Investigation On The Chemical Origin Of The Multicolour Bioluminescence And Chemical Mechanism

Many different organisms in nature, for example, bacteria, single-celled protists, fishes, and fireflies with very different luciferins, are endowed with the ability to emit light. Among all the bioluminescent organism, fireflies are the most studied and well-characterized. It is well known that there are more than 2000 firefly species found in temperate and tropical environments around the world. They use the same luciferin (LH2) substrate, (S)-2-(6-hydroxy-2-benzothiazolyl)-2-thiazoline-4- carboxylic acid, to naturally emit multicolor light from green (≈530 nm) to orange, and even to red (≈635 nm). This kind of bioluminescence has attracted the attention of both biologists and chemists and the origin of multicolor bioluminescence has been investigated by both experimental and theoretical methods. Considerable progress has been made. However, in this process two points have been less fully examined. One is the formation and decomposition mechanisms of the actual firefly dioxetanone (DO). A second is the ultimate conclusion about the relationship between the wide range of bioluminescent colors and the structure of the I i ght emitter. The present paper focuses on the two poi nts.1, In this chapter the formation and decomposition reactions of firefly dioxetanone (DO) have been investigated in the gas phase and in a solvent model with an appropriate dielectric constant. Firefly DO is identified as a key intermediate. The formation of DO involves a stepwise reaction mechanism: first the formation of the four-membered ring on the singlet potential surface but not on the triplet and then the departure of the PO4CH32- group. For the decomposition of DO, two possible reaction paths are proposed, one of which is reported for the first time.2, Is the resonance-based anionic keto form of oxyluciferin the chemical origin of multicolor bioluminescence? Can it modulate green into red luminescence? There is as yet no definitive answer from experiment or theory. The resonance-based anionic keto forms of oxyluciferin have been proposed as a cause of multicolor bioluminescence in the firefly. We model the possible structures by adding sodium or ammonium cations and investigating the ground- and excited-state geometries as well as the electronic absorption and emission spectra. A role for the resonance structures is obvious in the gas phase. The absorption and emission spectra of the two structures are quite different—one in the blue and another in the red. The differences in the spectra of the models are small in aqueous solution, with all the absorption and emission spectra in the yellow-green region. The resonance-based anionic keto form of oxyluciferin may be one origin of the red-shifted luminescence but is not the exclusive explanation for the variation from green (≈530 nm) to red (≈635 nm). We study the geometries, absorption, and emission spectra of the possible protonated compounds of keto(-1) in the excited states. A new emitter keto(-1)'-H is considered.3, The question whether the emitter of yellow-green firefly bioluminescence is the enol or keto-constrained form of oxyluciferin (OxyLH2) still has no definitive answer from experiment or theory. In this study, Arg220, His247, adenosine monophosphate (AMP), Water324, Phe249, Gly343, and Ser349, which make the dominant contributions to color tuning of the fluorescence, are selected to simulate the luciferase (Luc) environment and thus elucidate the origin of firefly bioluminescence. Their respective and compositive effects on OxyLH2 are considered and the electronic absorption and emission spectra are investigated with B3LYP, B3PW91, and PBE1KCIS methods. Comparing the respective effects in the gas and aqueous phases revealed that the emission transition is prohibited in the gas phase but allowed in the aqueous phase. For the compositive effects, the optimized geometry shows that OxyLH2 exists in the keto(-1) form when Arg220, His247, AMP, Water324, Phe249, Gly343, and Ser349 are all included in the model. Furthermore, the emission maximum wavelength of keto(-1)+Arg+His+AMP+H2O+Phe+Gly+Ser is close to the experimental value (560 nm). We conclude that the keto(-1) form of OxyLH2 is a possible emitter which can produce yellow-green bioluminescence because of the compositive effects of Arg220, His247, AMP, Water324, Phe249, Gly343, and Ser349 in the luciferase environment. Moreover, AMP may be involved in enolization of the keto(-1) form of OxyLH2. Water324 is indispensable with respect to the environmental factors around luciferin (LH2).4, The contributions of explicit water molecules to color-tuning mechanism of firefly were studied. The predictions explicit water molecules cause two different structures in the geometrical parameters of keto(-1) both in vacuo and aqueous solution. There are somewhat larger influences on absorption and emission spectra. When water molecules were added only on the side of benzothiazole ring the spectra shift to the blue. In contrast, when waters were added only on the side of the thiazoline ring the spectra shift to red. In a word, the color modulation of the emitted light depends on charge redistribution of molecule keto(-1), mainly the charge change of the benzothiazole and thiazole rings at the two terminal in keto(-1).