 3.0 at pH 9.1, 0.38 at pH 10.0, and 0.038 at pH 11.0. Because of the multiple -COO- and -CH2resonances and the resulting peak overlap, it is difficult to extract very accurate values of the Glu1-/Glu2- ratio from the deconvolutions. This is particularly noticeable for the samples synthesized at ∼pH 10, which give ratios in the range of 1 to 2, although the values are all of the same order of magnitude. A fit for the pH 11.0 sample assuming that the chemical shifts for the C0-COO- and β-CH2- sites are fixed at their values for the sample synthesized at pH 9.1 (values in parenthesis in Table 1) yields Glu1-/Glu2- ratios of 0.22 and 0.31 for C0-COO- and β-CH2- sites, respectively, which are only slightly less than those obtained by allowing the chemical shifts to titrate. Thus, although we cannot rule out the smaller-than-expected Glu1-/Glu2- ratio observed at pH 9.1 as being due to the accuracy of the deconvolutions, the values at pH 10.0 and 11.0 are, respectively, half an order of magnitude and an order of magnitude larger than expected. The calculations of the theoretical Glu1-/Glu2- ratios in solution use the initial pH values, and the final equilibrated pH may be somewhat different (see above).19 Except for the pH 9.1 sample, a higher final pH value would lead to an even greater difference between the theoretical and observed Glu1-/Glu2- ratios. These results, thus, unambiguously demonstrate a preferential adsorption of Glu1- relative to the bulk solution values under pH conditions higher than the pKa of the amine site (pKb ) 9.58).24 This preferential adsorption of the lower charged anion is in contrast to previous results for small inorganic species, which show that LDH compounds have a strong preference for higher-charge phosphate and carbonate species.25,26 On this basis, the possible preferential adsorption of Glu2- at pH values lower than the pKa is expected. The origin of the preferential adsorption of Glu1- at high pH values probably involves multiple factors. Potential contributing factors include the effects of the surface and interlayer nanoconfinement on pH and pKa and steric and energetic effects of Coulombic and H-bond interactions among the anions, the metal hydroxide substrate, and the interlayer and surface water molecules. Molecular dynamics simulations have shown that the formation of an integrated H-bond network among these sites dominates the energetics of carboxylate species in LDH interlayers.22,27 For Glu, purely electrostatic attraction should favor Glu2-,28,29 but electrostatics may be in competition with specific adsorption mechanisms, including hydrogen bonding, the need for an integrated H-bond network, and covalent bond formation.29 The similarity of the 13C NMR chemical shifts and the ease of Glu exchange indicate, however, that covalent bond formation is unlikely. H-bonding interactions for Glu at the pH values used in this study occur at -COO- sites (strong H-bond acceptors), -NH2 (an H-bond acceptor and donor), and -NH3+
3380 J. Phys. Chem. C, Vol. 113, No. 9, 2009
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TABLE 1: 13C MAS NMR Data for 13C-Enriched Glutamate Hydrotalcite Synthesized at pH 9.1, 10.0, and 11.0 and 65 °C, Where the Chemical Shift Assignments are Based on Reference 23 pH
componenta
chemical shift (ppm)
fwhm (Hz)
relative peak area (%)
9.1
β-CH2- (Glu1-) β-CH2- (Glu2-) γ-CH2Na-CH2-b R-CHNH2/NH3+C0-COO- (Glu1-) δ-COO- and C0-COO- (Glu2-)
27.1 30.0 33.0 36.2 54.3 175.6 180.7
720 900 530 400 550 620 560
14 7 12 1 19 15 31
10.0
β-CH2- (Glu1-) β-CH2- (Glu2-) γ-CH2Na-CH2-b R-CHNH2/NH3+C0-COO- (Glu1-) δ-COO- and C0-COO- (Glu2-)
28.0 30.5 34.0 37.8 55.2 176.4 181.9
770 720 600 310 590 730 550
16 8 12 1 19 15 29
11.0c
β-CH2- (Glu1-) β-CH2- (Glu2-) γ-CH2Na-CH2-b R-CHNH2/NH3+C0-COO- (Glu1-) δ-COO- and C0-COO- (Glu2-)
28.1 (27.1d) 30.4 (30.6) 33.3 (33.2) 36.6 (36.2) 55.2 (55.2) 178.2 (176.4d) 181.4 (181.3)
1350 (1180) 1000 (1020) 470 (460) 580 (590) 640 (640) 1230 (900) 445 (470)
5 (4) 12 (13) 18 (17) 2 (2) 19 (19) 7 (4) 37 (41)
a The 180-182 ppm chemical shift is assigned to the C0-COO- carbon from Glu2- and to the δ-COO- carbon from both Glu1- and Glu2-. Ratio calculations made from the relative peak areas of the 175-177 and 180-182 ppm chemical shifts take into account that the latter is a superposition of three components, one of them, δ-COO- (Glu1-), having the same contribution as the C0-COO- (Glu1-) signal, and the rest of the component is equally shared between the δ-COO- (Glu2-) and C0-COO- (Glu2-) signals. b This chemical shift is assigned to the -CH2- groups present in sodium glutamate species (noted Na-CH2-). c Values in parenthesis are those obtained for a fit processed with fixed chemical shifts. d Fixed chemical shift.
