Affiliations: Laboratory of Food Biochemistry, Division of Applied
Bioscience, Graduate School of Agriculture, Hokkaido University, Kita-ku,
Sapporo 060-8589, Japan. Fax.: +81 11 706 2496; E-mail:
junk@chem.agr.hokudai.ac.jp
Abstract: Solvent-dependency of the antiradical reaction of protocatechuic
acid (3,4-dihydroxybenzoic acid, PA) and its esters has been investigated. In
aprotic solvents, methyl protocatechuate (PAMe) was readily oxidized by two
molar equivalents of DPPH radical and converted to protocatechuquinone methyl
ester (PQMe). On the other hand, in alcoholic solvents such as methanol, PAMe
rapidly consumed five radicals in 30 min and changed to complex oxidized
mixtures. A ^1H-NMR analysis of the reaction mixture of PAMe
and DPPH radical in methanol showed that PAMe was rapidly converted to PQMe and
its 3-hemiacetal. In addition, a signal of 2-methoxy-PQMe 3-hemiacetal was also
detected in the reaction mixture. The results suggested that PQMe undergoes a
nucleophilic attack by the solvent alcohol molecule at the C-2 of the ring in
methanol, leading to a regeneration of catechol structure, which accounts well
for the higher DPPH radical scavenging activity of PAMe in alcohols than in
aprotic solvents.
