pH- and oxygen-sensitive photochemistry and photophysics of a model dipyrrin in solution

Phycobilins are a class of heterocyclic compounds ubiquitous in nature due to their remarkable absorption properties. Nature has utilized this class of molecules in unicellular photosynthetic organisms such as cyanobacteria and cryptophyte algae, where they serve as secondary pigments within phycobiliproteins. Phycobiliproteins have evolved to immobilize phycobilins, thereby reducing their inherent structural flexibility and protecting the bilin from unwanted photochemistry. To best investigate photochemical properties of bilins, their two model dipyrrolic subunits, dipyrrin and dipyrrinone, can be used as model compounds to ultimately reduce the extended nuclear degrees of freedom of bilins and potential photochemical reaction pathways. In this work, the pH-dependent and oxygen-sensitive photochemistry of a model dipyrrin (DPy) was investigated. Time-resolved UV-vis spectroscopy of a continuously irradiated sample revealed a pH- and oxygen-dependent conversion to photoproducts, which were analyzed using GC-MS to reveal photooxidative cleavage of the methine bridge. Density functional theory calculations of potential intermediates were carried out to validate the proposed mechanism leading to oxidative cleavage, predicting a dioxetane intermediate species. The experimental and computational results taken together support a photochemical mechanism that involves DPy sensitization of singlet O 2 in solution, dioxetane formation on the methine bridge, and [2+2] cycloreversion to result in the formation of pyrrole-2-carboxaldehyde and γ-lactam derivatives. The detrimental effects of this photochemistry on free dipyrroles in solution corroborates the need for a form of structural photoprotection for their use as light-harvesting pigments. Comparison with a representative phycobiliprotein structure indicates that encapsulation of bilins within the apoprotein, and proximal acidic residues at the pyrrole rings, help to retain a more photochemically resistive protonation state while preventing solvent and oxygen access at the susceptible methine bridges.