Time-resolved resonance Raman spectroscopy of α-tocopheroxyl and related radicals in solvent, micellar and membrane systems

Abstract

Time-resolved resonance Raman (TR³) spectroscopy has been used to study the α-tocopheroxyl radical which is an intermediate in the antioxidative reaction of α-tocopherol (vitamin E) in cellular membranes. TR³ spectra of the α-tocopheroxyl and a number of analogoue radicals in a range of solvents and in micellar and membrane environments have been measured. The spectra reflect the extent of delocalisation of the radical site due to interaction with the lone pair of the para-oxygen atom. The frequency and intensity of the CC stretch (v_8a) band at ca. 1590 cm^–1 reflect the hydrogen-bonding capacity and polarity (relative permittivity) of the solvents. The spectrum in sodium dodecyl sulfate micelles indicates that the chromanoxyl group of the α-tocopheroxyl radical occupies a very polar environment, whereas in hexadecyltrimethylammonium chloride and Triton X-100R-S micelles and in phospholipid bilayers a site of intermediate polarity is indicated. In concentrated HCl solutions, the spectrum of the radical cation of a water soluble α-tocopherol analogue (MDL 73404) has been recorded. The pK_a of the radical cation has been determined to be –1.4 ± 0.1.