Contrasting voltammetric behavior of different forms of vitamin A in aprotic organic solvents.

Six of the major vitamers and provitamins comprising vitamin A (ß-carotene, retinoic acid, retinol, retinyl palmitate, retinyl acetate, and retinal) were examined using voltammetric and controlled potential electrolysis techniques in the aprotic organic solvents acetonitrile and dichloromethane at glassy carbon and platinum electrodes. All of the compounds underwent oxidation and reduction processes and displayed a number of similarities and differences in terms of the number of redox processes and chemical reversibility of the voltammetric responses. The electrochemical properties of the compounds were strongly influenced by the functional groups on the unsaturated phytyl chains (carboxylic acid, alcohol, ester, or aldehyde groups), and not only on the fully conjugated hydrocarbon unit which is common to all forms of vitamin A. The compounds were reduced at potentials between approximately -1.7 and -2.6 vs (Fc/Fc(+))/V (Fc = ferrocene) and oxidized at potentials between approximately +0.2 and +0.7 vs (Fc/Fc(+))/V. The average number of electrons transferred per molecule under long time scale electrolysis experiments were found to vary between 0.4 and 4 electrons depending on the exact molecular structure and experimental conditions.

Competing hydrogen-bonding, decomposition, and reversible dimerization mechanisms during the one- and two-electron electrochemical reduction of retinal (vitamin A).

Retinal (R) can be sequentially voltammetrically reduced in CH3CN in two one-electron processes to form first the anion radical (R(•-)) at -1.75 (±0.04) V vs Fc/Fc(+) (Fc = ferrocene) then the dianion (R(2-)) at -2.15 (±0.04) V vs Fc/Fc(+). The anion radical undergoes a reversible dimerization reaction to form the dianion (R2(2-)) with a forward dimerization rate constant k(dim) = 8 × 10(2) L mol(-1) s(-1) and a reverse monomerization rate constant k(mon) = 2 × 10(-2) s(-1) at 295 K. 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. As the water content of the solvent increases, the fate of the reduced compounds is determined by chemically irreversible hydrolysis reactions with H2O and decomposition reactions of the highly charged R(2-). Bulk-controlled potential electrolysis experiments combined with NMR analysis of the reaction solutions indicate that the reduction occurs at the aldehyde group of retinal. The electrochemical data obtained under a range of experimental conditions (varying voltammetric scan rates, temperatures, H2O content of solutions, and retinal concentrations) were modeled by digital simulation techniques to determine the kinetic and thermodynamic parameters associated with all of the homogeneous reactions.

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).

Electron-transfer reactions between the diamagnetic cation of α-tocopherol (vitamin E) and ß-carotene.

ß-Carotene (ß-Car) was chemically oxidized in a -2e(-) process using 2 mol equiv of NOSbF(6) in a 4:1 ratio (v/v) of dichloromethane:acetonitrile to form the ß-carotene dication (ß-Car(2+)). Voltammetric monitoring of the chemical oxidation experiments over a range of temperatures indicated that the half-life of ß-Car(2+) was approximately 20 min at -60 °C, and approximately 1 min at -30 °C. α-Tocopherol (α-TOH) was chemically oxidized in a -2e(-)/-H(+) process using 2 mol equiv of NOSbF(6) to form the diamagnetic cation (α-TO(+)) which survives indefinitely at -60 °C in a 4:1 ratio (v/v) of dichloromethane:acetonitrile. Cyclic voltammetry experiments indicated that the oxidative peak potential for α-TOH was approximately +0.4 V more positive than the oxidative peak potential of ß-Car. When solutions of α-TO(+)/H(+) (prepared by chemical oxidation of α-TOH with 2 NO(+)) were reacted with solutions containing equal molar amounts of ß-Car, voltammetric monitoring indicated that α-TOH was quantitatively regenerated and ß-Car(2+) was formed in high yield in a homogeneous two-electron transfer, according to the reaction α-TO(+) + H(+) + ß-Car → α-TOH + ß-Car(2+).

Voltammetrically controlled electron transfer reactions from alkanethiol modified gold electrode surfaces to low molecular weight molecules deposited within lipid (lecithin) bilayers.

A procedure was developed for initiating electron transfer from a gold electrode to a low molecular weight electron acceptor present inside supported lipid (lecithin) bilayers, followed by further electron transfer to an electron acceptor present in an aqueous solution. The electron acceptors present in the lecithin bilayers and aqueous phase were 7,7,8,8-tetracyanoquinodimethane (TCNQ) and [Fe(III)(CN)(6)](3-), respectively. A polished planar gold disk electrode was first coated via self-assembly procedures with an alkanethiol monolayer. A phospholipid layer consisting of multiple bilayers of lecithin containing TCNQ was subsequently deposited onto the alkanethiol monolayer. The Au/alkanethiol/lecithin-TCNQ electrode was placed in an aqueous solution containing various amounts of [Fe(III)(CN)(6)](3-) and [Fe(II)(CN)(6)](4-), with 0.5 M KCl as the supporting electrolyte. In the absence of TCNQ inside the alkanethiol/lecithin layers, only a small background current was observed. When TCNQ was included in the alkanethiol/lecithin layers, the voltammetry showed features typical of a catalytic process, due to the TCNQ being reduced to TCNQ(-*) within the lecithin bilayers and then undergoing oxidation back to TCNQ via interaction with [Fe(III)(CN)(6)](3-) at the lecithin-aqueous solution interface. The procedures for preparing the alkanethiol/lecithin-TCNQ coatings were optimized in order to obtain the most reproducible voltammetric response. Experiments were also performed using tetrathiafulvalene (TTF) as an electron donor in the lipid bilayer phase.