Abstract
The emitted color in fireflies’ bioluminescent systems depends on the beetle species the system is extracted from and on different external factors (pH, temperature…) among others. Controlling the energy of the emitted light (i.e., color) is of crucial interest for the use of such bioluminescent systems. For instance, in the biomedical field, red emitted light is desirable because of its larger tissue penetration and lower energies. In order to investigate the influence of the protein environment and the AMP protonation state on the emitted color, the emission spectra of the phenolate-keto and phenolate-enol oxyluciferin forms have been simulated by means of MD simulations and QM/MM calculations, considering: two different protein conformations (with an open or closed C-terminal domain with respect to the N-terminal) and two protonation states of AMP. The results show that the emission spectra when considering the protein characterized by a closed conformation are blue-shifted compared to the open conformation. Moreover, the complete deprotonation of AMP phosphate group (AMP2−) can also lead to a blue-shift of the emission spectra but only when considering the closed protein conformation (open form is not sensitive to changes of AMP protonation state). These findings can be reasoned by the different interactions (hydrogen-bonds) found between oxyluciferin and the surrounding (protein, AMP and water molecules). This study gets partial insight into the possible origin of the emitted color modulation by changes of the pH or luciferase conformations.
Highlights
The bioluminescence of fireflies is a quite efficient natural process which consists of light emission produced by an enzymatic-chemical reaction
In this study we propose the analysis of the influence of both the protein conformation and the protonation state of adenosine monophosphate (AMP) on the emission
We analyze the different interactions between the protein, AMP, water and oxyluciferin by performing MD simulations
Summary
The bioluminescence of fireflies is a quite efficient natural process which consists of light emission produced by an enzymatic-chemical reaction. Apart from its outstanding efficiency, this process shows a high substrate specificity. For these reasons, fireflies’ bioluminescence has found numerous applications in the last decades [1,2,3,4,5,6,7,8]. The substrate (luciferin) inside the enzyme (luciferase) undergoes a multistep oxidative reaction in presence of molecular oxygen, adenosine triphosphate (ATP) and Mg2+ (Scheme 1). The high in energy intermediate (dioxetanone) is produced by reaction with the dioxygen molecule and release of adenosine monophosphate (AMP). Afterwards, decomposition of dioxetanone leads to the oxidation of luciferin producing the so-called oxyluciferin in the excited state (reaction (2) in Scheme 1). Oxyluciferin decays to the ground state, emitting light (hν)
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