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

Abstract

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 × 10(4) and 1.3 × 10(4) M(-1) cm(-1), in accord with the values found first through similar means.

Kinetic Study of the Scavenging Reaction of the Aroxyl Radical by Eight Kinds of Vegetable Oils in Solution

“…(4)): where 1,1‐diphenyl‐2‐picrylhydrazine (DPPHH) is a reduced form of DPPH•. The DPPH• radical and DPPHH show two absorption maxima ( λ max = 325 and 517 nm) and an absorption maximum ( λ max = 323 nm) in ethanol, respectively (Foti, ; Nagaoka et al, ; Naqvi et al, ). UV–vis absorptions of DPPH• and DPPHH overlap with each other in the wavelength region of 250–600 nm.…”

“…Antioxidant activity of food extracts has been measured using several different methods such as (1) TRAP (total radical‐trapping antioxidant parameter), (2) TEAC (trolox equivalent antioxidant capacity), (3) FRAP (ferric‐ion‐reducing antioxidant power), (4) ORAC (oxygen radical absorption capacity), and (5) SOAC (singlet oxygen absorption capacity) assay methods. Stable free radicals such as (6) DPPH• (2,2‐diphenyl‐1‐picrylhydrazyl), (7) galvinoxyl•, and (8) ABTS + • (2,2′‐azinobis(3‐ethylbenzothiazoline‐6‐sulfonic acid)) radicals are generally used to evaluate free radical scavenging activity of natural AOH and food extracts (Espin, Soler‐Rivas, & Wichers, ; Foti, ; Kawashima, Ohkubo, & Fukuzumi, ; Nagaoka, Nagai, Fujii, Ouchi, & Mukai, ; Naqvi, Li, Melø, & Webster, ).…”

Primary‐Colored Electrochromism of 1,4‐Phenylenediamines

“…The path length of the OSTLE cell (0.0609 cm) was determined by using a-tocopherol as ap robe molecule with known molar absorptivity (extinction coefficient). [53] Approximatem olar absorptivities for the ten PPDs were then also calculated using Beer-Lambert's law,a nd are provided in Table 1.…”

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

“…See DOI: 10.1039/c1cp20579j transfer which has been shown to occur for a number of other phenols, [22][23][24] because the radical cation has been positively identified by a range of spectroscopic techniques. 9,12,14,25 It is uncertain whether the second electron transfer step occurs heterogeneously at the electrode surface as shown in Scheme 1 for an ECE process, or whether it occurs by a homogeneous disproportionation reaction (DISP). 18,24 For some systems it is possible to distinguish the two mechanisms (ECE and DISP) via electrochemical modelling of the voltammetric data, 26 although for a-TOH there are additional steps that complicate the detailed modelling of the experiments.…”