Abstract
We examined the effects of caffeic acid (CA) and related compounds on 7-carboxyheptyl radical formation. This analysis was performed using a standard D2O reaction mixture containing 4.3 mM oleic acid, 25 μM flavin mononucleotide (FMN), 160 mM phosphate buffer (pH 7.4), 10 mM cholic acid, 100 mM α-(4-pyridyl-1-oxide)-N-tert-butylnitrone, and 1 mM Fe(SO4)2(NH4)2 during irradiation with 7.8 J/cm2 at 436 nm. 7-Carboxyheptyl radical formation was inhibited by CA, catechol, gallic acid, chlorogenic acid, ferulic acid, noradrenalin, 2-hydroxybenzoic acid, 3-hydroxybenzoic acid, and 4-hydroxybenzoic acid. Quinic acid, benzoic acid, and p-anisic acid had no effect on radical formation. These results suggest that a phenol moiety is essential for these inhibitory effects. The fluorescence intensity of FMN decreased by 69% ± 2% after CA addition, suggesting that CA quenches the singlet excited state of FMN. When 1 mM CA was added to a standard reaction mixture containing 25 μM FMN, 140 mM phosphate buffer (pH 7.4), and 10 mM 4-oxo-2,2,6,6-tetramethylpiperidine, the electron spin resonance signal of 4-oxo-2,2,6,6-tetramethylpiperidinooxy disappeared. This finding suggests that singlet oxygen was scavenged completely by CA. Therefore, CA appears to inhibit 7-carboxyheptyl radical formation by scavenging singlet oxygen and quenching the excited state of FMN.
Highlights
Oxidative stress can result from many different unfavorable environmental factors such as ultraviolet (UV) light [1,2], smoking [3], and air pollutants [4,5]
The free radicals were previously identified as α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN)/7-carboxyheptyl radical adducts [30]
To improve the signal:noise ratio of the Electron Spin Resonance (ESR) signal, the reaction was performed in D2O, since D2O increases the half-life of singlet oxygen [6]
Summary
Oxidative stress can result from many different unfavorable environmental factors such as ultraviolet (UV) light [1,2], smoking [3], and air pollutants [4,5]. Oxidative stress occurs when levels of reactive oxygen species (ROS), including superoxide anion (O2−), hydrogen peroxide (H2O2), and hydroxyl radicals (OH), are elevated. These species can target lipid-rich membranes, cellular DNA, and proteins to exert an array of toxic effects. Many beverages and foods are major sources of polyphenols that can protect against oxidative stress via their antioxidant activity. Other studies have suggested that polyphenols inhibit the formation of free radicals and the propagation of free radical reactions by chelating transition metal ions [11,12,13]. We examined the antioxidant properties of CA and related compounds on the formation of oleic acid-derived radicals during flavin mononucleotide (FMN) photosensitization
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