Comprehensive evaluation of the photo-transformation routes of trans-resveratrol.

Liquid chromatography (LC) combined with accurate mass spectrometry (MS), based on the use of a hybrid quadrupole time-of-flight (TOF) MS system, is employed to systematically investigate the photo-transformation routes of trans-resveratrol. Experiments were performed in quartz tubes, containing ethanolic solutions (12% v/v) of the precursor compound, exposed to different ultraviolet (UV) sources and to solar light. Time-courses of trans-resveratrol and transformation products (TPs) were investigated by direct injection of different reaction times aliquots in the LC-QTOF-MS system. Structural elucidation of detected TPs was derived from interpretation of their accurate product ion scan spectra. Trans-resveratrol labelled with (13)C6 in the mono-hydroxylated ring was also employed to further confirm the exact positions of some substituents in the generated TPs. In addition to the well-known trans-/cis-isomerization process, three different main reactions pathways were noticed under all the investigated conditions: (1) water addition to the exocycle double bond followed by oxidation to a ketone and cleavage of the molecule, (2) intramolecular cyclization to render a trihydroxylated phenanthrene, and (3) oxidation of the phenanthrene-like derivative to generate an orto-diquinone. Both, the trihydroxylated phenanthrene and the orto-diquinone underwent further aromatic hydroxylation reactions. The above transformation routes were also noticed for cis-resveratrol and the two analogue phytoalexins piceid and piceatannol. In addition to above transformation pathways, under solar light exposure, resveratrol underwent a molecular re-arrangement rendering the so-called resveratrone, whose structure consists of two fused aromatic rings bonded to a linear chain containing a carbonyl group conjugated with a double bond.