13C CPMAS NMR and DFT calculations of anthocyanidins

Abstract

Anthocyanidins, red dyes from flower petals and fruits, are beneficial to human health. They attract considerable attention owing to their strong antioxidant and radical scavenging properties, however they are unstable in solution and available in small amounts only. 13C CP MAS NMR spectra were recorded to characterize solid-state conformation of nine anthocyanidins: apigenidin, pelargonidin, cyanidin, delphinidin, peonidin, malvidin robinetidin, luteolinidin and diosmetinidin chlorides. For some carbons, the solid-state chemical shifts were different from those obtained for solutions, indicating differences in conformation and intermolecular interactions. The principal elements of the 13C chemical shift tensor were measured for pelargonidin, cyanidin, delphinidin and malvidin chlorides using PASS-2D NMR technique. DFT GIAO calculations of shielding constants were performed for apigenidin and several geometric isomers of pelargonidin. Comparison of experimental 13C δ ii with the theoretical shielding parameters was helpful in predicting the most reliable geometry in the solid state. The cross-polarization parameters were obtained from variable-contact time experiments; T CH are longer and the values of T 1 ρ H are shorter in the order: pelargonidin<cyanidin<delphinidin. It is probable that solid anthocyanidins become less ordered as the number of OH groups increases.