Glutamic Acid Adsorption and Transformations on Silica

Abstract

We report here on the adsorption and deposition of L-glutamic acid (Glu) on an amorphous silica and its subsequent thermal transformations. When adsorbed from aqueous solutions, Glu only has a low affinity for the silica surface but at high concentrations the adsorbed molecules serve as nuclei for the formation of small glutamic acid crystallites. As long as the Glu loading remains inferior to a saturation limit (about 0.5 molecule nm À2 ), the thermal behavior of adsorbed Glu is significantly different from that of bulk glutamic acid. Two successive condensation steps are observed upon mild thermal activation, and their products have been identified by a combination of techniques including thermogravimetry, in situ IR, solid-state NMR, and ESI-MS. The first step at about 120 °C is a lactam ring closure quantitatively yielding pyroglutamic acid in a first-order reaction. The second step, at 150 °C, probably results in the formation of a highly activated tricyclic imide, PyroGluDKP, and is easily reversible in the presence of water. These reactions have implications for prebiotic peptide formation and for synthetic chemistry.

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

“…This pH-dependent adsorption has been also observed by other authors via acid-base titration experiments. Bouchoucha et al 211 and Gao et al 253 also observed low amounts of Glu adsorption on amorphous silica and SBA-15, respectively, which was attributed to the negligible difference in pH between the pI of Glu and the PZC of these silica materials (around 3).…”

“…For instance, bulk amino acid crystals were evidenced both in Gly/SiO 2 210 and Glu/SiO 2 . 211 An interesting phenomenon that will be discussed in Section 9.1.1.2 is that in the case of Gly, excess glycine precipitated as the metastable  polymorph, while the  form is normally observed upon precipitation from a clean solution.…”

“…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.…”

“…As far as Glu is concerned, Bouchoucha et al 211 recorded the IR spectrum for the Glu/silica system, results showing that at pH 3 the predominant species was the H 2 Glu  zwitterionic state, as the dominant bands were those involving the backbone NH 3 + and COO -groups and the protonated side chain COOH (see Figure 75a). As expected, at pH 8-9 the C=O band associated to COOH was absent indicating that the predominant species was the HGlu -anion.…”

“…Noticeably, the peak at 150 ºC was not present for the DKP/silica system as no amide bond formation could occur (there are no free amine or carboxylic groups in this molecule). In spite of these general similarities between AA, the case of Glu has been studied in more detail by Bouchoucha et al 211 because, in addition to the peak at 150-160 ºC, another well defined peak at 110-120 ºC was also detected in some conditions (see Figure 80a). Experimental measurements performed with FTIR, ssNMR and ESI-MS (ElectroSpray Ionization-Mass Spectrometry) techniques clearly indicated that this first peak was due to an intramolecular lactam ring closure of Glu to yield pyroglutamic acid (PyroGlu, see Figure 80b).…”