Deciphering the protonation and tautomeric equilibria of firefly oxyluciferin by molecular engineering and multivariate curve resolution

The mysterious flashes of light communicated by fireflies conceal a rich and exciting solution spectrochemistry that revolves around the chemiexcitation and photodecay of the fluorophore, oxyluciferin. A triple chemical equilibrium by double deprotonation and keto–enol tautomerism turns this simple molecule into an intricate case where the relative spectral contributions of six chemical species combine over a physiologically relevant pH range, rendering physical isolation and spectral characterization of most of the species unmanageable. To disentangle the individual spectral contributors, here we demonstrate the advantage of chemical oriented multivariate data analysis. We designed a set of specific oxyluciferin derivatives and applied a multivariate curve resolution-alternating least squares (MCR-ALS) procedure simultaneously to an extensive set of pH-dependent spectroscopic data for oxyluciferin and the target derivatives. The analysis provided, for the first time, the spectra of the pure individual components free of contributions from the other forms, their pH-dependent profiles and distributions, and the most accurate to date values for the three equilibrium constants.