Mini review on Keto-Enol ratio of curcuminoids

Abstract

Introduction. Curcumin, demethoxycurcumin and bisdemethoxycurcumin collectively constitute curcuminoids, exhibit different rates of degradation as well as therapeutic properties. Their remarkable dissimilar behaviour is affiliated to their tendency to undergo keto-enol tautomerism whereas their rates of degrading and imparting therapeutic properties depend on the ratio of keto and enol tautomers of individual curcuminoid. Materials and Methods. The aim of the current review is to focus on the ratio of keto and enol tautomers and to explain how the ratio of the two tautomers affects the rate of activities of the curcuminoids. The papers regarding variations in the rate of activities of curcuminoids were scrutinized. The factors affecting the ratio of keto-enol tautomerism were studied. Results and discussion. Structurally, curcuminoids are the compounds possessing β-diketone moiety. The characteristic property of compounds containing β-diketone moiety is to exhibit keto-enol tautomerism. Hence, each curcuminoid is a mixture of keto and enol tautomers. The keto tautomer of curcumioids is responsible for their therapeutic activities. On the other hand, enol tautomer of curcuminods has tendency to undergo degradation. Hence, the ratio of keto and enol tautomer needs to be considered while studying the activities of curcuminoids. Nevertheless, the ratio of keto and enol tautomer depends on the existence of other functional groups. Because of the presence of electron donating hydroxyl group, the proportion of enol tautomer predominates in curcuminoids. Further, due to the presence of two more electron donating methoxy groups in curcumin, the ratio of the enol tautomer is higher and hence, the susceptibility to undergo degradation is greater than demethoxycurcumin, which contains only one methoxy group. The ratio of the enol tautomer is minimum in bisdemethoxycurcumin amongst all the curcuminoids, which is devoid of methoxy group. Consequently, bisdemethoxycurcumin has the least rate of degradation amongst curcuminoids. Methoxy group enhances the activity of the functional sites in curcuminoids in keto form. Therefore, curcumin shows maximum beneficial activities in keto form amongst all the curcuminoids, followed by demethoxycurcumin and bisdemthoxycurcumin. Thus, the rate of the activities displayed by curcuminoids is governed by their keto and enol proportion. Conclusion. It can be concluded from this review article that besides β-diketone moiety, responsible for keto-enol tautomerism, other groups also play an important role in determining the ratio of keto and enol tautomers.

Comparative effects of curcumin versus nano-curcumin on histological, immunohistochemical expression, histomorphometric, and biochemical changes to pancreatic beta cells and lipid profile of streptozocin induced diabetes in male Sprague–Dawley rats

“…Curcumin (CUR) is a natural polyphenol, with two o-methoxy phenolic groups, one enone moiety, and an α, β-unsaturated diketone group (Indira Priyadarsini 2013 ). It has keto-enol tautomerism (Rege et al 2019 ). CUR decreases blood glucose as well as glycosylated hemoglobin levels and prohibited weight loss (Jain et al 2009 ).…”

Curcumin: Biological Activities and Modern Pharmaceutical Forms

“…Solvent polarity, pH, and temperature significantly affect curcumin’s keto–enol balance [ 48 ]. The ratio of keto and enol tautomers of curcumin, on the other hand, strongly influences pharmacological activities [ 49 ]. Manolova et al examined the tautomerism of curcumin in ethanol/water binary mixtures using ultraviolet–visible (UV–VIS) spectroscopy and advanced quantum chemical calculations.…”

4D Printing of Sago Starch with Turmeric Blends: A Study on pH-Triggered Spontaneous Color Transformation

“…Given its anti-inflammatory, antibacterial, and antitumor properties, several engineering approaches have been explored for the delivery of this nutraceutical in the human body . Keto–enol tautomerism in curcumin occurs with variations in pH; curcumin serves as a proton donor under acidic conditions due to the presence of a strongly activated carbon atom in the heptadienone region between the two methoxy phenol rings in its bis-keto form . It changes color from yellow to red in different pH solutions (yellow in acidic and neutral pH and red in alkaline conditions), and these reactions can also be reversible …”

“…22 Keto−enol tautomerism in curcumin occurs with variations in pH; curcumin serves as a proton donor under acidic conditions due to the presence of a strongly activated carbon atom in the heptadienone region between the two methoxy phenol rings in its bis-keto form. 23 It changes color from yellow to red in different pH solutions (yellow in acidic and neutral pH and red in alkaline conditions), and these reactions can also be reversible. 16 Accordingly, the objective of this study was to efficiently use an underutilized material (tapioca sago) as the constrain material for 4D printing, with curcumin-based spontaneous color transformation with sodium bicarbonate as the stimulus.…”