Interaction of lactose with individual milk proteins and Thermodynamicla parameters. I.K-Casein

Abstract

A thermodynamic study of K-casein - lactose interaction was made.  Electrophoretically pure K-casein (5 X 10^-5 M) was heated at pH 6,9 and ionic strength = 0,2 (phosphate buffer) in presence of lactose (2,9 X 10^-3M -- 2,9 X 10^- M) for 20 min at 60 to 98 C. Heated samples were dialyzed and bound lactose determined. Mathematical analysis by the Scatchard equation, assuming equal intrinsic association constants for all binding sites of the protein molecule, allowed calculation of free energy, enthalpy and enthropy changes ( _Fº, _Hº and _ Sº). At 60C, binding is barely detectable. From 70 to 98 C the protein-Lactose association cons­tant decreases as well as the extent of the binding, proving that this is an exothermic process. The thermodynamic parameters of the binding suggests that the main contribution to the interaction mechanism is due to van der Waals forces. From the highly negative value of the enthalpy change (--12.1 Kcal mol^-1) it is deduced that the implied forces are strongly stabilised by dipole-dipole interaction. The change of enthropy (--- _Sº = 30 enthropy units) was negative as expected.