(principally an H-bond donor). The -COO- groups accept H-bonds from the surface -OH groups, water molecules, and the amine sites of the Glu species. The presence of -COOgroups at both ends of the Glu backbone may allow the surface adsorption of both groups, as observed for TiO2 surfaces.30 Interactions involving the amine sites may be critical. The pKa of the amine site (pKb ) 9.58)24 is related to the deprotonation of -NH3+ to -NH2, and the preference for Glu1- may be due to the formation of a more integrated H-bond network with -NH3+ sites.29 -NH2 may donate H-bonds to -COO- groups, surface -OH groups, and water molecules and receive H-bonds from surface -OH groups and water molecules. Similar H-bonding sites are possible for -NH3+, but it is more likely to act as an H-bond donor than an -NH2 group. Additionally, because -NH3+ sites are positive, they are more likely to be more privileged H-bond donors to -COO- groups. Glu species have a strong affinity for water,19,22,31 and their solvation must also play a critical role in this H-bonding network. Such network stabilization is known to modify the pKa values of the carboxylic acid groups in amino acids, proteins, enzymes, and humic substances via either H-bonding with water molecules or between the biomolecules themselves.32-35 This effect, frequently referred to as a pKa shift, may increase or reduce the acidity of different sites. For instance, a pKa shift of carboxylic acid groups by H-bond network stabilization has been observed for amido acid-functionalized fused quartz/water interfaces.36 For our samples, there may be a similar effect on the amine groups of the adsorbed Glu species, shifting the pKa of the -NH3+/NH2 couple toward higher values and rendering the amine group more basic. To our knowledge, this is the first time that such behavior has been proposed for the amine group of an adsorbed amino acid. The solution pH may also be affected by nanoconfinement in the HT interlayers and by the presence of external surfaces.
Indeed, previous studies of inorganic surfaces show that both pH and pKa values can change at surfaces and interfaces.37-40 pH changes near interfaces relative to bulk values have been attributed to the presence of elevated counterion concentrations39-41 and to kinetic effects.40 H bonding may play a key role by modifying the Glu pKa value.39 Monitoring pH and pKa changes at interfaces has been possible in recent years using second-harmonic generation, which allows the determination of interfacial potentials and surface charge densities.36-40 The nature of the counterion is also significant because dispersion effects can lead to ions with high polarizability (e.g., NO3-) having higher binding energies than those having lower polarizability (e.g., acetate).41 This has an important impact in our case because there is competition between Glu species, NO3-, OH-, and CO32- for surface and interlayer sites during sample synthesis.19 Conclusion In this work, we have used 13C MAS NMR of isotopically enriched samples to investigate the proportion of glutamate anions with different charges (Glu1- and Glu2-) adsorbed on or intercalated into coprecipitated HT. The Glu1-/Glu2- ratios are determined for a set of samples synthesized at pH values from 9.1 to 11.0. At pH 10.0 and 11.0, this ratio is significantly higher than expected in bulk solution, demonstrating for the first time the preferential adsorption of a lower-charged anion on an LDH. This unexpected discrepancy may be due to complex effects involving an H-bond network favoring the adsorption of Glu containing the positively charged -NH3+ functional group instead of the neutral -NH2 group. Such H-bonding network stabilization is referred to as a pKa shift, but this is the first time that this phenomenon has been proposed for the amine group of an amino acid adsorbed on an LDH.
Letters Effects of nanoconfinement in the LDH interlayers and the presence of the LDH surfaces may also affect the local pH and the pKa of the adsorbed Glu. This phenomenon is strongly dependent on the counterion concentration in the bulk, especially of highly polarizable ions such as NO3-, which is a significant competitor for adsorption in our system. Thus, Glu adsorbed on HT-like compounds may undergo pKa changes as a result of both interfacial pH changes and H bonding. Acknowledgment. This work was supported by grant DOEFG02-00ER15028 from the geoscience program of the U.S. Department of Energy Division of Basic Energy Sciences. References and Notes (1) Bank, S.; Yan, B.; Edwards, J. C.; Ofori-Okai, G. Langmuir 1994, 10, 1528. (2) Hill, A. R., Jr.; Bo¨hler, C.; Orgel, L. E. Orig. Life EVol.Biosph. 1998, 28, 235. (3) Liu, R.; Orgel, L. E. Orig. Life EVol.Biosph. 1998, 28, 245. (4) Fischer, K. Water, Air, Soil Pollut. 2002, 137, 267. (5) Ding, X.; Henrichs, S. M. Mar. Chem. 2002, 77, 225. (6) Whilton, N. T.; Vickers, P. J.; Mann, S. J. Mater. Chem. 1997, 7, 1623. ´ .; Pa´linko´, I.; Kiricsi, I. Inorg. Chem. 1999, 38, 4653. (7) Fudala, A (8) Aisawa, S.; Takahashi, S.; Ogasawara, W.; Umetsu, Y.; Narita, E. J. Solid State Chem. 2001, 162, 52. (9) Nakayama, H.; Wada, N.; Tsuhako, M. Int. J. Pharm. 2004, 269, 469. (10) Aisawa, S.; Sasaki, S.; Takahashi, S.; Hirahara, H.; Nakayama, H.; Narita, E. J. Phys. Chem. Solids 2006, 67, 920. (11) Wei, M.; Shi, Z.; Evans, D. G.; Duan, X. J. Mater. Chem. 2006, 16, 2102. (12) Silverio, F.; dos Reis, M. J.; Tronto, J.; Valim, J. B. Appl. Surf. Sci. 2007, 253, 5756. (13) Wei, M.; Guo, J.; Shi, Z.; Yuan, Q.; Pu, M.; Rao, G.; Duan, X. J. Mater. Sci. 2007, 42, 2684. (14) Choy, J.-H.; Kwak, S.-Y.; Park, J.-S.; Jeong, Y.-J.; Portier, J. J. Am. Chem. Soc. 1999, 121, 1399. (15) Ambrogi, V.; Fardella, G.; Grandolini, G.; Nocchetti, M.; Perioli, L. J. Pharm. Sci. 2003, 92, 1407. (16) Tyner, K. M.; Schiffman, S. R.; Giannelis, E. P. J. Controlled Release 2004, 95, 501.
