Structural properties of anthocyanins: rearrangement of C-glycosyl-3-deoxyanthocyanidins in acidic aqueous solutions.

Seven C-glycosyl-3-deoxyanthocyanidins 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-glycosyldeoxyanthocyanidins established fixed equilibrium proportions with their corresponding A-ring isomer in aqueous solutions, even under relatively strong acidic conditions (pH approximately 1), whether one started with pure 6-C- or 8-C-glycosyl-3-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-glycosyldeoxyanthocyanidins 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.