The behavior of L-histidine (His) in an aqueous buffer solution (pH 7.4) is studied via densimetry and differential scanning calorimetry. The densities and specific heat capacities of solutions of the amino acid are determined in the 288.15-313.15 K range of temperatures at His concentrations of 0.00215 to 0.03648 mol kg −1 . The apparent and partial molar parameters of L-histidine in the buffer solution are found, and features of the temperature and concentration dependences are identified. It is shown that the partial molar properties of transferring the amino acid from water to the buffer solution (both volume and heat capacity) have positive values in the investigated range of temperatures. Different types of molecular interaction between the solute and the solvent are considered using the Gurney model.

The influence of temperature and concentration of L-histidine on the heat capacity properties of its aqueous buffer solutions was studied by differential scanning calorimetry. The investigations were carried out in aqueous buffer solutions (pH 7.4) containing monobasic sodium phosphate and dibasic sodium phosphate, which brings the environment closer to the conditions of real biological systems. The pH values of the solutions were fixed with a digital pH meter Mettler Toledo, model Five-Easy (Switzerland). The differential scanning microcalorimeter SCAL-1 (Biopribor, Pushchino, Russia) was used for measure the specific heat capacity of the system under study. It was equipped with Peltier thermoelectric elements, two measuring glass cells with an internal volume of 0.377 cm3, as well as a computer terminal and software for calculating heat capacity. The standard error of measurement of the specific heat for the studied solutions was within ±7·10-3 J·K-1·g-1. The experimental values of the specific heat of solutions of the amino acid in a phosphate buffer solvent in the temperature range (283.15 – 343.15) K were obtained. The concentration of histidine was varied from (0.00215 to 0.03648) mol·kg-1. All the studied solutions were prepared by the gravimetric method using Sartorius-ME215S scales (with a weighing accuracy of 1·10-5 g). The apparent molar heat capacities of L-histidine in the buffer solution, as well as its partial molar heat capacities at infinite dilution, were determined. The calculated molar parameters increase with an increase in both temperature and amino acid concentration. It was shown that the partial molar heat capacities transfers of L-histidine from water to the buffer solution have positive values in the temperature range studied. The results are discussed on base of the Gurney model.