J. Phys. Chem. C, Vol. 113, No. 9, 2009 3381 (17) del Arco, M.; Cebadera, E.; Gutie´rrez, S.; Martı´n, C.; Montero, M. J.; Rives, V.; Rocha, J; Sevilla, M. A. J. Pharm. Sci. 2004, 93, 1649. (18) Mohanambe, L.; Vasudevan, S. J. Phys. Chem. B 2005, 109, 15651. (19) Reinholdt, M. X.; Kirkpatrick, R. J. Chem. Mater. 2006, 18, 2567. (20) Hou, X.; Kalinichev, A. G.; Kirkpatrick, R. J. Chem. Mater. 2002, 14, 2078. (21) Hou, X.; Bish, D. L.; Wang, S.-L.; Johnston, C. T.; Kirkpatrick, R. J. Am. Mineral. 2003, 88, 167. (22) Kumar, P. P.; Kalinichev, A. G.; Kirkpatrick, R. J. J. Phys. Chem. B 2006, 110, 3841. (23) Quirt, A. R.; Lyerla, J. R., Jr.; Peat, I. R.; Cohen, J. S.; Reynolds, W. F.; Freedman, M. H. J. Am. Chem. Soc. 1974, 96, 570. (24) “Properties of Amino Acids”, in CRC Handbook of Chemistry and Physics, 88th Edition (Internet Version 2008), Lide, David R., ed., CRC Press/Taylor and Francis, Boca Raton, FL. (25) Dutta, P. K.; Puri, M. J. Phys. Chem 1989, 93, 376. (26) Hou, X.; Kirkpatrick, R. J. Inorg. Chem. 2001, 40, 6397. (27) Kumar, P. P.; Kalinichev, A. G.; Kirkpatrick, R. J. J. Phys. Chem. C 2007, 111, 13517. (28) Stievano, L.; Piao, L. Y.; Lopes, I.; Meng, M.; Costa, D.; Lambert, J.-F. Eur. J. Mineral 2007, 19, 321. (29) Lambert, J.-F. Orig. Life EVol.Biosph. 2008, 38, 211. (30) Roddick-Lanzilotta, A. D.; McQuillan, A. J. J. Colloid Interface Sci. 2000, 227, 48. (31) Li, Q.; Kirkpatrick, R. J. Am. Mineral. 2007, 92, 397. (32) Lim, C.; Bashford, D.; Karplus, M. J. Phys. Chem. 1991, 95, 5610. (33) Zscherp, C.; Schlesinger, R.; Tittor, J.; Oesterhelt, D.; Heberle, J. Proc. Natl. Acad. Sci. USA 1999, 96, 5498. (34) Leenheer, J. A.; Wershaw, R. L.; Brown, G. K.; Reddy, M. M. Appl. Geochem. 2003, 18, 471. (35) Schutz, C. N.; Warshel, A. Proteins: Struct., Funct., Bioinf. 2004, 55, 711. (36) Gibbs-Davis, J. M.; Hayes, P. L.; Scheidt, K. A.; Geiger, F. M. J. Am. Chem. Soc. 2007, 129, 7175. (37) Zhao, X.; Subrahmanyan, S.; Eisenthal, K. B. Chem. Phys. Lett. 1990, 171, 558. (38) Eisenthal, K. B. Chem. ReV. 1996, 96, 1343. (39) Konek, C. T.; Musorrafiti, M. J.; Al-Abadleh, H. A.; Bertin, P. A.; Nguyen, S. B.; Geiger, F. M. J. Am. Chem. Soc. 2004, 126, 11754. (40) Gibbs-Davis, J. M.; Kruk, J. J.; Konek, C. T.; Scheidt, K. A.; Geiger, F. M. J. Am. Chem. Soc. 2008, 130, 15444. (41) Bostro¨m, M.; Williams, D. R. M.; Ninham, B. W. Langmuir 2002, 18, 8609.
JP8109786
