The role of phenolic hydroxy groups in the free radical scavenging activity of betalains.

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.