Kinetic and thermochemical study of the antioxidant activity of sulfur-containing analogues of vitamin E

Sulfur-containing analogues of vitamin E (thiachromanols), either linked or not to a catechol moiety, were synthesized and their hydrogen-atom donating ability evaluated. The determination of the O-H bond dissociation enthalpy (BDE) of the a-tocopherol analogue 4 by the electron paramagnetic resonance (EPR) equilibration technique provided a value of 78.9 kcalmol(-1), that is, approximately 1.8 kcal mol(-1) higher than that of cc-tocopherol. The kinetic rate constants for the reaction with peroxyl radicals (k(inh)), measured by inhibited autoxidation studies, showed that thiachromanols react 2.5 times slower than the corresponding tocopherols, in agreement with the higher BDE value. Ibis behavior was explained, on the basis of crystallographic analyses and DFT calculations, in terms of a change in the molecular geometry caused by insertion of a sulfur atom into the frame-work of vitamin E. This behavior implies a greater deviation of the condensed ring from coplanarity with the aromatic ring, thus giving rise to a decrease in the conjugative stabilization of the phenoxyl radical and consequently to an increase in the O-H bond strength. Although less reactive than tocopherols, thiachromanols may, however, act as bimodal antioxidants as a result of the hydroperoxide decomposing ability of the sulfur atom.