Although 2,3- and 3,4-endiol forms of diketogulono-delta-lactone (2,3- or 3,4-End.DKGL), which are formed from 2,3-diketo-L-gulonic acid (DKG), showed an anodic wave on d.c. polarogram, the half-wave potentials of both 2,3-End.DKGL and 3,4-End.DKGL were slightly different from that of L-ascorbic acid (AsA). The anodic waves of both 2,3- and 3,4-End.DKGL had the characteristic of kinetic current, and their cyclic voltammograms in neutral aqueous buffer solutions showed anodic peaks, but no cathodic peaks, probably due to the hydration of carbonyls of their oxidized forms. DKG showed two small anodic waves in neutral buffer solutions, the more positive h1 wave being an overlapping wave of the oxidation waves of 2,3- and 3,4-End.DKGL. The more negative h2 wave may be an oxidation wave due to the oxidation and weak adsorption of the enol form of DKG. The two anodic waves disappeared upon the addition of excess o-phenylenediamine; only one cathodic wave due to the quinoxaline ring was newly obtained. The cathodic wave may be used for the determination of DKG and 2,3- and 3,4-End.DKGL.
