Mechanisms of dimer and trimer formation from ultraviolet-irradiated alpha-tocopherol

Alpha-Tocopherol (alpha-TH) undergoes ultraviolet (UV)-induced photooxidation on the surface of mouse skin to produce a dihydroxydimer, a spirodimer, and trimers as the major products. To study the photochemistry involved, we UV-irradiated alpha-TH in a thin film on a glass petri dish. Photooxidation yielded a mixture of dihydroxydimer, spirodimer, and trimers. In the time-course studies, the dihydroxydimer accumulated and then was further oxidized, whereas the spirodimer and trimers accumulated more gradually. Reaction of two tocopheroxyl radicals forms the dihydroxydimer, whereas the spirodimer may be formed either by photooxidation of alpha-TH to an orthoquinone methide (o-QM) followed by a Diels-Alder reaction or by photooxidation of alpha-TH to the dihydroxydimer, followed by two-electron oxidation. Irradiation of a mixture of d10-labeled and unlabeled (d0) dihydroxydimer produced a mixture of labeled and unlabeled spirodimers as detected by positive atmospheric pressure chemical ionization-mass spectrometry. The absence of mixed label spirodimers among products indicated that direct oxidation of the dihydroxydimer is a facile route to the spirodimer and is probably the major spirodimer-forming reaction in alpha-TH photooxidations. Trimer formation from the dihydroxydimer and the spirodimer was observed, however, and requires an o-QM intermediate. Photooxidation of dl0-labeled and unlabeled (d0) dihydroxydimers yielded mixed isotopomers of the trimer products, thus demonstrating that the dihydroxydimer and spirodimers underwent conversion to o-QM intermediates. Photochemical conversion of alpha-TH to UV-absorbing dimer and trimer products may contribute to photoprotection by topically applied alpha-TH.