title: Kinetic study of the radical scavenging of capsaicin in homogeneous solutions and aqueous Triton X-100 micellar suspensions.
abstract: A kinetic study of capsaicin (CAP) toward radicals has been performed using stopped-flow spectrophotometry in detail. The second-order rate constants (k2) for the reaction of CAP toward 2,2-diphenyl-1-picrylhydrazyl (DPPH) and galvinoxyl have been measured in methanol, ethanol, 2-propanol/water (5:1, v/v), and aqueous micellar suspensions containing 5% Triton X-100 (pH 4.0 to 10.0), respectively. The decay rates of DPPH and galvinoxyl for the reaction with CAP increased linearly in a concentration-dependent manner in homogeneous solutions and aqueous micellar suspensions. However, the k2 for CAP obtained in an aqueous micellar suspension showed notable pH dependence; that is, the reactivity of CAP increased with an increasing pH value from 4 to 10. In addition, a good correlation between the k2 value and the molar fraction of CAP (phenolate anion (CAP-O(-))/undeprotonated form (CAP-OH)) was observed. These properties are associated with the pKa of CAP. Furthermore, it was found that the CAP-O(-) reacts with galvinoxyl about 6 times as fast as the CAP-OH. These results indicate that sequential proton loss electron transfer from the phenolic hydrogen of CAP may be responsible for the scavenging of radicals in an aqueous micellar suspensions.
authors: Dept. of Physiology, Biological Engineering & Medical Information Engineering, Faculty of Health Sciences, Kyorin Univ, 476 Miyashita-cho, Hachioji, Tokyo, 192-8508, Japan.
authors: Atsushi
authors: A
authors: Watanabe
authors: Shin-Ichiro
authors: S
authors: Seno
authors: Eiki
authors: E
authors: Kogure
authors: Kensuke
authors: K
authors: Seki
authors: Takeshi
authors: T
authors: Sakamoto
authors: Youji
authors: Y
authors: Okada
authors: Hideaki
authors: H
authors: Shimazu
mesh_terms: D000975:Antioxidants / Q000494:pharmacology*; D001559:Benzhydryl Compounds / Q000378:metabolism; D001713:Biphenyl Compounds / Q000378:metabolism; D002211:Capsaicin / Q000494:pharmacology*; D016166:Free Radical Scavengers / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008823:Micelles; D017830:Octoxynol / Q000378:metabolism*; D010084:Oxidation-Reduction; D010851:Picrates / Q000378:metabolism; D012996:Solutions; D012997:Solvents; D013535:Suspensions
chemical_list: D000975:Antioxidants; D001559:Benzhydryl Compounds; D001713:Biphenyl Compounds; D016166:Free Radical Scavengers; D008823:Micelles; D010851:Picrates; D012996:Solutions; D012997:Solvents; D013535:Suspensions; D017830:Octoxynol; C004931:1,1-diphenyl-2-picrylhydrazyl; D002211:Capsaicin; C020338:galvinoxyl
keywords: DPPH; antioxidative site; capsaicin; galvinoxyl; pH dependence; pKa; phenolate anion; phenolic OH group; rate constant