Enthalpy of transfer of amino acids from water to aqueous glucose solutions at 298.15 K

The enthalpies of solution of amino acids, glycine, L-alanine, L-serine and L-proline were measured in water and in aqueous glucose solutions at 298.15 K with a microcalorimeter. Enthalpies of transfer of amino acids from water to aqueous glucose solutions were derived. The processes of solution of glycine, L-alanine and L-serine are endothermic, while that of L-proline is exothermic. Enthalpies of transfer of glycine and L-serine are negative, while those of L-alanine and L-proline are positive. The transfer enthalpies suggest that the highly polar zwitterion group and OH group of the molecule of amino acids cause a breakdown of structure in the aqueous glucose solutions. The non-polar side chains produce order in proportion to the size of the side chain. The breakdown and ordering effects are greater in water than in glucose solutions. L-Serine is quite special; there are a minimum and a maximum in the plot of transfer enthalpies vs. glucose concentrations. All results are discussed using hydrophobic interactions and structure-breaking-structure-making theory. (C) 1998 Elsevier Science B.V.