Solid-liquid phase behaviour of novel kosmotrope/chaotrope binary mixtures involving sarcosine

The concept of deep eutectic solvents (DESs) has been recently applied to osmolytes (kosmotropes or chaotropes), small organic molecules responsible for stabilization of proteins and other biomolecules in various biological systems. In this study the combinations of the kosmotrope sarcosine (Sar) with chaotrope urea (U) or guanidine hydrochloride (G) across various molar ratios was studied experimentally by the differential scanning calorimetry and infrared spectroscopy. Theoretical prediction of thermodynamic interaction terms based on screening of the charge density was used to support interpretation of experimental findings. Phase diagrams were constructed for both Sar-U and Sar-G systems and eutectic behaviour was identified. Neither of the two systems showed depression of the eutectic temperature compared to its ideal solution values, and therefore neither of the systems could be classified as a deep eutectic solvent in the classical sense. The Sar-G system did show at least partial crystallization of its eutectic solution upon cooling. However, the eutectic solution of the Sar-U system remained amorphous at decreased temperatures, and the phenomenon was attributed to be kinetic in nature and originating from strong intermolecular hydrogen bonding interactions of the components. Thus, it was speculated that the role of osmolytes in stabilizing cellular biomolecules might not come from the thermodynamics only.