Features of the application of methods for determining the total content of biologically active substances (polyphenols and flavonoids, total antioxidant and antiradical activities) in the presence of deep eutectic solvents

Deep eutectic solvents (DES) are biologically safe solvents containing donors and acceptors of hydrogen bonds.They are considered promising alternative solvents used in the extraction of biologically active substances from plants.To characterize extracts, spectrophotometric methods based on various complexation reactions are used, such as quantitative determination of the total content of flavonoids, polyphenols, determination of the total antioxidant activity. However, the articles indicate calibration characteristics for aqueous or ethanol solutions of standard substances, and no calibration dependencies were found in the DES medium. The question of the validity of the application of calibration characteristics using ethanol solutions for DES remains open. Therefore, the aim of our work was to assess the effect of the presence of deep eutectic solvents on the applicability of methods for determining polyphenols, flavonoids and total antioxidant activity. In the course of the work, calibration dependences were obtained in the medium of deep eutectic solvents based on choline chloride and malonic, malic, citric and tartaric acids. It has been established that the nature of the solvent strongly influences the calibration coefficients, which indicates the need to use separate graduations for each type of solvent.