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Spectroscopic Characterization of Neutral and Cation Radicals of α-Tocopherol and Related Molecules: A Satisfactory Denouement

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Department of Physics, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway, and Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371
* To whom correspondence should be addressed. E-mail: [email protected]
†Norwegian University of Science and Technology.
‡Nanyang Technological University.
Cite this: J. Phys. Chem. A 2010, 114, 40, 10795–10802
Publication Date (Web):September 16, 2010
https://doi.org/10.1021/jp106736x
Copyright © 2010 American Chemical Society

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    Abstract

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    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) × 103 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.

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    Cited By

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    10. Luca Bamonti, Takashi Hosoya, Katharina F. Pirker, Stefan Böhmdorfer, Francesco Mazzini, Francesco Galli, Thomas Netscher, Thomas Rosenau, Lars Gille. Tocopheramines and tocotrienamines as antioxidants: ESR spectroscopy, rapid kinetics and DFT calculations. Bioorganic & Medicinal Chemistry 2013, 21 (17) , 5039-5046. https://doi.org/10.1016/j.bmc.2013.06.050
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    12. Christopher J. Rhodes. Electron spin resonance. Annual Reports Section "C" (Physical Chemistry) 2011, 107 , 47. https://doi.org/10.1039/c1pc90002a

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