Simulation and Analysis of the Spectroscopic Properties of Oxyluciferin and Its Analogues in Water

Abstract

Firefly bioluminescence is a quite efficient process largely used for numerous applications. However, some fundamental photochemical properties of the light emitter are still to be analyzed. Indeed, the light emitter, oxyluciferin, can be in six different forms due to interexchange reactions. In this work, we present the simulation of the absorption and emission spectra of the possible natural oxyluciferin forms in water and some of their analogues considering both the solvent/oxyluciferin interactions and the dynamical effects by using MD simulations and QM/MM methods. On the one hand, the absorption band shapes have been rationalized by analyzing the electronic nature of the transitions involved. On the other hand, the simulated and experimental emission spectra have been compared. In this case, an ultrafast excited state proton transfer (ESPT) occurs in oxyluciferin and its analogues, which impairs the detection of the emission from the protonated state by steady-state fluorescence spectroscopy. Transient absorption spectroscopy was used to evidence this ultrafast ESPT and rationalize the comparison between simulated and experimental steady-state emission spectra. Finally, this work shows the suitability of the studied oxyluciferin analogues to mimic the corresponding natural forms in water solution, as an elegant way to block the desired interexchange reactions allowing the study of each oxyluciferin form separately.

Exploring the Possibility of Excited State Keto‐Enolate Transformation of the Oxyluciferin‐Luciferase Complex with QM/MM Free Energy Simulations

“…All the data presented up to this point is based on the time dependent (TD) density functional theory (DFT) calculations with the range‐separated ωB97X exchange‐correlation functional. While (TD)DFT has been widely adopted due to its computational efficiency and relatively good reliability, one may criticize its use for characterizing reaction free energies as it does not properly describe the multi‐configurational characters associated with bond breaking and bond formation . Of course, this is a separate issue from the use of the range separated functional for handing charge transfer states .…”

“…OLU can exist in six different chemical forms through keto/enol tautomerization combined with the deprotonation of both phenol and enol forms (Scheme ). Quite naturally, different chemical species will have different emission energies . The condition that each species prefers will also be different.…”

Modeling absorption spectra of molecules in solution

“…Recent studies demonstrated good agreement between experimental absorption and emission spectra and calculated ensemble spectra for oxyluciferin and its analogues. [136,137] In these systems, the spectral lineshape is dominated by solute-solvent interactions and low frequency motions of the chromophore, such as twisting around a dihedral angle, which often correspond to anharmonic regions of the potential energy surface. In systems where contributions from configurations on anharmonic areas of the potential energy surface have a much stronger spectral contribution than vibronic effects, an ensemble approach based on explicit solvent representation and vertical excitation energies can yield accurate lineshapes.…”

pH‐Dependent Absorption Spectrum of Oxyluciferin Analogues in the Presence of Adenosine Monophosphate

“…For that reason, oxyluciferin analogues have been proposed and synthesized for inhibiting certain forms . In this work, we are interested in a particular set of analogues (Figure , right), studied both experimentally and theoretically . In a nutshell, such analogues are meant to avoid any keto‐enol tautomerization.…”