Mechanism of isomerization of 4-propyl-o-quinone to its tautomeric p-quinone methide

In previous work, we showed that o-quinones (3,5-cyclohexadiene-1,2-diones) can isomerize to p-quinone methides (4-alkyl-2,5-cyclohexadien-1-one) at rates which depend on the type of substituent at the para position [Iverson, S. L., Hu, L. Q., Vukomanovic, V., and Bolton, J. L. (1995) Chem. Res. Toxicol. 8, 537-544]. In the present investigation, we explored the mechanism of this isomerization reaction using 4-propyl-3,5-cyclohexadiene-1,2-dione (PQ) and its benzyl dideuterio analog 4-(1',1'-dideuteriopropyl)-3,5-cyclohexadiene-1,2-dione (DPQ). The results show that the isomerization reaction is general base-catalyzed, which suggests that amino acids on proteins with basic side chains could catalyze the reaction in vivo. The Bronsted beta value was determined to be 0.23 +/- 0.02, consistent with the transfer of a proton in the rate-determining step. The rate/pH profile generated from the buffer dilution plots showed dependence on hydroxide ion concentration from pH 7.8 to 9, indicative of base catalysis. From pH 6 to 7.8, the reaction was independent of pH, suggesting that other processes compete at low buffer concentration in this pH region. Substitution of the benzyl CH2 group with CD2 dramatically slows the isomerization reaction. The kinetic deuterium isotope effect on quinone methide formation was determined by measuring the amount of quinone methide trapped as GSH conjugates from PQ compared with DPQ. The isotope effect on product formation was 5.5 +/- 0.6, 37 degrees C. These data provide further evidence that formation of these electrophilic quinone methides from o-quinones could be catalyzed by basic residues in vivo and that the reaction could be inhibited by deuterium substitution at the benzyl methylene group.