Toward a rationalization of the sensitizing potency of substituted p-benzoquinones: reaction of nucleophiles with p-benzoquinones.

MNDO calculations have been carried out for the contact sensitizers 2,6-dimethoxy-1,4-benzoquinone (6) and 2-methoxy-6-methyl-1,4-benzoquinone (10) and for 2,5-dimethoxy-1,4-benzoquinone (7), which is nonallergenic in contrast to thymoquinone (8) (2-methyl-5-isopropyl-1,4-benzoquinone), which is a relatively strong contact allergen. Theoretical results indicate that the conformational flexibility of methoxy groups substituted at the quinone rings influences the electronic properties of these compounds, in particular their reactivity with regard to nucleophiles. According to theory, 6, 10, and 8 should possess a pronounced reactivity toward nucleophiles while 7 should resist nucleophilic attack. Hence, the allergenic capacity of a quinone seems to depend on their binding interactions with nucleophiles such as amino or thio groups of amino acids.