Mode of action of betaine on some amino acids and globular proteins: Thermodynamic considerations

The interactions of the amino acids glycine, L-alanine, DL-alpha-amino-n-butyric acid, L-valine, and L-leucine have been studied based on values of apparent molar volume (V-2,V-m,V-phi) and apparent molar compressibility K-s,K-2,K-m,K-phi) in 1 mol.kg (1) and 2 mol.kg (1) aqueous betaine solutions at T = 298.15 K. These thermodynamic quantities have been calculated from density and sound velocity measurements. Isothermal titration calorimetry has been employed to determine the values of enthalpies of interaction of aqueous solutions of these amino acids with betaine. These data have been further used to calculate values of infinite dilution standard partial molar volumes (V-2,m(0)), standard partial molar isentropic compressibilities (K-s,2,m(0)) and limiting enthalpies of dilution (Delta H-dil(m)0) of these amino acids in aqueous betaine solutions. The standard partial molar volumes of transfer (Delta V-tr(2,m)0), isentropic compressibilities of transfer (Delta K-tr(s,2,m)0) and enthalpies of dilution of transfer Delta(tr)(Delta H-dil(m)0) of amino acids from water to aqueous betaine solutions have been calculated from the measured data in order to understand possible intermolecular interactions such as ion-ion, ion-polar, hydrophilic-hydrophobic and hydrophobic-hydrophobic group interactions. The contributions of the end groups{(NH3+, COO), CH2 groups and the other alkyl chains of the amino acids to V-2,m(0)} have also been determined. The interactions of betaine with the globular proteins a-lactalbumin and bovine serum albumin have also been studied. Such studies are important in obtaining information on the action of osmolytes on proteins and their constituents and establishing possible correlations. (C) 2017 Elsevier Ltd