THE SOLUBILITIES OF l-PROLINE AND l-HYDROXYPROLINE IN WATER, THE CALCULATED HEATS OF SOLUTION, AND THE PARTIAL MOLAL VOLUME OF l-HYDROXYPROLINE

The present work is a continuation of the previously reported studies on the solubilities of various amino acids in water and their thermodynamic properties (1). No precise solubility determinations on l-proline and/-hydroxyproline appear to have been carried out previously. Due to the high solubility of l-proline and l-hydroxyproline, it was not feasible to carry out solubility determinations over as great a temperature range as in the previously reported studies. There is a tendency for these amino acids, especially hydroxyproline, to darken at higher temperatures. I t was necessary to modify somewhat the previously reported technique for the determination of solubility and partial molal volumes. A micro pipette of about 100 c.mm. capacity was used. I t possessed a uniform capillary stem whose diameter was about 0.3 ram.; 1 ram. of the capillary corresponded to about 0.07 c.mm. The volume of the pipette was calibrated with the aid of mercury. The amount of amino acid solution which was contained in the pipette was estimated by measuring the distance on the stem which was not filled with the solution. For the estimation of density, the pipette was filled to a certain point on the stem and the tip of the pipette was wiped with wet cotton. The pipette was left in the balance room at 25 ° for 15 to 30 minutes, after which it was weighed on a Kuhlmann micro balance. The capillary stem and the tip of the pipette were so