Abstract
Oil palm (Elaeis guineensis Jacq.) leaf (OPL) is abundantly generated from oil palm plantations as biomass that is rich in bioactive metabolites, primarily flavonoids. Six natural deep eutectic solvents (NaDES) were synthesized using a direct heating technique from different combinations of choline chloride with 1,2-propanediol (PD), 1,4-butanediol (BD), glycerol (GLY), glucose (GLU), maltose (MAL) and lactic acid (LA). The synthesized NaDES were subjected to physicochemical and biological evaluations comprising physical appearance, density, water activity, viscosity, polarity, thermal behaviors, spectroscopic analysis, cytotoxicity, radical scavenging activities and solubility tests. Compared to aqueous methanol, the synthesized NaDES, which appeared as a slightly to moderately viscous transparent liquid, showed favorable physicochemical properties as extraction solvents with a low cytotoxicity profile on cultured fibroblast cells. Further, the NaDES obtained from the choline chloride:lactic acid (LA) combination showed high free radical scavenging characteristics. Hydrogen bonding interactions were shown to play a significant role in the formation of the NaDES. Further, ultra-high-performance liquid chromatography ultraviolet/photodiode array (UHPLC-UV/PDA) analysis revealed that the NaDES from the choline chloride:glycerol (GLY) combination had comparable efficiencies with aqueous methanol regarding extracting flavonoids (luteolin and apigenin derivatives) from OPLs. The results of the present study suggested that the tailor-made NaDES were not only easy-to-use, stable and safe solvents but also suitable for extracting bioactive phytochemical compounds. The study highlighted their potential as an alternative green technology for applications in oil palm biomass utilization programs.
Highlights
In parallel with the development of sustainable chemistry, natural deep eutectic solvents (NaDES) have recently been receiving tremendous attention from scientists globally in accordance with the efforts on the minimal utilization of conventional organic solvents (COS) in industrial applications [1]
NaDES are commonly composed of quaternary salts, such as choline derivatives, acting as a hydrogen bond acceptor (HBA), with sugars, organic acids or alcohols acting as a hydrogen bond donor (HBD)
Continuing our efforts toward maximizing the efficiency of phytochemical extraction from oil palm leaves (OPLs), we further explored the application of NaDES to extract the main flavonoids from the agrobiomass
Summary
In parallel with the development of sustainable chemistry, natural deep eutectic solvents (NaDES) have recently been receiving tremendous attention from scientists globally in accordance with the efforts on the minimal utilization of conventional organic solvents (COS) in industrial applications [1]. The number of articles reported in the literature on the application of NaDES as a green solvent for extraction and separation of phenolics and flavonoids has increased in the recent decade [2,6,9,13,14,15,16], more detailed information, such as on their physical appearance, solubility, density, water activity, polarity, viscosity, thermal behaviors, functional group interactions, radical scavenging activities and cytotoxicity profile, is still required for tailor-made NaDES toward a better understanding and assessment of their potential application as an alternative medium for extracting key components from plant matrices. HBDs, namely, 1,2-propanediol, 1,4-butanediol, glycerol, glucose, maltose and lactic acid (Figure 1), with appropriate molar ratios and water contents This was followed by the assessment of their physicochemical properties (physical appearance—color and clarity, density, water activity, viscosity, polarity, thermal behaviors, spectroscopic analysis and solubility), and their cytotoxic and radical scavenging activities.
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