Enthalpic interactions of some amino acids with monomethylurea in aqueous solutions at 298.15 K

The enthalpies of mixing of monomethylurea(MMU) with six kinds of amino acids (glycine, L-alanine, L-serine, L-proline, L-threonine, L-valine) in aqueous solutions have been determined by the LKB-2277 flow microcalorimetric system 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 coefficients (h(xy), h(xxy), etc.) in terms of the McMillan-Mayer theory. The pairwise cross interaction coefficients (h(xy)) of amino acids and MMU have been discussed. The results show that different side-groups of amino acids have different contributions to h(xy). The hydrophobic-hydrophobic interactions of glycine, L-alanine and L-valine with MMU increase with the number of - CH2- groups on alpha -carbon. The hydroxyl group gives a negative contribution to h(xy). The five-membered pyrolidine ring in proline exhibits some hydrophobicity. The introduction of CH3 makes the hydrophobicity of MMU stronger than that of urea.