Microwave-assisted extraction of polysaccharides from Micromelum minutum leaves using citric acid monohydrate-glycerol based deep eutectic solvents and evaluation of biological activities

Background

The extraction of polysaccharides using an acidic extraction media has been extensively reported, highlighting its effectiveness in yielding high-quality polysaccharides. A higher concentration of acidic solution could hydrolyze the structure of polysaccharide, while a low concentration reduces the extraction efficiency. Despite this challenges, deep eutectic solvents (DES) were introduced as an alternative extraction medium due to additional interactions such as inter and intra-molecular interactions, Van de Waals, hydrogen bond, and electrostatic interactions, which could improve the polysaccharide extraction efficiency and biological activities. Furthermore, the extraction conditions such as extraction medium and extraction parameters could affect the properties of polysaccharides as well as influence their structure-activity relationship for biological activities.

Results

The result showed that the microwave-assisted extraction of Micromelum minutum leaf polysaccharide (MMLP) using DES as an extraction media (MMLP-DES) gave a higher yield (improvement of 101.20 %) than citric acid monohydrate (CAM) (MMLP-CAM) and required a lower percentage of microwave power (19.83 % less) and time (0.78 min less). The properties of MMLPs significantly differ based on their pH, molecular weight, viscosity, degree of esterification and monosaccharide molar ratio which influenced the biological activities. Compared to MMLP-CAM, MMLP-DES had a more branched and less linear structure. The bioactivities study revealed that MMLP-DES exhibited higher antioxidant and anti-α-amylase activities (i.e., DPPH: 74.52 %, FRAP: 2.87 mM FeSO₄ and α-amylase inhibition: 86.23 %) compared to MMLP-CAM (i.e., DPPH: 49.33 %, FRAP: 1.49 mM FeSO₄, and α-amylase inhibition: 81.76 %). The mechanism and structure-activity relationship of MMLPs on bioactivities were also hypothesized.

Significance

Based on our previous study, the citric acid monohydrate-glycerol based DES as an extraction medium has enhanced the extraction yield of polysaccharides from M. minutum. This study highlights the DES combined with microwave-assisted extraction to improve the yield of MMLP and evaluate the biological activities compared to CAM as a classical solvent. In conclusion, the DES showed the advantages for extraction of polysaccharides with desired biological activities.