Photoinduced dissociation mass spectroscopy of firefly oxyluciferin anions

The oxyluciferin molecule in its anionic form is responsible for light emission from fireflies and some railroad worms and click beetles. Here we have studied the breakdown of the ions after photoexcitation by 550-nm light, and identified the atom composition of eight fragment ions based on mass spectrometric experiments on isotope-labeled compounds. A sector instrument with an electrospray ion source and a pulsed laser system was used for the experiments. After photoexcitation the time for dissociation was up to about 15 μs, which is much shorter than the 100-μs time constant for dissociation after one-photon absorption. The laser power was therefore kept high to allow the oxyluciferin anions to absorb two photons to produce enough fragment ions on the instrumental relevant time scale. The reaction energies leading to these ions were obtained from density functional theory calculations. The dominant fragment ion was deprotonated 2-cyano-6-hydroxybenzothiazole. Interestingly this behavior mirrors that of oxyluciferin both in vivo in insects, where the same nitrile is an intermediate in the postulated regeneration of d-luciferin from oxyluciferin or in vitro in near-neutral aqueous buffer. Dissociation of the oxyluciferin anion into this fragment ion was calculated to require 1.86 eV, which is less than the energy of one photon (2.25 eV). Experiments done on 5,5-dimethyloxyluciferin revealed a similar fragmentation pattern.