Aqueous solutions of proline and NaCl studied by differential scanning calorimetry at subzero temperatures

The hydration properties of proline are studied by differential scanning calorimetry (DSC) in aqueous solutions during freezing to -60 degrees C and subsequent heating to +20 degrees C. The concentration of proline in the freeze concentrated solution was estimated to approximately 50 wt% (w/w) indicating a high water solubility of proline at subzero temperatures. No glass transition was observed within the concentration range 0.9-40.1 wt% (w/w), neither at a low scanning rate of 2.5 degrees C/min nor at a higher scanning rate of 10 degrees C/min. Eutectic crystallization of proline was not observed during freezing or melting which shows that proline has the ability to stay in solution at subzero temperatures. Samples containing proline-NaCl-water were also investigated by DSC and it was shown that the solubility of proline is maintained in aqueous salt solutions at temperatures as low as -60 degrees C. From DSC measurements it was found that the eutectic crystallization of NaCl is prevented by the presence of proline, even when NaCl (initially) is present in molar excess ([NaCl]/[proline] = 2.6). The possible association of these findings with the occurrence of proline accumulation in some plants and insects living under water stress conditions is discussed. (C) 1997 Elsevier Science B.V.