Shizuka Saito

Effects of alcoholic solvents on antiradical abilities of protocatechuic acid and its alkyl esters.

DPPH radical scavenging reactions of protocatechuic acid and its methyl ester were investigated in various solvents. In alcoholic solvents, methyl protocatechuate rapidly scavenged more than four equivalents of the radical, whereas approximately two equivalents were consumed in aprotic solvents. Methyl, ethyl, butyl, isopropyl, and tert-butyl protocatechuates were examined for their DPPH radical scavenging abilities in methanol or ethanol. As a result, the radical scavenging equivalence of sterically bulky esters tended to decrease compared to that of methyl or ethyl ester. The ABTS radical scavenging ability of those esters in water also showed the same tendency. Since 2-methoxy derivatives were detected in the reaction mixture of methyl protocatechuate and DPPH radical in methanol, a nucleophilic attack of an alcoholic molecule on the o-quinone intermediate, which is sensitive to steric hindrance from alkyl groups of both esters and alcoholic solvents, must be crucial for total radical scavenging abilities.

Inhibitory Effects of 5,6,7-Trihydroxyflavones on Tyrosinase

Baicalein (1), 6-hydroxyapigenin (6), 6-hydroxygalangin (13) and 6-hydroxy-kaempferol (14), which are naturally occurring flavonoids from a set of 14 hydroxy-flavones tested, exhibited high inhibitory effects on tyrosinase with respect to L-DOPA, while each of the 5,6,7-trihydroxyflavones 1, 6, 13 or 14 acted as a cofactor to monophenolase. Moreover, 6-hydroxykaempferol (14) showed the highest activity and was a competitive inhibitor of tyrosinase compared to L-DOPA. 5,6,7-Trihydroxyflavones 1, 6, 13 or 14 showed also high antioxidant activities. Hence, we conclude that the 5,6,7-trihydroxy-flavones are useful as good depigmentation agents with inhibitory effects in addition to their antioxidant properties.

Quinone Hemiacetal Formation from Protocatechuic Acid during the DPPH Radical Scavenging Reaction

Protocatechuic acid was rapidly converted to protocatechuquinone 3-methyl hemiacetal and protocatechuquinone during the reaction with DPPH radical in methanol. The structure of the acetal was determined by comparing the NMR data with those of an authentic compound prepared by (diacetoxy)iodobenzene oxidation of protocatechuic acid.

A Novel Oxidative Dimer from Protocatechuic Esters: Contribution to the Total Radical Scavenging Ability of Protocatechuic Esters

A novel oxidative dimer was isolated as a major product from a reaction mixture of methyl protocatechuate and DPPH radical in methanol. Its unusual benzobicyclo[3.2.1]octane structure was elucidated by extensive spectral analysis. This result suggests that the regeneration of catechol structures by the nucleophilic addition of an alcohol molecule on o-quinones and subsequent dimerization is one of the key reactions in the high radical-scavenging activity of protocatechuic esters in an alcoholic solvent.