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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.

Betalamic acid, the chromophore of betaxanthins, was enzymatically synthesized on a large scale from l-dihydroxyphenylalanine (L-DOPA) using recombinant Mirabilis jalapa DOPA 4,5-dioxygenase. After synthesis, proline was directly added to the concentrated reaction mixture to generate proline-betaxanthin. The molecular mass and nuclear magnetic resonance spectrum of the purified product were identical to those previously reported for proline-betaxanthin. Twenty-four betaxanthin species were synthesized by the condensation reaction of purified betalamic acid and amino acids or amines. An HPLC protocol was established for identifying the different betaxanthin species. Proline-, dopamine-, and γ-aminobutyric acid (GABA)-betaxanthins were prepared as representative betaxanthins under large-scale conditions, and their 1,1-diphenyl-2-picrylhydrazyl radical-scavenging activities were compared against those of known antioxidants. GABA-betaxanthin showed comparatively low activity, whereas dopamine-betaxanthin had similar activity to the red pigment betanin and the anthocyanin cyanidin 3-glucoside. Proline-betaxanthin had the highest activity of the three synthesized compounds and was similar to the flavonoid quercetin.