Oxidative dimers produced from protocatechuic and gallic esters in the DPPH radical scavenging reaction.
DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging reactions of protocatechuic and gallic acids, and their methyl esters, have been investigated by NMR. In acetone, methyl protocatechuate was gradually converted to a Diels-Alder adduct of two molecules of the intermediate quinone in the reaction with DPPH radical, whereas methyl gallate rapidly gave a symmetrical dimer via a putative quinone precursor. Both dimers are rather unstable and their structures have been deduced by in situ NMR measurements of the reaction mixtures. Gallic acid also gave a corresponding symmetrical dimer in the same reaction as methyl gallate, although protocatechuquinone produced from protocatechuic acid did not yield a Diels-Alder adduct, unlike its methyl ester. Interestingly, these dimer formations were not observed in methanol solution.
Laboratory of Food Biochemistry, Division of Applied Bioscience, Graduate School of Agriculture, Hokkaido University, Sapporo 060-8589, Japan. junk@chem.agr.hokudai.ac.jp
Jun
J
Kawabata
Yasuko
Y
Okamoto
Asuka
A
Kodama
Terumasa
T
Makimoto
Takanori
T
Kasai
D000096:Acetone; D016227:Benzoquinones / Q000737:chemistry; D001713:Biphenyl Compounds; D019281:Dimerization; D016166:Free Radical Scavengers / Q000737:chemistry*; D005707:Gallic Acid / Q000737:chemistry*; D062385:Hydroxybenzoates / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008745:Methylation; D010084:Oxidation-Reduction; D010851:Picrates / Q000737:chemistry*
D016227:Benzoquinones; D001713:Biphenyl Compounds; D016166:Free Radical Scavengers; D062385:Hydroxybenzoates; D010851:Picrates; D000096:Acetone; C009091:protocatechuic acid; C004532:quinone; D005707:Gallic Acid; C004931:1,1-diphenyl-2-picrylhydrazyl