In situ Raman study of the crystallization of glycine

The process of crystallization of organic molecules is a well-known effect. However, a full understanding of this process is still lacking because it is very challenging to monitor fast changes in the molecular interactions. In this work, we perform an in situ study on the crystallization of glycine upon H2O and D2O evaporation by micro-Raman spectroscopy. The magnitude of the frequency splitting between the high-frequency asymmetric and the low-frequency symmetric CH2 stretching components is used to determine the dihedral angle xi, which measures the deviation from the planar glycine conformation through rotation around the C-C bond, hence providing insights on the molecular arrangement. Our results show that the average xi of the glycine molecules in both H2O and D2O is similar to 20 degrees, and it does decrease to similar to 18 degrees when the mature crystal is formed, corresponding to the molecular arrangement of alpha-glycine. However, right before the crystal is fully formed, a complex CH peak lineshape is observed, indicating that some of the molecules have xi similar to 22 degrees, which is close to the molecular arrangement of beta-glycine. Our results show that glycine molecules in water do not crystallize directly into a perfect alpha-glycine crystal, but a metastable state is first formed, where some of the molecules are more bent as compared to the alpha-glycine configuration.