Chemistry of Isoeugenol and Its Oxidation Products: Mechanism and Kinetics of Isoeugenol as a Skin Sensitizer

Structurally similar phytochemical compounds may elicit markedly different skin sensitization responses. Eugenol and isoeugenol are natural phenylpropanoids found in various essential oils are frequently used as fragrance ingredients in consumer products due to their pleasing aromatic properties. Both compounds are also skin sensitizers with isoeugenol being a stronger sensitizer than eugenol. The most commonly accepted mechanisms for haptenation by eugenol involve formation of a quinone methide or an ortho-quinone intermediate. The mechanism for the increased skin response to isoeugenol remains elusive, although quinone methide intermediates have been proposed. The recent identification of diastereomeric 7,4′-oxyneolignans as electrophilic, thiol-depleting isoeugenol derivatives has revived interest in the possible role of elusive reactive intermediates associated with the isoeugenol's haptenation process. In the present work, integrated non-animal skin sensitization methods were performed to determine the ability of syn-7,4′-oxyneolignan to promote haptenation and activation of further molecular pathways in keratinocytes and dendritic cells, confirming it as a candidate skin sensitizer. Kinetic NMR spectroscopic studies using dansyl cysteamine (DCYA) confirmed the first ordered nature of the nucleophilic addition for the syn-7,4′-oxyneolignan. Computational studies reaffirmed the “syn” stereochemistry of the isolated 7,4′-oxyneolignans along with that of their corresponding DCYA adducts and provided evidence for the preferential stereoselectivity. A plausible rationale for isoeugenol’s strong skin sensitization is proposed based on the formation of a hydroxy quinone methide as a reactive intermediate rather than the previously assumed quinone methide.