Theoretical photodynamic study of the photoprotolytic cycle of firefly oxyluciferin

Firefly oxyluciferin presents a pH-sensitive fluorescence in aqueous solutions. Its fluorescence spectra are composed of two green peaks at different pH values, despite the enolate anion being the only emitter. A computational approach was used to further elucidate the photoprotolytic cycle of oxyluciferin and investigate its pH sensitivity. It was found that oxyluciferin forms π-π stacking complexes both in the ground and excited states, at basic and acidic/neutral pH. However, at different pH values, these complexes adopt a different conformation, which explains the lower energy of the emission at acidic/neutral pH, in comparison with the emission at basic pH. A pH-sensitive fluorophore: Different conformations of the π-π stacking complex of the excited-state enolate species of firefly oxyluciferin lead to pH-sensitive fluorescence. This means that the enolate anion possesses different emission energies at acidic/neutral and basic pH.