Electrochemical Reduction of Thymine in Dimethyl Sulfoxide Autoprotonation of the Radical Anion and Reduced Free Radical

Thymine is reduced in dimethyl sulfoxide (E 1/2 of -2.4 V); the decrease in apparent faradaic IZ from one at infinite dilution to two-thirds at concentrations above 2 m&Z is due to proton transfer by thymine to the initially formed radical anion and the anion formed on reduction of the resulting free radical, which is more easily reduced than the parent compound. The proton transfer, which is more rapid than the radical anion dimerization, deactivates part of the thymine, forming the more difficuitly reducible conjugate base. The latter forms insoluble mercury salts, producing up to three oxidation waves (IS,,, between -0.08 and -0.3 V). The free radical reduction, which is not observed for uracil or 2-hydroxypyrimidine, occurs due to steric hindrance by the 5-methyl group toward free radical dimerization, thereby increasing the half-life of the free radical. The effect of added base on the electrochemical behavior is described.