The Tautomerism of Quinones and the Question of Quinone Methide Intermediates in Oxidative Phosphorylation

Aviva Lapidot,Brian L Silver,David Samuel

doi:10.1016/s0021-9258(18)96376-3

Abstract

A series of quinones, including vitamin K1(20), have been shown not to exhibit isotopic hydrogen exchange. This finding is strong evidence against both the hypothesis of quinone methide tautomerism and the suggested occurrence of quinone methide intermediates in oxidative phosphorylation and various chemical reactions. The results of previous studies on duroquinone with the use of nuclear magnetic resonance had been interpreted in terms of hydrogen isotope exchange in basic solution. This interpretation has been shown to be mistaken. The nuclear magnetic resonance results have now been shown to be due to the formation of a free radical in basic solution. The radical was also detected with the use of electron spin resonance spectroscopy. The relevance of these results to the mechanism of oxidative phosphorylation is discussed.

Highlights

We have extended our studies to other quinones and have examined the amination of duroquinones as described by Cameron, Scott, and Todd (II), since this reaction has been assumed to proceed via a quinone methide intermediate (Scheme 2)
The results show that when the above named quinones are dissolved in deuterated solvents under various conditions and recovered from solution, no hydrogen isotope exchange can be detected by NMR spectroscopy
The results show that the changes in the NMR and ESR spectra of duroquinone following the addition of base are completely reversed by subsequent acidification and occur in both the presence and absence of deuterium in the solvent

Summary

AND RESULTS

Materials-Duroquinone, 2,3-dimethyl-l , 4 - naphthoquinone, and vitamin K1 [20] were obtained commercially. 2, S-Dimethyl-. The NMR spectrum of a weighed amount of the sublimate in CC14 gave a peak due to the methyl group of duroquinone with an intensity indicating no significant deuterium exchange. When duroquinone (10 mg) is dissolved in alkaline solution (0.014 to 1.0 N NaOH) in methanol-d or normal methanol (0.5 ml), the methyl resonance broadens and appears to lose intensity, as described by Scott [9]. This loss is illusory, since integration of the broadened peak shows that there is no decrease in the total area under the peak. In their study of the side chain cyclization in vitamin Kl~o,

Findings

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DISCUSSION