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

alpha -Tocopherol (alpha -TH) undergoes ultraviolet (UV)induced photooxidat ion on the surface of mouse skin to produce a dihydroxydimer, a spirodimer, and trimers as the major products. To study the photochemistry involved, w e UV-irradiated alpha -TH in a thin film on a glass peiri dish. Photooxidat ion yielded a mixture of dihydroxydimer, spirodimer, and trimers. In the ti me-course studies, the dihydroxydimer accumulated and then was further oxid ized, whereas the spirodimer and trimers accumulated more gradually. Reacti on of two tocopheroxyl radicals forms the dihydroxydimer, whereas the spiro dimer may be formed either by photooxidation of alpha -TH to an orthoquinon e methide (o-QM) followed by a Diets-Alder reaction or by photooxidation of alpha -TH to the dihydroxydimer, followed by two-electron oxidation. Irrad iation of a mixture of d(10) labeled and unlabeled (d(0)) dihydroxydimer pr oduced a mixture of labeled and unlabeled spirodimers as detected by positi ve atmospheric pressure chemical ionization-mass spectrometry. The absence of mixed label spirodimers among products indicated that direct oxidation o f the dihydroxydimer is a facile route to the spirodimer and is probably th e major spirodimer-forming reaction in alpha -TH photooxidations. Trimer fo rmation from the dihydroxydimer and the spirodimer was observed, however, a nd requires an o-QM intermediate. Photooxidation of d(10)-labled and unlabe led (d(0)) dihydroxydimers yielded mixed isotopomers of the trimer products , thus demonstrating that the dihydroxydimer and spirodimers underwent conv ersion to o-QM intermediates. Photochemical conversion of alpha -TH to UV-a bsorbing dimer and trimer products may contribute to photoprotection by top ically applied alpha -TH.