Characterization of Frozen Solutions of Glycine

Abstract

The broad objective of this research was to better understand the physical chemistry of freeze drying of the system glycine/water, with emphasis on the role of polymorphism of glycine on freezing and freeze-drying behavior. Frozen solutions of glycine were characterized by differential scanning calorimetry (DSC) and by freeze-dry microscopy. Cooling rates ranged from 0.1 degrees C/min to quench-cooling by immersing samples in liquid nitrogen. During slow cooling, only a beta-glycine/ice eutectic mixture is formed, melting at -3.60 degrees C. For quench-frozen solutions, the low-temperature thermal behavior is more complex. A complex glass transition region is observed on the DSC thermogram, with midpoint temperatures at about -73 degrees C and -60 degrees C, as well as two separate crystallization exotherms. Use of very low heating rates in the DSC experiment allows resolution of four separate endotherms in the temperature range just below the melting of ice. The experimental data support the conclusion that these endotherms arise from melting of the beta-glycine/ice eutectic mixture at -3.6 degrees C, dissolution of crystals of alpha-glycine at -2.85 degrees C, and melting of the gamma-glycine/ice eutectic mixture at -2.70 degrees C. One of the endotherms could not be characterized because of inadequate resolution from the beta-glycine/ice eutectic melting endotherm. Freeze-dried solids were characterized by X-ray powder diffraction after annealing under conditions established by the DSC and freeze-dry microscopy experiments. Annealing at controlled temperatures in the melting region prior to recooling the system was useful not only in interpreting the complex DSC thermogram, but also in controlling the glycine polymorph resulting from freeze drying.