Singlet oxygen quenching by anthocyanin's flavylium cations

Area of Science:

• Photochemistry
• Natural Product Chemistry
• Biochemistry

Background:

• Singlet molecular oxygen ((1)O(2)) is a reactive oxygen species implicated in various biological processes.
• Anthocyanins are natural pigments with potential antioxidant properties.
• Understanding the interaction between anthocyanins and (1)O(2) is crucial for their biological applications.

Purpose of the Study:

• To investigate the quenching efficiency of six anthocyanin derivatives in their flavylium cation form on singlet molecular oxygen ((1)O(2)).
• To elucidate the mechanism underlying the quenching process.
• To determine the structure-activity relationship influencing the quenching efficacy.

Main Methods:

• Time-resolved phosphorescence detection (TRPD) was employed to monitor (1)O(2) decay kinetics.
• Photostationary actinometry using perinaphthenone and methylene blue as sensitizers was utilized.
• Kinetic studies were performed in a 1% HCl methanol solution.

Main Results:

• The studied anthocyanins demonstrated effective quenching of (1)O(2) with total quenching rate constants (k(0)) around 4 x 10(8) M(-1) s(-1) and chemical quenching rate constants (k(c)) around 3 x 10(6) M(-1) s(-1).
• Malvidin derivatives exhibited higher quenching efficiency compared to cyanidin derivatives, attributed to the electron-donating methoxy group.
• Quenching efficiency was influenced by the type and number of glycosylated substitutions and modulated by the total number of -OR substituents.

Conclusions:

• Anthocyanins act as efficient catalytic quenchers of singlet molecular oxygen ((1)O(2)).
• The quenching mechanism involves a charge-transfer process modulated by the electron-donating ability of substituents.
• Structural features, including methoxy groups and glycosylation patterns, significantly impact the antioxidant efficacy of anthocyanins against (1)O(2).