title: Caffeic acid inhibits the formation of 7-carboxyheptyl radicals from oleic acid under flavin mononucleotide photosensitization by scavenging singlet oxygen and quenching the excited state of flavin mononucleotide.
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.
authors.affiliation: Department of Chemistry, Wakayama Medical University, 580 Mikazura, Wakayama 641-0011, Japan. asano@wakayama-med.ac.jp.
authors.forename: Marie
authors.initials: M
authors.lastname: Asano
authors.affiliation: Department of Chemistry, Wakayama Medical University, 580 Mikazura, Wakayama 641-0011, Japan. chem1@wakayama-med.ac.jp.
authors.forename: Hideo
authors.initials: H
authors.lastname: Iwahashi
mesh_terms: D002109:Caffeic Acids / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D005486:Flavin Mononucleotide / Q000737:chemistry*; D016166:Free Radical Scavengers / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry; D007501:Iron / Q000737:chemistry; D007502:Iron Chelating Agents / Q000737:chemistry; D019301:Oleic Acid / Q000737:chemistry*; D026082:Singlet Oxygen / Q000737:chemistry*
chemical_list: D002109:Caffeic Acids; D016166:Free Radical Scavengers; D005609:Free Radicals; D007502:Iron Chelating Agents; D026082:Singlet Oxygen; D019301:Oleic Acid; D005486:Flavin Mononucleotide; D007501:Iron; C040048:caffeic acid