Adsorption of Amino Acids on Oxide Supports: A Solid-State NMR Study of Glycine Adsorption on Silica and Alumina

Abstract

The adsorption of glycine on a well-characterized silica surface was investigated at the molecular level by means of solid-state 13 C and 15 N NMR spectroscopy, combined with macroscopic level information such as adsorption isotherms, pH dependence, or TGA. At least three different forms of glycine were observed and could be distinguished on the basis of their NMR properties: two bulk crystalline forms (R-and β-glycine, both containing the amino acid in the zwitterionic form) and a molecularly adsorbed glycine species. The latter is formed by adsorption of zwitterionic glycine to the silica surface through the formation of cooperative hydrogen-bond networks in a kind of molecular recognition phenomenon, yielding a glycine/surface adduct. In addition, crystallites of metastable β-glycine nucleate at the silica/solution interface already during the initial adsorption, in a phenomenon of surface-induced precipitation. On γ-alumina, glycine does not form well-defined adducts but may coordinate the Al 3+ ions upon thermal activation.

Hydration–dehydration interactions between glycine and anhydrous salts: Implications for a chemical evolution of life

“…For example, recent IR and NMR observations revealed that speciations and orientations of amino acids adsorbed on alumina surface in dry conditions are not significantly different from those observed without alumina (Garcia et al, 2007;Lambert et al, 2009;Lopes et al, 2009). These observations mean that the mechanism (1) and (2) are not essential factors for amino acids' polymerizations on alumina surface.…”

“…These observations mean that the mechanism (1) and (2) are not essential factors for amino acids' polymerizations on alumina surface. The mechanism (3) also cannot explain the promotion effects of alumina, because such covalent bonds are not formed on alumina surface except in special conditions, such as adsorption of amino acids from the vapor phase at high temperature (>150°C) (Basiuk et al, 1990(Basiuk et al, -1991Basiuk and Gromovoy, 1993;Meng et al, 2004;Lambert et al, 2009;Lopes et al, 2009). The mechanisms (4) can be expected to decrease the D r G 0 of amino acids' polymerizations because adsorption energies of peptides are generally higher than those of amino acids (e.g., the adsorption energies of GlyGly toward amorphous silica and calcium montmorillonite are 0.23 kJ/mol (Basiuk et al, 1995) and 1.76 kJ/mol (Greenland et al, 1965) higher than those of Gly, respectively).…”

“…As suitable inorganic materials for the interaction experiment, silicate minerals may be first considered because they are present abundantly in the Earth' crust. However, silicate minerals are known to influence orientation and speciation of adsorbed amino acids through interactions between their surface OH groups ("Si-OH and "Al-OH) and functional groups of amino acids (-NH 3 + and -COO À ) (Meng et al, 2004;Garcia et al, 2007;Stievano et al, 2007;Kitadai et al, 2009a,b;Lambert et al, 2009;Lopes et al, 2009). These interactions may also influence polymerization reactivity of amino acids.…”

Silica Surface Features and Their Role in the Adsorption of Biomolecules: Computational Modeling and Experiments

“…In spite of this, the application of solid-state NMR to understand biomolecules adsorption on silica is recent. 211,[224][225][226][227][228] This is due to the low intrinsic sensitivity of the 13 C nucleus. In conjunction with the fact that the biomolecule is a minority species in the samples, it causes NMR signals to be very weak, so that long accumulation times are necessary -unless one uses expensive isotopically enriched biomolecules.…”

“…The rewards are high however because the 13 C isotropic shifts are very sensitive to the molecular environment: for instance it has been demonstrated that the C=O signal position responds in a predictable way to the different Hbonding states of HGly ± zwitterions in the three polymorphs -, -and -glycine, 229 and for Gly/Aerosil a specific signal was attributed to glycine zwitterions adsorbed by H-bonding. 224 A word of caution is in order again about the difficulty in quantifying 13 In spite of these divergences, the studies just mentioned represent a breakthrough in that they started investigating dynamic processes in adsorbed biomolecules systems instead of provinding a static picture -the very strong dependence of the adsorption behavior upon the hydration state of the surface revealed in this way, and the experimental determination of new observables such as dipolar coupling strengths, represent new challenges for molecular modeling.…”