Enthalpy of solution of glycine, L-alanine, L-serine in water and aqueous solutions of urea was measured at 298.15 K with an RD496-III microcalorimeter. Enthalpy of transfer of amino acids from water to aqueous solutions of urea was derived and interpreted qualitatively with structural hydration model. The results show that the process of transfer is governed by the electrostatic interaction and structure interaction between urea and the zwitterionic group of amino acids with a negative enthalpy, while that with the side chain gives a positive or negative contribution depending on its hydrophobicity or hydrophilicity. In the solvent composition range studied, the enthalpy of transfer of amino acids in general decreased with the increasing concentration of urea, with the relative order of L-serine＜glycine＜L-alanine. The difference in the enthalpy of transfer of amino acids reflected the change in the structure interaction between solute and cosolvent.

Key words: urea, glycine, L-alanine, L-serine, enthalpy of transfer