Further insights into the electrooxidation of N-methyluric acids and correlation of oxidation potentials with frontier MO energies

The electrochemical oxidation of various N-methylated uric acids has been s tudied at a pyrolytic graphite electrode at physioiogical pH 7.2. The obser ved behavior was compared with uric acid to evaluate the effect of a methyl group. The E-p value was found to shift to less positive potentials when a methyl group is present at the N-1 position and to more positive potential s when substitution is at the N-3 position or at nitrogens of the imidazole ring. The values of Delta E-p followed the additivity of the substituents effect only when two methyl groups are present in different rings. The meth yl groups were also found to increase the electron density at N atoms of ur ic acids, and an excellent correlation between the oxidation potential and energy of the highest occupied molecular orbital was observed. On the basis of these studies it is concluded that the presence of a methyl group in th e pyrimidine ring restricts the formation of allantoin. The peroxidase-cata lysed oxidation of the N-methylated derivatives of uric acid was found to f ollow a pathway similar to that observed during electrochemical oxidation.