Low-Vacuum Deposition of Glutamic Acid and Pyroglutamic Acid: A Facile Methodology for Depositing Organic Materials beyond Amino Acids

Thin layers of pyroglutamic acid (Pygl) have been deposited by thermal evaporation of the molten L-glutamic acid (L-Glu) through intramolecular lactamization. This deposition was carried out with the versatile handmade low-vacuum coater, which was simply composed of a soldering iron placed in a vacuum degassing resin chamber evacuated by an oil-free diaphragm pump. Molecular structural analyses have revealed that thin solid film evaporated from the molten L-Glu is mainly composed of L-Pygl due to intramolecular lactamization. The major component of the L-Pygl was in -phase and the minor component was in -phase, which would have been generated from partial racemization to DL-Pygl. Electron microscopy revealed that the L-Glu-evaporated film generally consisted of the 20？nm particulates of Pygl, which contained a periodic pattern spacing of 0.2？nm intervals indicating the formation of the single-molecular interval of the crystallized molecular networks. The DL-Pygl-evaporated film was composed of the original DL-Pygl preserving its crystal structures. This methodology is promising for depositing a wide range of the evaporable organic materials beyond amino acids. The quartz crystal resonator coated with the L-Glu-evaporated film exhibited the pressure-sensing capability based on the adsorption-desorption of the surrounding gas at the film surface. 1. Introduction L-glutamic acid (L-Glu) has been widely investigated due to its greatly beneficial properties in the food and pharmaceutical industries where its polymorphism and crystalline shape have received considerable attention [1, 2]. It should be noted that the transformation between the metastable α form of L-Glu and its stable β form has been widely studied not only out of scientific interest, but also from the application aspect [3–5]. The polymorphs of amino acids are ascribed to the complex interactions between the hydrogen bonding moieties (amino and carboxy groups) and side chains, which permit the steric and electrostatic interactions that specifically form the well-ordered aggregates in the solid phase. The L-Glu polymorph is substantially governed by the degree of charge separation in the main interaction moieties of the amino and carboxy groups, whose charging state is either zwitterionic or neutral [6]. Because the molecular packing arrangements of two polymorphs (α and β forms) of L-Glu differ significantly, the transformation between polymorphs in the solid state would hardly occur at room temperature [7]. The thermal effects on the polymorphs of L-Glu have been an intriguing subject in