Electrochemical studies of vitamin A and vitamin E in organic solvents
Tan, Ying Shan
Date of Issue2013
School of Physical and Mathematical Sciences
Electrochemical studies were conducted on 6 major forms of vitamin A, namely; retinol, retinal, retinoic acid, retinyl acetate, retinyl palmitate and beta-carotene (β-Car) under similar experimental conditions using cyclic voltammetry (CV) and bulk controlled potential electrolysis (CPE) with coulometry. The compounds could all be reduced and oxidized in the aprotic organic solvents acetonitrile (CH3CN) and dichloromethane (CH2Cl2), but the voltammetric responses varied depending on the solvent and different electrode surfaces (Pt and GC). Among the six different forms of vitamin A, retinal displays chemically reversible reductive behavior during CV experiments. Retinal can be voltammetrically reduced in CH3CN in two one-electron processes to first form the anion radical (R•–) and then the dianion (R2–). The anion radical undergoes a reversible dimerization reaction to form the dimer dianion (R22–). All three anion species (anion radical, dianion and dimer dianion) undergo hydrogen-bonding interactions with water that is present at millimolar levels in the solvent. Interactions between vitamin species were examined. α-Tocopherol (vitamin E) is known to undergo 2e– oxidation while β-Car (pro-vitamin A) also undergoes a 2e– oxidation process. Cyclic voltammetry experiments indicated that the oxidative peak potential for α-tocopherol (α-TOH) was approximately +0.40 V more positive than the oxidative peak potential of β-Car. A solution of α-TO+/H+ (prepared by chemical oxidation of α-TOH with 2 NO+) was reacted with a solution containing an equal molar amount of β-Car. Voltammetric monitoring indicated that α-TOH was quantitatively regenerated and β-Car2+ was formed in high yield in a homogeneous two-electron transfer. The electrochemical oxidation of α-TOH was performed in CH3CN in the presence of varying amounts of water to understand how the water affected the overall oxidation mechanism. α-TOH can be electrochemically oxidised in a –2e–/–H+ process to form a diamagnetic cation that is long-lived in dry organic solvents but in the presence of water quickly reacts to form a hemiketal. The oxidation peak potential of α-TOH measured during cyclic voltammetry experiments was found to shift to less positive potentials as increasing amounts of water were added to the CH3CN, which was interpreted based on hydrogen-bonding interactions between the phenolic hydrogen atom and water. The results obtained for α-TOH have wide implications on how the electrochemical behavior of other phenols in organic solvents are interpreted.