Kinetic and Thermochemical Study of the Antioxidant Activity of Sulfur-Containing Analogues of Vitamin E

Authors

  • Riccardo Amorati Dr.,

    1. Dipartimento di Chimica Organica “A. Mangini” Università di Bologna, Via San Giacomo 11, 40126 Bologna, Italy, Fax: (+39) 051-209-5688
    Search for more papers by this author
  • Andrea Cavalli Dr.,

    1. Dipartimento di Scienze Farmaceutiche, Università di Bologna, Via Belmeloro 6, 40126 Bologna, Italy
    Search for more papers by this author
  • Maria Grazia Fumo Dr.,

    1. Dipartimento di Chimica Organica “A. Mangini” Università di Bologna, Via San Giacomo 11, 40126 Bologna, Italy, Fax: (+39) 051-209-5688
    Search for more papers by this author
  • Matteo Masetti Dr.,

    1. Dipartimento di Scienze Farmaceutiche, Università di Bologna, Via Belmeloro 6, 40126 Bologna, Italy
    Search for more papers by this author
  • Stefano Menichetti Prof.,

    1. Dipartimento di Chimica Organica e Laboratorio di Progettazione, Sintesi e Studio di Eterocicli Bioattivi (HeteroBioLab), Polo Scientifico e Tecnologico Università di Firenze, Via della Lastruccia 13, 50019 Sesto Fiorentino, Italy, Fax: (+39) 055-457-3531
    Search for more papers by this author
  • Chiara Pagliuca Dr.,

    1. Dipartimento di Chimica Organica e Laboratorio di Progettazione, Sintesi e Studio di Eterocicli Bioattivi (HeteroBioLab), Polo Scientifico e Tecnologico Università di Firenze, Via della Lastruccia 13, 50019 Sesto Fiorentino, Italy, Fax: (+39) 055-457-3531
    Search for more papers by this author
  • Gian Franco Pedulli Prof.,

    1. Dipartimento di Chimica Organica “A. Mangini” Università di Bologna, Via San Giacomo 11, 40126 Bologna, Italy, Fax: (+39) 051-209-5688
    Search for more papers by this author
  • Caterina Viglianisi Dr.

    1. Dipartimento di Chimica Organica e Laboratorio di Progettazione, Sintesi e Studio di Eterocicli Bioattivi (HeteroBioLab), Polo Scientifico e Tecnologico Università di Firenze, Via della Lastruccia 13, 50019 Sesto Fiorentino, Italy, Fax: (+39) 055-457-3531
    Search for more papers by this author

Abstract

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[BOND]H bond dissociation enthalpy (BDE) of the α-tocopherol analogue 4 by the electron paramagnetic resonance (EPR) equilibration technique provided a value of 78.9 kcal mol−1, that is, approximately 1.8 kcal mol−1 higher than that of α-tocopherol. The kinetic rate constants for the reaction with peroxyl radicals (kinh), measured by inhibited autoxidation studies, showed that thiachromanols react 2.5 times slower than the corresponding tocopherols, in agreement with the higher BDE value. This 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 framework 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[BOND]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.

Ancillary