Glycine phases formed from frozen aqueous solutions: Revisited

Abstract

Glycine phases formed when aqueous solutions were frozen and subsequently heated under different conditions were studied by Raman scattering, x-ray diffraction, and differential scanning calorimetry (DSC) techniques. Crystallization of ice I(h) was observed in all the cases. On cooling at the rates of 0.5 K∕min and 5 K∕min, glassy glycine was formed as an intermediate phase which lived about 1 min or less only, and then transformed into β-polymorph of glycine. Quench cooling of glycine solutions (15% w∕w) in liquid nitrogen resulted in the formation of a mixture of crystalline water ice I(h) and a glassy glycine, which could be preserved at cryogenic temperatures (80 K) for an indefinitely long time. This mixture remained also quite stable for some time after heating above the cryogenic temperature. Subsequent heating under various conditions resulted in the transformation of the glycine glass into an unknown crystalline phase (glycine "X-phase") at 209-216 K, which at 218-226 K transformed into β-polymorph of glycine. The "X-phase" was characterized by Raman spectroscopy; it could be obtained in noticeable amounts using a special preparation technique and tentatively characterized by x-ray powder diffraction (P2, a = 6.648 Å, b = 25.867 Å, c = 5.610 Å, β = 113.12[ordinal indicator, masculine]); the formation of "X-phase" from the glycine glassy phase and its transformation into β-polymorph were followed by DSC. Raman scattering technique with its power for unambiguous identification of the crystalline and glassy polymorphs without limitation on the crystallite size helped us to follow the phase transformations during quenching, heating, and annealing. The experimental findings are considered in relation to the problem of control of glycine polymorphism on crystallization.

Insights into the Crystallization and Structural Evolution of Glycine Dihydrate by In Situ Solid‐State NMR Spectroscopy

“…The evolution of the solid crystalline phases observed in our in situ solid‐state NMR studies mirrors the behavior reported in the powder XRD study of Xu et al . In their work, using the same procedure of flash cooling an aqueous solution of glycine below 145 K followed by slow warming to 209 K, a new powder XRD pattern was observed and the crystal structure of the new crystalline phase was determined from the powder XRD data as a dihydrate of glycine (GDH). On further warming of the sample to 250 K, the powder XRD pattern of GDH disappeared and was replaced by that of the β polymorph of glycine .…”

“…The recent work that proved the existence of glycine dihydrate (GDH) relied on a preparation procedure reported by Pyne and Suryanarayanan and later by Surovtsev et al . In this procedure, an aqueous solution of glycine is quench frozen in liquid nitrogen to form a glycine/water glass phase (frozen solution).…”

“…In the present work, the procedure reported previously has been carried out inside an NMR spectrometer, exploiting in situ solid‐state NMR techniques to monitor the structural evolution, as a function of temperature and time, of the glycine/water glass formed after flash cooling an aqueous solution of glycine. In recent years, in situ NMR techniques for monitoring crystallization processes have been applied to a range of systems, including crystallization of glycine from aqueous solution near ambient temperature ,.…”

Amino Acid Structures

“…Most interestingly, another polymorph has been discovered and tentatively characterized recently, found when heating previously frozen glycine solutions. This phase has been dubbed glycine "X-phase," crystallizes in space group P2, and exists in the temperature range between 209 and 216 K. At higher temperatures, this phase was found to transform into β-glycine (Surovtsev et al 2012).…”

“…4 These subtleties might be responsible for the fact that some experimental preparations following the literature do not always lead to reproducible results. Somehow frustratingly, despite the vast amount of research on crystal growth of these polymorphs, the knowledge is still not complete and the control of the outcome of crystallization is still not perfectly understood (Surovtsev et al 2012).…”