Deep eutectic solvents (DESs) exhibit numerous advantages over conventional ones used in several chemical and biochemical processes. Besides addressing most of the principles of green chemistry, DESs can also act as catalyst depending on their nature. The use of DESs as acid catalyst has several advantages such as utilization at stoichiometric amount, non-toxicity, possibility of the recovery, similar or higher catalytic effect than the acid itself alone, the recyclability and reusability without a significant loss of activity. In this mini-review, the state- of-the-art in the use of DESs as catalyst is presented. The DESs, which show Lewis type acidity, Brønsted type acidity and other types of catalytic influence in various types of reactions including esterification, organic synthesis, glycolysis and depolymerisation are presented and their roles in the reactions are discussed.

Olive leaf as an agricultural waste contains valuable bioactive compounds that are mainly used for pharmaceutical and cosmetic industries. Lately the major component, oleuropein, has gained extra attention due to the anti-viral activity against SARS-CoV-2 that causes Coronavirus disease (Covid-19). In this study, extraction of the bioactive compounds from olive leaves was conducted using a non-conventional and green method. New generation green solvents, natural deep eutectic solvents (NADES) were used in combination with ultrasound assisted extraction. Screening of NADES type, temperature, and particle size were investigated using one-pot-at-a-time method while, NADES amount and liquid-to-solid ratio were optimized using experimental design. The results were evaluated in terms of total polyphenol yield (Y TP), total flavonoid yield (Y TF) and antiradical activity (A AR). At the optimized conditions, the highest total polyphenol yield and the highest total flavonoid yield were achieved with choline chloride-fructose-water (CFW) (5:2:5) as 187.31 ± 10.3 mg gallic acid equivalent g −1 dw and 12.75 ± 0.6 mg apigenin equivalent g −1 dw, respectively. The extracts were also analyzed for oleuropein, caffeic acid and luteolin contents. The highest amount of oleuropein and caffeic acid were extracted by glucose-fructose-water (GFW) (1:1:11) as 1630.80 mg kg −1 dw and 112.77 mg kg −1 dw, respectively.

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Flavonoids are polyphenolic secondary plant metabolites which possess antioxidant and anti-inflammatory properties. Besides, they have been shown to exhibit increased antioxidant properties in their polymerized form. Catechins are one of the attractive class of flavonoids which belong to the group of flavan-3-ols. Polymerization of catechins have been investigated in numerous studies indicating the requirement of certain amount of organic solvent to provide the solubility of the monomer. However, many research projects have been conducted recently to replace toxic organic contaminants of the processes with environmentally friendly solvents. In this aspect, deep eutectic solvents (DESs) that are regarded as "green solvents" have been studied extensively in various enzyme catalyzed reactions. In the present study, we focused on establishing a green pathway for laccase catalyzed polycatechin synthesis by replacing organic solvent content with DESs as green solvents. For this aim, various parameters were investigated, such as DES types and concentrations laccase amount and reaction time. Consequently, the highest molecular weight polycatechin was obtained using 5% (v/v) B-M, 125 U laccase in 1 hr of reaction time, at 30°C, as 4,354 ± 678 g mol. Corresponding X/XO inhibitory activity and superoxide radical scavenging activities were achieved as, 59 and 50%, respectively.