Isotope effect on the solubility of amino acids in water

The solubilities of glycine, alanine, phenylalanine, and proline in H₂O and D₂O from 283 K to 335 K were determined. It was found that glycine and alanine are less soluble in heavy water than in light water but proline is more soluble in heavy water over the whole temperature range studied. Phenylalanine is more soluble in H₂O than D₂O below 310 K but above that temperature heavy water becomes a better solvent. An influence of H/D isotope substitution on the enthalpies of solution is also observed. In the case of glycine and alanine enthalpies of solution in heavy water increase by a small amount and in the same time the solution enthalpy for phenylalanine in D₂O increases markedly. No change in the solution enthalpy for proline was observed. The isotope effects on solubility and the solution enthalpy are qualitatively discussed.