Voltammetry of the liposoluble vitamins (A, D, E and K) in organic solvents

A review summarizing the voltammetric literature of the liposoluble vitamins A, D, E and K in organic solvents containing supporting electrolyte is presented. Electrochemical studies that were performed by attaching the vitamins to electrode surfaces and performing voltammetric scans in aqueous solutions are also summarized. Vitamins A (retinol and retinal) and D (cholecaliferol and ergocalciferol) undergo chemically irreversible voltammetric oxidation processes in organic solvents to form complicated or unknown compounds that cannot be electrochemically converted back to the starting materials. In contrast to vitamins A and D, vitamins E and K undergo chemically reversible electron-transfer processes that are often coupled to proton-transfer reactions. Vitamin E (a phenol) is voltammetrically oxidized in aprotic organic solvents in a -2e(-)/-H(+) process to form a diamagnetic cation, which is unusually long-lived compared to the analogous cations produced during the oxidation of other phenols. In an aqueous environment, vitamin E is electrochemically oxidized to the hydroquinone in a chemically irreversible -2e(-) process. In low moisture content aprotic solvents, vitamin K (a quinone) is reduced in two one-electron chemically reversible steps to form first a radical anion (semiquinone, at E(1)) and then at more negative potentials a dianion is formed (at E(2)). The dianion is especially prone to strong hydrogen-bonding interactions with trace water present in the organic solvents, resulting in a shift in the formal reduction potential of E(2) to more positive potentials as more water is added to the solvent.