Many terpenes used as fragrance compounds autoxidize when exposed to air, forming allylic hydroperoxides that have the potential to be skin contact allergens. To trigger the immunotoxicity process that characterizes contact allergy, these hydroperoxides are supposed to bind covalently to proteins in the skin via radical pathways. We investigated the formation of reactive radical intermediates from 7-hydroperoxy-3,7-dimethylocta-1,5-dien-3-ol and 2-hydroperoxylimonene, responsible for the sensitizing potential acquired by autoxidized linalool and limonene. Both compounds were synthesized through new short and reproducible synthetic pathways. The hydroperoxide decomposition catalyzed by Fe(II)/Fe(III) redox systems, playing a key role in degradating peroxides in vivo, was examined by spin-trapping-EPR spectroscopy. Alkoxyl and carbon-centered free radicals derived from the hydroperoxides were successfully trapped by the spin-trap 5,5-dimethyl-1-pyrroline N-oxide, whereas peroxyl radicals were characterized by spin-trapping studies with 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide. Using liquid chromatography combined with mass spectrometry, we demonstrated the formation of adducts, via radical mechanisms induced by Fe(II)/Fe(III), between the hydroperoxides and N-acetylhistidine methyl ester, a model amino acid that is prone to radical reactions. Free radicals derived from these hydroperoxides can thus induce amino acid chemical modifications via radical mechanisms. The study of these mechanisms will help to understand the sensitizing potential of hydroperoxides.

studies on the reactivity of sensitizing allylic hydroperoxides: investigation of the covalent modification of amino acids by carbon-radical intermediates. a Many fragrance terpenes autoxidize yielding allylic hydroperoxides that are strong skin allergens. These chemicals are supposed to bind through radical reactions to proteins in the skin to start the immunotoxic process characterizing skin sensitization, and further allergic contact dermatitis. In order to get more insight into this reactivity, we investigated the chemical behavior of the allylic hydroperoxides responsible for the allergenicity of autoxidized limonene and linalool. We studied by liquid chromatography-mass spectrometry the reactivity in Fe(II)/Fe(III) systems, able to degrade peroxides in vivo, toward amino acids prone to radical reactions, together with glutathione. We validated that carbon radicals issued from these compounds alter directly the lateral chain of some of the amino acids tested forming adducts via radical processes. Though, in parallel, due to the high oxidative strength of the media, we show that redox processes can also be induced and engender chemical modifications on the amino acids themselves. Amino acids chemically modified by the oxidant stress can then react with radicals issued from the hydroperoxides through what we call a "non-direct" reactivity process. Understanding protein binding processes to allergenic allylic hydroperoxides needs thus to consider both, covalent binding of carbon radicals to the amino acids, and additional oxido-reduction processes contributing to their sensitizing potential.