Vitamins play very important role in human and animal bodies. They have great significance of complex formation with proteins. The present work reports on interactions of L-asparagine (Asn) with pyridine-3-carboxylic acid, i.e. nicotinic acid or vitamin B3 (NA), in phosphate buffered saline, pH 7.4. The thermodynamic parameters of complex formation between the reagents were determined calorimetrically at temperature 298.15 K in buffer saline medium. The resultant values of stability constants in buffer and aqueous solutions were compared and discussed. In order to supplement this information, the apparent molar volumes, standard apparent molar volumes at infinite dilution of the reagents (NA, Asn) and their derivatives with respect to temperature have been calculated for buffer saline as well as for a system of (NA – Asn – buffered saline) using the measured density values in temperature range of 288–313 K. The results revealed formation of Asn complex with vitamin B3. The effects of solute concentration and temperature on the volume properties were discussed in terms of predominant types of molecular interactions (hydrophilic, hydrophobic, ionic and zwitterionic) on the basis of co-spheres overlap model. The interaction of nicotine acid with asparagine is accompanied by an exothermic effect and an increase in standard molar volume of NA. The increase in the standard molar volume, the decrease in the molar expansibility and the change in the sign of the Heppler parameter confirm the restructuring of the hydrate shell of NA under the influence of Asn additives.
