Phenolic betalain as antioxidants: meta means more

Abstract

Betalains are phytochemicals of nutraceutical importance that emerged as potent antioxidants, preventing radical chain propagation and the deleterious health effects of oxidative stress. However, despite the wide application of betalains as color additives in products for human consumption, little is known about the relationship between their structure and antioxidant potential. Here we investigate the mechanism of antioxidant action of three regioisomeric phenolic betalains and show that the meta isomer has higher antiradical capacity than most natural betalains, anthocyanins and flavonoids. Structural and pH effects on redox and antiradical properties were investigated and the results are rationalized in light of quantum chemical calculations. Our results demonstrate that hydrogen atom transfer/proton-coupled electron transfer or sequential proton loss electron transfer mechanisms are plausible to explain the radical chain breaking properties of phenolic betalains in water. Furthermore, mesomeric effects are responsible for the stabilization of the resulting radical phenolic betalains. These findings are useful for the design of biocompatible antioxidants and for the development of novel additives for functional foods and cosmetics with high antioxidant potential.

Design of an Antioxidant Nitrone Scaffold Based on Plant Pigments

“…The product was isolated in 50% yield after purification in the presence of oxygen, which is twice as high as the values reported for other betalains. 20,26 The high-resolution mass spectrum was consistent with the postulated structure of OxiBeet (Fig. 1b).…”

“…Last, these findings highlight the importance of the 1,7-diazaheptamethinium system of betalains for their striking antioxidant properties. 20,34 Ultrafast transient absorption experiments were carried out to determine whether photoexcitation plays a role in the autoxidation of OxiBeet. In aerated water, the electronic excitation of OxiBeet at 485 nm results in ground state bleaching (Fig.…”

“…[13][14][15][16][17] Both phenolic and non-phenolic betalains are highly efficient dietary antioxidants, 14 outperforming vitamin C, vitamin E and many flavonoids in terms of antioxidant capacity (thermodynamics, number of radicals scavenged) and activity (kinetics, reactivity towards radicals). 14,[18][19][20] Here, we describe the semisynthesis and properties of OxiBeet, the first betalain-nitrone. This bioinspired compound is stable towards hydrolysis under neutral and slightly alkaline conditions and is not cytotoxic to the human hepatic cell line HepaRG at concentration up to 1 mM.…”

Rationale on the High Radical Scavenging Capacity of Betalains

“…b Species refer to the parent species and the deprotonation site (Figure 3c). c from Reference [27].…”

“…The antioxidant capacity of betanin is comparable to that of ECG due to the presence of a 5- O -glucosyl catechol moiety and a 1,7-diazaheptamethinium [23] system [24,25,26]. We have found that N -(3-hydroxylphenyl)betalain ( m -OH-pBeet) is an adequate model to study the mechanism of antioxidant action of betanin because they share the same radical scavenging capacity [27]. Although the antioxidant action of m -OH-pBeet was rationalized in terms of proton-coupled electron transfer (PCET), the importance of the 1,7-diazaheptamethinium system for the antioxidant potential of betalains could not be elucidated [27].…”