Preliminary UHPLC–PDA–ESI-MS screening of light-accelerated autoxidation products of the tetrapyrrole biliverdin

The application of phycobiliproteins, e.g. blue C-phycocyanin, as natural water-soluble food colourants is emerging. The chromophore of these proteins comprises a number of tetrapyrroles (or phycocyanobilins), which have an extensive conjugated system, vulnerable to autoxidation. To assess the autoxidation products, a simplified model system was used in which the free tetrapyrrole biliverdin, instead of phycobiliprotein, was subjected to light-accelerated autoxidation. Degradation products of biliverdin were subsequently annotated by reversed-phase ultra-high performance liquid chromatography with a photo diode array and positive mode in-line electrospray ionization mass spectrometry. To facilitate the analysis of degradation products, autoxidation of the three methine bridges in biliverdin was mimicked in silico. It was found that both the peripheral and central methine bridges of biliverdin were susceptible to light-accelerated autoxidation. Scission products tentatively annotated with MS2 and MS3 were propionic acid-containing pyrroles. From this, it can be speculated that tetrapyrroles attached to phycobiliproteins are susceptible to autoxidative degradation.