Structure-activity study on the quinone/quinone methide chemistry of flavonoids

A structure-activity study on the quinone/quinone methide chemistry of a se ries of 3 ' ,4 ' -dihydroxyflavonoids was performed. Using the glutathione trapping method followed by HPLC, H-1 NMR, MALDI-TOF, and LC/MS analysis to identify the glutathionyl adducts, the chemical behavior of the quinones/q uinone methides of the different flavonoids could be deduced. The nature an d type of mono- and diglutathionyl adducts formed from quercetin, taxifolin , luteolin, fisetin, and 3,3 ' ,4 ' -trihydroxyflavone show how several str uctural elements influence the quinone/quinone methide chemistry of flavono ids. In line with previous findings, glutathionyl adduct formation for quer cetin occurs at positions C6 and C8 of the A ring, due to the involvement o f quinone methide-type intermediates. Elimination of the possibilities for efficient quinone methide formation by (i) the absence of the C3-OH group ( luteolin), (ii) the absence of the C2=C3 double bond (taxifolin), or (iii) the absence of the C5-OH group (3,3 ' ,4 ' -trihydroxyflavone) results in g lutathionyl adduct formation at the B ring due to involvement of the o-quin one isomer of the oxidized flavonoid. The extent of di- versus monoglutathi onyl adduct formation was shown to depend on the ease of oxidation of the m onoadduct as compared to the parent flavonoid. Finally, unexpected results obtained with fisetin provide new insight into the quinone/quinone methide chemistry of flavonoids. The regioselectivity and nature of the quinone add ucts that formed appear to be dependent on pH. At pH values above the pK(a) for quinone protonation, glutathionyl adduct formation proceeds at the A o r B ring following expected quinone/quinone methide isomerization patterns. However, decreasing the pH below this pK(a) results in a competing pathway in which glutathionyl adduct formation occurs in the C ring of the flavono id, which is preceded by protonation of the quinone and accompanied by H2O adduct formation, also in the C ring of the flavonoid. All together, the da ta presented in this study confirm that quinone/quinone methide chemistry c an be far from straightforward, but the study provides significant new data revealing an important pH dependence for the chemical behavior of this imp ortant class of electrophiles.