Betalains as Antioxidants Erick L. Bastos and Willibald Schliemann Contents 1 Introduction and Scope ....................................................................... 2 2 A General Survey of Betalains ............................................................... 3 3 Antioxidant Properties of Betalains .......................................................... 6 3.1 Betalain-Rich Plant Extracts ........................................................... 8 3.2 Isolated Betalains ....................................................................... 10 4 Chemical Mechanisms of Antioxidant Action ............................................... 21 4.1 Model Pseudo-natural Betaxanthins ................................................... 25 5 Conclusions and Perspectives ................................................................ 26 References ........................................................................................ 27 Abstract An overview is provided of the status of research on the antioxidant and radical- scavenging properties of betalains, water-soluble pigments found in plants, fungi, and bacteria. Together with anthocyanins, betalains are responsible for most of the red, purple, and blue colors of fruits and flowers, although both classes of secondary metabolites are mutually exclusive in nature. The 1,7-diazaheptamethinium scaffold of betalains promotes their radical-scavenging properties, which involve the occur- rence of proton-coupled electron transfer. Betalains derived from cyclo-DOPA, namely betacyanins, are antioxidants as potent as epicatechin gallate from green tea. The other betalains, classified as betaxanthins, also show high antioxidant potential whether they are phenolic or not. Since much of the current understanding E. L. Bastos (*) Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, São Paulo, SP, Brazil e-mail: elbastos@iq.usp.br W. Schliemann (*) Leibniz Institute of Plant Biochemistry, Department of Secondary Metabolism, Halle (Saale), Germany e-mail: willibald.schliemann@gmail.com © Springer Nature Switzerland AG 2021 H. M. Ekiert et al. (eds.), Plant Antioxidants and Health, Reference Series in Phytochemistry, https://doi.org/10.1007/978-3-030-45299-5_9-2 1