Thermodynamic properties of crystalline L-carnosine and its aqueous solutions

Abstract

In this work, complex investigations of crystalline L-carnosine (Car) and its solutions in water and aqueous buffer solvent have been conducted by the methods of differential scanning calorimetry (DSC), Knudsen’s effusion mass spectrometry (KEMS), calorimetry of dissolution, and densimetry. DSC and KEMS have revealed no phase transitions in the temperature range 208–471 K. The phenomenon that the sublimation of Car is accompanied by cyclization of fragment ions in the temperature range (298–360) K has been observed. Calorimetric and volumetric methods were employed to investigate molecular interactions of L-carnosine in water (pH 5.4) and in aqueous buffer solutions (pH 7.4) at various temperatures. The effects of solute structure, concentration, pH and temperature on the thermodynamic properties derived were discussed. In case of aqueous solutions with different pH values, the data have been discussed in terms of predominant types of molecular interactions (hydrophilic, hydrophobic, ionic and zwitterionic) on the basis of co-spheres overlap model. For neutral molecule of carnosine, DFT theoretical calculations at B3LYP/6–31++G(d,p) level of theory have been performed. Analysis of natural bond orbitals (NBO) has been carried out for the four most stable conformers.