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

• Time-of-flight mass spectrometry investigation of resveratrol photo-transformation.
• Identification of four different transformation routes.
• Formation of hydroxylated phenanthrene and diquinone derivatives.
• Confirmation of same transformation routes for other natural phytoalexins.

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 13C6 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.