Intermolecular interactions in natural deep eutectic solvents and their effects on the ultrasound-assisted extraction of artemisinin from Artemisia annua

A variety of natural deep eutectic solvents (NADES) were synthesized and subjected to structural characterization using NMR and FTIR, physical characterization (viscosity, pH, thermal gravimetric analysis, and differential scanning calorimetry), and cytotoxicity tests. From the results of experiments and quantum chemical calculations, hydrogen bonds were confirmed as the driving force for the formation of the NADESs. Then, the NADESs were used to extract artemisinin from Artemisia annua using an ultrasound-assisted technique (UAEA), and the artemisinin yield was evaluated using high-performance liquid chromatography. Of the prepared NADES, that formed of l-carnitine and isosorbide (CaIs) in a 1:2 M ratio was found to possess the best extraction efficiency. The response surface methodology (RSM) combined with central composite design (CCD) was used for experimental design, and, for the CaIs NADES, the best extraction efficiency was obtained when the solid–liquid ratio was 1:20, the extraction temperature was 48 °C, the extraction time was 32.62 min, and the concentration of aqueous NADES solution (w/v) was 62.81%. To explore the extraction mechanism in depth, quantum chemical calculations and scanning electron microscopy were used. The van der Waals and hydrogen-bonding interactions are the main driving forces for the UAEA, and the structure of the plant cell wall is destroyed by the NADES, which results in an increase in extraction efficiency. Thus, NADES ultrasound-assisted extraction of artemisinin is an efficient, green, and feasible extraction method.