Spectroscopic characterization of neutral and cation radicals of α-tocopherol and related molecules: A satisfactory denouement

Notwithstanding the facile occurrence of one-electron oxidation in α-tocopherol and its acetate (TOH and TOAc, respectively), and despite the remarkable stability, under appropriate conditions, of the oxidation products (TOH^̇+, TO^̇, and TOAc^̇+), their spectroscopic characterization is in an unsatisfactory state, calling for a fresh attempt to acquire reliable data. A new, model-free method is developed for analyzing time-resolved spectra showing the progress of the reaction TOH + R^̇ → TO^̇ + RH, where R^̇ is a stable free radical. The resulting absorption coefficients of TO^̇ in dichloromethane and hexane are in severe disagreement with some recent values derived from stopped-flow spectrophotometry. The discrepancy is traced to the imposition of boundary conditions that do not take proper account of the dead time of the apparatus; when multiplied by a factor of two, the stopped-flow data fall mostly in the range ε = (7.5 ± 0.5) × 10 ³ M^-1 cm^-1, conforming with the results of this study and the values found by Boguth and Niemann in 1969. Absorption spectra of the radical cations produced (electro)chemically are found to be reliable only in the visible region. Incomplete conversion of the parent compound to the radical cation, an obstacle to the determination of absorption coefficients from electrochemical studies, is circumvented by combining EPR and optical spectroscopy. The absorption coefficients of TOH^̇+ and TOAc ^̇+, determined in this manner, are found to be, respectively, 1.6 × 10⁴ and 1.3 × 10⁴ M^-1 cm ^-1, in accord with the values found first through similar means.