A Time‐Dependent Density Functional Theory Investigation on the Origin of Red Chemiluminescence

Abstract

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 (approximately 530 nm) to red (approximately 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.

Computational Investigation of the Effect of pH on the Color of Firefly Bioluminescence by DFT

“…[13,21,39] Analyzing the results obtained for the open conformation model, we can see that at about pH 5-6 the sole species is Keto-(À1), that is, this anion may also be the red emitter. These results further contradict the hypothesis of White et al [4] and also do not support the studies of Min et al, [13,14] which presented the possibility of a Keto-(À1)-H red emitter.…”

“…The absence of enolic light emitters is in agreement with experiment [6] and with the latest theoretical studies. [13,14,21,[39][40][41] However, while informative, the work of Branchini et al [6] with the OxyLH 2 analogue only demonstrates that the keto species could produce yellow-green light, and to date no experimental data excluded the enolic species from the bioluminescence reaction. Also, many of the theoretical studies [13,14,21,40] that consider Keto-(À1) to be the sole light emitter already assume this, due to the findings of Branchini et al, [6] and do not include the enolic species in their simulations.…”

“…More recently, a seventh form was hypothesized by Min et al, [13,14] whose theoretical studies indicated that this species could be formed by…”

Study on the Effects of Intermolecular Interactions on Firefly Multicolor Bioluminescence

“…[20] Calculations by Nakatani et al [21] discarded the resonance-based structure as an explanation, as was corroborated by our own time-dependent DFT (TD-DFT) calculations. [22] Also, several TD-DFT calculations [22][23][24] demonstrated that interaction of a cation with the benzothiazole moiety of OxyLH 2 results in a blueshift, thus refuting the hypothesis formulated by Hirano et al [18] Theoretical [25] and experimental [18] studies also discarded the hypothesis formulated by Nakatsu et al, [17] which stated that the rigidity of the active site modulates the color of light emitted. More recently, we began to pursue clarification of this topic, having reviewed computational studies on multicolor bioluminescence.…”

Theoretical Study of the Relationships between Excited State Geometry Changes and Emission Energies of Oxyluciferin

“…30,31 This geometry is the initial form of excited substrate. The energy minimum geometry in the excited state (point E f ) can be found with time-dependent B3LYP (TD B3LYP).…”