Abstract: The enthalpies of mixing of monomethylurea（MMU）with six kinds of ami no acids (glycine,L-alanine,L-serine,L-proline,L-threonine,L-valine) in aqu eous solutions have been determined by the LKB-2277 flow microcalorimetric syst em at 298.15 K.These results along with enthalpies of dilution of these solutes for the initial solutions were used to determine the enthalpic interaction coeff icients (hxy,hxxy,etc.) in terms of the McMillan-Mayer theory.The pairwise cros s interaction coefficients (hxy) of amino acids and MMU have been discussed.The results show that different side-groups of amino acids have different contribut ions to hxy.The hydrophobic-hydrophobic interactions of glycine,L-alanine and L-valine with MMU increase with the number of -CH2- groups on α-carbon.The hydroxyl group gives a negative contribution to hxy.The five-membered pyrolidin e ring in proline exhibits some hydrophobicity.The introduction of CH3 makes the hydrophobicity of MMU stronger than that of urea.

Key words: α-amino acid, Monomethylurea (MMU), Enthalpic interaction