Phenolic betalain as antioxidants: meta means more

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.