Rationale on the High Radical Scavenging Capacity of Betalains

Abstract

Betalains are water-soluble natural pigments of increasing importance as antioxidants for pharmaceutical use. Although non-phenolic betalains have lower capacity to scavenge radicals compared to their phenolic analogues, both classes perform well as antioxidants and anti-inflammatory agents <i>in vivo</i>. Here we show that <i>meta</i>-hydroxyphenyl betalain (<i>m</i>-OH-pBeet) and phenylbetalain (pBeet) show higher radical scavenging capacity compared to their <i>N</i>-methyl iminium analogues, in which proton-coupled electron transfer (PCET) from the imine nitrogen atom is precluded. The 1,7-diazaheptamethinium system was found to be essential for the high radical scavenging capacity of betalains and concerted PCET is the most thermodynamically favorable pathway for their one-electron oxidation. The results provide useful insights for the design of nature-derived redox mediators based on the betalain scaffold.