Purification and antiradical properties of the structural unit of betalains.

Betalamic acid [4-(2-oxoethylidene)-1,2,3,4-tetrahydropyridine-2,6-dicarboxylic acid] is a naturally occurring compound that is normally found condensed with amino acids, amines, cyclo-DOPA, and cyclo-DOPA derivatives to form the betalains. Betalains are the pigments responsible for the yellow to violet color of the fruits and flowers of plants belonging to the order Caryophyllales. Betalamic acid is the structural feature common to all of these pigments and contains the electron resonance system responsible for the spectroscopic properties. Betalamic acid was purified by chromatography and identified by UV-vis spectrophotometry and ESI mass spectrometry. The antioxidant and free radical scavenging capacities of betalamic acid were assessed using the FRAP and ABTS(·+) radical assays. A pK(a) of 6.8 was found for the deprotonation equilibrium involved in the nucleophilic activity of betalamic acid; this pK(a) explains the observed pH effect on the free radical scavenging capacity of these pigments.