Thermodynamic Properties of L-Methionine and Nicotinic Acid in an Aqueous Buffer Solution

Abstract

Interaction between L-methionine (Met) and nicotinic acid (NA) in an aqueous buffer solution (pH 7.4) is studied via dissolution calorimetry and densimetry. The thermodynamic characteristics of the formation of a 1 : 1 molecular complex between Met and L at Т = 298.15 K are determined. The experimental density of the Met–L–buffer system is measured at T = 288.15, 298.15, 303.15, 308.15, and 313.15 K in the 0.0036 to 0.0367 mol/kg range of NA concentrations at a constant Met concentration (0.01249 mol/kg). It is shown that the extreme character of apparent molar volume–concentration dependences suggests the structure-strengthening effect NA has on solutions containing the amino acid (Met) with a slight shift of maxima with increasing temperature. The results are discussed in the context of the Gurney model.