The enthalpies of mixing of aqueous urea solutions and eight kinds of aqueous amino acid solutions (glycine, L-alanine, l-serine, L- proline, L-hydroxyproline, L-methionine, L-threonine and L-valine) have been determined at 298.15K by LKB-2277 flow microcalorimetric system. The data have been analyzed in terms of the McMillan-Mayer formalism to obtain the enthalpic virial coefficients for heterotactic interactions. The pairwise interactions between amino acids and urea have been studied by a group contribution additivity aproach. the results show that the zwitterionic groups and different side-groups of the amino acids, such as non-polar alkyl groups, hydroxy group and five-membered pyrolidine ring, make their different contributions to enthalpic pair interactions.

Key words: AMINO ACID, UREA, ENTHALPY, INTERACTIONS, GLYCINE, ALANINE, SERINE, PROLINE, GROUP CONTRIBUTION