A DFT and QSAR study of the role of hydroxyl group, charge and unpaired-electron distribution in anthocyanidin radical stabilization and antioxidant activity

Abstract

Density function theory study was conducted to examine the structure, planarity, and charge and unpaired electron delocalization of some anthocyanidins and monohydroxy flavylium ions as well as all their possible phenoxyl radicals. Bond dissociation energy of O–H bond was in the order of 3-<5-<4′-<7-OH bond except for in malvidin where 4′-OH has less bond dissociation energy than 5-OH bond because of the extra stability provided by the two o-OMe groups. Stabilization by both charge and unpaired electron resonances were suggested for each radical. The present work revealed that the 3-anthocyanidin radicals were stabilized by three types of resonance through 5-, 7- or 4′-oxygen atoms while 5-, 7- or 4′-radicals were stabilized only by resonance through pyrylium or 3-oxygen atom. Positive charge was found delocalized on carbons 2, 5, 7, 9, and 4′ by resonance and moves towards the radical oxygen upon radical formation. In all radicals, radical oxygen carries the highest spin density and least negative charge therefore, its charge, spin density and the least bond dissociation energy (O–H) in each anthocyanidin were well quantitatively correlated with the experimental radical scavenging activity.