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–2), 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 ...