THERMODYNAMICS OF COMPLEXATION WITH 18-CROWN-6 WITH L-CARNOSINE AND ITS CONFORMATIONAL ANALYSIS

The study of model biological "guest-host" processes between dipeptides and crown ethers allows us to understand the nature of molecular recognition in living systems. The reaction of the formation of the molecular complex of the crown ether 18-crown-6 (18K6) with L-carnosine (L-Carn) in water at T = 298.15 K was studied using isothermal titration calorimetry. The mixing heats of aqueous solutions of L-carnosine with aqueous solutions of 18-Carn-6 were determined on a TAM III titration calorimeter system (TA Instruments) equipped with a 20 mL titration cell. In the main experiment, the heat of mixing of aqueous 18K6 solution with aqueous L-Carn solution was determined. In separate experiments the heat effect of dilution of L-Carn solution in water was measured, which was taken into account in determining the thermal effect of complex formation. From the thermochemical data, the thermodynamic parameters of the molecular complex for-mation reaction were calculated. It was found that the presence of a histidine fragment in the L-Carn structure leads to an increase in the stability of the molecular complex L-Carn with 18K6 and to a decrease in the exothermicity of complex formation as compared to the complex formation of alanine with 18K6. Thermodynamic parameters of [L-Carn 18K6] formation reaction are similar to thermodynamics of 18K6 complex formation with histidine. Quantum-chemical calculations were performed within the framework of the electron density functional theory using the GAUSSSIAN 03 software package. The results were visualized using the ChemCraft software. We performed the conformational analysis of L-Carn within the framework of the density functional theory (version B3LYP/6-31G**) to analyze the reactivity of L-Carn structural fragments. For three stable conformers in the M062X/ aug-cc-pVTZ approximation the geometric parameters were obtained and the electron density distribution was analyzed within the framework of the QTAIM method as well as the NBO scheme.