Shining light on the electronic structure and relaxation dynamics of the isolated oxyluciferin anion.

Anand M Patel,James C Anderson,Michael A Parkes,Graham A Worth,Mariana Assmann,Helen H Fielding,Alice Henley

doi:10.1039/d0cp03276j

Abstract

Firefly bioluminescence is exploited widely in imaging in the biochemical and biomedical sciences; however, our fundamental understanding of the electronic structure and relaxation processes of the oxyluciferin that emits the light is still rudimentary. Here, we employ photoelectron spectroscopy and quantum chemistry calculations to investigate the electronic structure and relaxation of a series of model oxyluciferin anions. We find that changing the deprotonation site has a dramatic influence on the relaxation pathway following photoexcitation of higher lying electronically excited states. The keto form of the oxyluciferin anion is found to undergo internal conversion to the fluorescent S1 state, whereas we find evidence to suggest that the enol and enolate forms undergo internal conversion to a dipole bound state, possibly via the fluorescent S1 state. Partially resolved vibrational structure points towards the involvement of out-of-plane torsional motions in internal conversion to the dipole bound state, emphasising the combined electronic and structural role that the microenvironment plays in controlling the electronic relaxation pathway in the enzyme.

Highlights

Bioluminescence is the production and emission of light by a living organism
The keto form of the oxyluciferin anion is found to undergo internal conversion to the fluorescent S1 state, whereas we find evidence to suggest that the enol and enolate forms undergo internal conversion to a dipole bound state, possibly via the fluorescent S1 state
3.1 Photoelectron spectra 359 nm (3.45 eV) photoelectron spectra of M-phenolate-keto, M-phenolate-enol and M-phenol-enolate model analogues are presented in the top panel of Fig. 4; corresponding photoelectron spectra of OLÀ generated by Electronic supplementary information (ESI) from MeOH and MeCN are presented in the bottom panel

Summary

Introduction

Bioluminescence is the production and emission of light by a living organism It is one of the most spectacular processes in nature and is observed widely in terrestrial and marine organisms.[1,2] The light is produced by catalytic oxidation of a small molecule (luciferin) by an enzyme (luciferase). Subsequent relaxation to the ground electronic state, S0, results in the emission of yellow-green light (lmax = 558 nm).[5] Despite the fact that all bioluminescent beetle species use the same small molecule and the same reaction to produce light, the colour of the emission varies from green to red depending on the species,[6,7] or in the case of the Pyrophorus plagiophthalamus click beetle, even between individuals.[6,8] Wavelength shifts are observed when changing Luc in vitro and have been attributed to different amino acid residues.[9] it is clear that the microenvironment of the luciferin plays a key role in defining its electronic properties

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Conclusion