Partially Crystalline Systems in Lyophilization: I. Use of Ternary State Diagrams to Determine Extent of Crystallization of Bulking Agent

Two model ternary systems: water-glycine-raffinose and water-glycine-trehalose were investigated to determine the extent of glycine crystallization in frozen solutions. The use of such partially crystalline systems allows primary drying to be carried out substantially above the collapse temperature. Differential scanning calorimetry (DSC) and variable temperature X-ray diffractometry (XRD) were used to monitor phase transitions in frozen systems as well as to determine the Tg′. Aqueous solutions containing different glycine to carbohydrate weight ratios were first cooled to −60°C and then warmed to room temperature. In both raffinose and trehalose systems, when the initial glycine to sugar (raffinose pentahydrate or trehalose dihydrate) ratio was <1, glycine crystallization was not detected. When the ratio was ≥1, partial glycine crystallization was observed during warming. The presence of amorphous glycine caused the Tg′ to be substantially lower than that of the solution containing only the carbohydrate. To determine the extent of glycine crystallization, the solutions were annealed for 5 h just above the temperature of glycine crystallization. The Tg′ observed in the second warming curve was very close to that of the carbohydrate solution alone, indicating almost complete glycine crystallization. These studies enabled the construction of the water-rich sections of the raffinose-glycine-water and trehalose-glycine-water state diagrams. These diagrams consist of a kinetically stable freeze-concentrated solution and a doubly unstable glassy region, which readily crystallizes during cooling or subsequent warming. In addition, there is an intermediate region, where during the experimental timescale, there appears to be hindered glycine nucleation but unhindered crystal growth. To obtain substantially crystalline glycine in the frozen solutions, the glycine to carbohydrate ratios should be ≥1.