The Role of Phenolic Hydroxy Groups in the Free Radical Scavenging Activity of Betalains

Abstract

Free radical scavenging compounds play important roles as health-protecting factors. Betalains are natural water-soluble pigments present in most plant families belonging to the order Caryophyllales. They are the subject of increasing attention following the discovery of their antiradical capacity, but a systematic analysis of the structural features involved in the activity is necessary. In this paper, both natural and previously unconsidered betaxanthins were obtained in order to study the role of phenolic hydroxy groups in the high free radical scavenging activity of betalains. Pigments were characterized spectrophotometrically, chromatographically, and by ESI-MS, and their antiradical and antioxidant properties were studied under the ABTS(*+) radical and FRAP assays. A high intrinsic activity is described that is not linked to the presence of hydroxy groups or aromaticity in the pigment structure. In addition, the presence of phenolic hydroxy groups implies an enhancement of the antiradical activity, reaching a TEAC value in the ABTS(*+) assay of 5.8 +/- 0.2 for the pure compound with two hydroxy groups.

Two-electron photo-oxidation of betanin on titanium dioxide and potential for improved dye-sensitized solar energy conversion

“…9 Wybraniec et al 10,11,12 have studied the oxidation of betanin (Bt) and its aglycone betanidin (Bd). They report that in the pH range 3 to 5, electrochemical oxidation of Bd is a reversible two-electron, one-proton process, while at somewhat higher pH this changes to a twoelectron, two-proton process.…”

Indicaxanthin Dietetics: Past, Present, and Future

“…The reducing activity of indicaxanthin has been evaluated by the reaction with the ABTS radical cation, generated by reacting ABTS with potassium persulfate [21]. When expressed as Trolox equivalents, the radical-scavenging activity of indicaxanthin was one order of magnitude lower than the betacyanin betanin [21], which was in accordance with the structural features [52,53] and the redox potential of these two betalains [21]. The higher scavenging capacity of betanin can be explained by the ease with which it is possible to withdraw an electron from its phenolic hydroxyl group, and by the stability of the resulting delocalized radical [24].…”

“…Several studies in the latest decade investigated the antiradical activity of betalains [21,[24][25][26][52][53][54]. The cyclic amine of betalamic acid, the building block of the betalain pigments, may be envisaged as the reactive group acting similarly to the amine group of ethoxyquin, a potent antioxidant in lipid systems [55,56] and in vivo [57].…”

“…The cyclic amine of betalamic acid, the building block of the betalain pigments, may be envisaged as the reactive group acting similarly to the amine group of ethoxyquin, a potent antioxidant in lipid systems [55,56] and in vivo [57]. A recent systematic analysis [52,53] explored the relations between structures, spectroscopic properties and antiradical activity of various betalains, measured by the assay of decolorization of the 2,2′-azino-bis[3-ethylbenzthiazoline-6-sulfonic acid (ABTS) radical cation [58]. The studies considering both natural and synthetic compounds, with or without hydroxyl groups or aromaticity in the pigment structure, ascertained that in betaxanthins there is an "intrinsic activity" not linked to the presence of hydroxyl groups or aromaticity, which might be associated with the common electronic resonance system supported between the two nitrogen atoms, and be general to all betalains that contain a similar resonance system.…”

“…The studies considering both natural and synthetic compounds, with or without hydroxyl groups or aromaticity in the pigment structure, ascertained that in betaxanthins there is an "intrinsic activity" not linked to the presence of hydroxyl groups or aromaticity, which might be associated with the common electronic resonance system supported between the two nitrogen atoms, and be general to all betalains that contain a similar resonance system. The presence of phenolic hydroxy group, however, enhances significantly the radicalscavenging activity of betaxanthins [52]. In this context, the proline-derivative of betalamic acid (indicaxanthin) should act as a radical scavenger less effective than betacyanins, e.g., betanin, which has indeed been shown [21].…”