The hydrogen-bonded dianion of vitamin K1 produced in aqueous-organic solutions exists in equilibrium with its hydrogen-bonded semiquinone anion radical.

When the quinone, vitamin K1 (VK1), is electrochemically reduced in aqueous-acetonitrile solutions (CH3CN with 7.22 M H2O), it undergoes a two-electron reduction to form the dianion that is hydrogen-bonded with water [VK1(H2O)y(2­)]. EPR and voltammetry experiments have shown that the persistent existence of the semiquinone anion radical (also hydrogen-bonded with water) [VK1(H2O)x(­â€¢)] in aqueous or organic­aqueous solutions is a result of VK1(H2O)y(2­) undergoing a net homogeneous electron transfer reaction (comproportionation) with VK1, and not via direct one-electron reduction of VK1. When 1 mM solutions of VK1 were electrochemically reduced by two electrons in aqueous-acetonitrile solutions, quantitative EPR experiments indicated that the amount of VK1(H2O)x(­â€¢) produced was up to approximately 35% of all the reduced species. In situ electrochemical ATR-FTIR experiments on sequentially one- and two-electron bulk reduced solutions of VK1 (showing strong absorbances at 1664, 1598, and 1298 cm(­1)) in CH3CN containing <0.05 M H2O led to the detection of VK1(­â€¢) with strong absorbances at 1710, 1703, 1593, 1559, 1492, and 1466 cm(­1) and VK1(H2O)y(2­) with strong absorbances at 1372 and 1342 cm(­1).

The role of low levels of water in the electrochemical oxidation of α-tocopherol (vitamin E) and other phenols in acetonitrile.

The phenol, α-tocopherol, can be electrochemically oxidised in a -2e(-)/-H(+) process to form a diamagnetic cation that is long-lived in dry organic solvents such as acetonitrile and dichloromethane, but in the presence of water quickly reacts to form a hemiketal. Variable scan rate cyclic voltammetry experiments in acetonitrile with carefully controlled amounts of water between 0.010 M-0.6 M were performed in order to determine the rate of reaction of the diamagnetic cation with water. The water content of the solvent was accurately determined by Karl Fischer coulometric titrations and the voltammetric data were modelled using digital simulation techniques. The oxidation peak potential of α-tocopherol measured during cyclic voltammetry experiments was found to shift to less positive potentials as increasing amounts of water (0.01-0.6 M) were added to the acetonitrile, which was interpreted based on hydrogen-bonding interactions between the phenolic hydrogen atom and water. Several other phenols were examined and they displayed similar voltammetric features to α-tocopherol, suggesting that interactions of phenols with trace amounts of water were a common occurrence in acetonitrile. The H-bonding interactions of α-tocopherol with water were also examined via NMR and UV-vis spectroscopies, with the voltammetric and spectroscopic studies extended to include other coordinating solvents (dimethyl sulfoxide and pyridine).