Singlet oxygen quenching by anthocyanin's flavylium cations

Abstract

The quenching of singlet molecular oxygen (¹O₂) by the flavylium cation form of six widespread anthocyanin derivatives: cyanidin 3-glucoside (CG), cyanidin 3-rutinoside (CR), cyanidin 3-galactoside (CGL), malvidin (M), malvidin 3-glucoside (MG) and malvidin 3,5-diglucoside (MDG) was studied in 1% HCl methanol solution by time-resolved phosphorescence detection (TRPD) of ¹O₂ and photostationary actinometry using perinaphthenone and methylene blue as sensitizers, respectively. The average value of the total (k₀) and chemical (k_c) quenching rate constants were ∼ 4×10⁸ m⁻¹ s⁻¹ and 3×10⁶ m⁻¹ s⁻¹, respectively, indicating the good performance of the studied anthocyanins as catalytic quenchers of ¹O₂. The quenching efficiency was larger for malvidin than for cyanidin derivatives, probably by the extra electron-donating methoxy group in ring B of the malvidin derivatives; and it was also dependent on the number and type of glycosylated substitution. As observed for other phenolic-like derivatives, the quenching of ¹O₂ by anthocyanins was mediated by a charge-transfer mechanism, which was modulated by the total number of –OR substituents that increases the electron-donating ability of these compounds.

• Anthocyanins
• singlet oxygen
• flavylium cation
• pigments
• antioxidant capacity