Structural Properties of Anthocyanins: Rearrangement of C-Glycosyl-3-deoxyanthocyanidins in Acidic Aqueous Solutions

Seven C glycosyl-3-deoxyanthocyanid ins were made from their corresponding C-glycosylflavones. The structures of their rearrangement products, which were formed in acidic aqueous solutions, were elucidated. Rotameric conformers were detected for all of the 8-C-glycosyldeoxyanthocyanidins but were absent for their isomeric 6-C analogues in acidified methanolic NMR solvent. A correlation method based on HPLC-DAD and NMR integration of similar samples made it possible for the first time to determine accurately the proportions of two isomeric 6-C and 8-C-glycosylflavonoids occurring in mixtures. Each of the C-glycosyldeoxyanthocyanid ins established fixed equilibrium proportions with their corresponding A-ring isomer in aqueous solutions, even under relatively strong acidic conditions (pH similar to 1), whether one started with pure 6-C- or 8-C-glycosyl3-deoxyanthocyanidin. The nature of the aglycone, C-glycosyl moiety, and temperature were found to affect the equilibrium proportions. Increased water content (to a certain level) and temperatures were shown to increase the isomerization rates. The flavylium cations were the only equilibrium forms present at detectable quantities. The significance of rotation of the A-ring during isomerization was confirmed by lack of rearrangement of both 6-C and 8-C-glycosyl-3-deoxy-5-carboxypyranoanthocyanidins. The intermediary C-ring open forms of the C-glycosyideoxyanthocyanidins experience fast ring closure to their cyclic forms, which may reduce irreversible degradation reported for open chalcone, forms of the common anthocyanins. The stable C glycosyl-3-deoxyanthocyanidins may thus attract interest as possible colorants in the food industry, etc.