Phase transitions of glycine in frozen aqueous solutions and during freeze-drying.
OBJECTIVE
To study the solid-state and phase transitions of glycine, (i) in frozen aqueous solutions, and (ii) during freeze-drying.


METHODS
X-ray powder diffractometry (XRD) and differential scanning calorimetry (DSC) were used to analyze the frozen systems. In situ freeze-drying in the sample chamber of the diffractometer enabled characterization of phase transitions during freeze-drying.


RESULTS
Transitions in frozen systems. Rapid (20 degrees C/min) or slow (2 degrees C/min) cooling of aqueous solutions of glycine (15% w/w) to -70 degrees C resulted in crystallization of beta-glycine. Annealing at -10 degrees C led to an increase in the amount of the crystalline phase. When quench-cooled by immersing in liquid nitrogen, glycine formed an amorphous freeze-concentrate. On heating, crystallization of an unidentified phase of glycine occurred at approximately -65 degrees C which disappeared at approximately -55 degrees C, and the peaks of beta-glycine appeared. Annealing caused a transition of beta- to the -gamma- form. The extent of this conversion was a function of the annealing temperature. Slower cooling rates and annealing in frozen solutions increased the crystalline beta-glycine content in the Iyophile. Freeze-drying of quench-cooled solutions led to the formation of gamma-glycine during primary drying resulting in a lyophile consisting of a mixture of beta- and -gamma-glycine. The primary drying temperature as well as the initial solute concentration significantly influenced the solidstate of freeze-dried glycine only in quench-cooled systems.


CONCLUSIONS
The cooling rate, annealing conditions and the primary drying temperature influenced the solid-state composition of freeze-dried glycine.
Pyne
A
A
College of Pharmacy, University of Minnesota, 55455, USA.
Suryanarayanan
R
R
D002152:Calorimetry, Differential Scanning; D003460:Crystallization; D005612:Freeze Drying; D005615:Freezing; D005998:Glycine / Q000737:chemistry*; D012996:Solutions; D013696:Temperature; D014961:X-Ray Diffraction
D012996:Solutions; D005998:Glycine