Theoretical study for absorption spectra of oxyluciferin in aqueous solutions

Hiyama, Miyabi; Akiyama, Hidefumi; Wang, Yu; Koga, Nobuaki

doi:10.1016/j.cplett.2013.05.053

Cited by 21 publications

(49 citation statements)

References 39 publications

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“…This situation is different from the situation in the ground state, where the p K E exp (S 0 ) of −0.39 signals that the two forms are of similar stability. Indeed, for the ground state, there are both experimental and computational data for a variety of solvents from which the presence of enol‐OxyLH 2 can be inferred.…”

Section: Resultsmentioning

confidence: 99%

“…One approach to help deducing the most probable form of the chemiexcited OxyLH 2 light emitter is to measure or calculate the ground‐ and/or excited‐state equilibrium constants for the keto–enol and acid–base reactions connecting the various species of Figure in solution. Although it is clear that the protein environment surrounding OxyLH 2 in firefly luciferase is different from, for example, an aqueous solution, such data reveal the intrinsic tendency of OxyLH 2 to prefer a particular tautomeric form and a particular protonation state, and have been reported in a number of studies . For example, ground‐state p K a measurements in water have shown that the enolic hydroxyl group of enol‐OxyLH 2 is more acidic than the phenolic hydroxyl group, which may indicate that the enolate‐OxyLH – monoanion is a likelier emitter than the phenolate‐enol‐OxyLH – monoanion.…”

Section: Introductionmentioning

confidence: 98%

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Distinguishing between keto-enol and acid-base forms of firefly oxyluciferin through calculation of excited-state equilibrium constants

Falklöf

Durbeej

2014

J. Comput. Chem.

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While recent years have seen much progress in the elucidation of the mechanisms underlying the bioluminescence of fireflies, there is to date no consensus on the precise contributions to the light emission from the different possible forms of the chemiexcited oxyluciferin (OxyLH2) cofactor. Here, this problem is investigated by the calculation of excited-state equilibrium constants in aqueous solution for keto-enol and acid-base reactions connecting six neutral, mono-anionic and di-anionic forms of OxyLH2.Particularly, rather than relying on the standard Förster equation and the associated assumption that entropic effects are negligible, these equilibrium constants are for the first time calculated in terms of excited-state free energies of a Born-Haber cycle.Performing quantum chemical calculations with density functional theory methods and using a hybrid cluster-continuum approach to describe solvent effects, a suitable protocol for the modeling is first defined from benchmark calculations on phenol. Applying this protocol to the various OxyLH2 species and verifying that available experimental data (absorption shifts and ground-state equilibrium constants) are accurately reproduced, it is then found that the phenolate-keto-OxyLH -mono-anion is intrinsically the preferred form of OxyLH2 in the excited state, which suggests a potential key role for this species in the bioluminescence of fireflies. Keywords Graphical Table of ContentsAqueous keto-enol and acid-base excited-state equilibrium constants between six neutral, mono-anionic and di-anionic forms of oxyluciferin, the cofactor responsible for the bioluminescence of firefly luciferase, are for the first time calculated from free energies of a Born-Haber cycle, rather than using the Förster equation. Thereby, it is found that the phenolate-keto-OxyLH -mono-anion is the preferred excited-state form of oxyluciferin in aqueous solution, attributing a potential key role to this species in the bioluminescence of fireflies.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 98%

Distinguishing between keto-enol and acid-base forms of firefly oxyluciferin through calculation of excited-state equilibrium constants

Falklöf

Durbeej

2014

J. Comput. Chem.

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“…Thus, we successfully reproduced experimental absorption spectra by utilizing the following correction for the p K a values :

Δ G_{corr} ({BH}^{+}) = Δ G_{calc} ({BH}^{+}) + Δ Δ G ({CH}^{+}),

where CH + is a reference compound that is the same as or similar to the molecules under investigation. Note that the correction for p K a values using these experimentally determined p K a values corresponds to that for the relative free energies of chemical species in the S 0 states . The second term of the right side represents the correction term evaluated by utilizing a reference molecule:

Δ Δ G ({CH}^{+}) = Δ G_{expl} ({CH}^{+}) - Δ G_{calc} ({CH}^{+})

The same correction can be used for the p K a values of molecules in the S 1 state as shown in Appendix and thus we used the following equation:

Δ G_{corr}^{*} ({BH}^{+}) = Δ G_{calc}^{*} ({BH}^{+}) + Δ Δ G ({CH}^{+}),

where

Δ G_{corr}^{*} ({BH}^{+})

and

Δ G_{calc}^{*} ({BH}^{<...>}

…”

Section: Methodsmentioning

confidence: 99%

“…In the latter calculations, we have nearly degenerate keto-enol tautomers and charge-transfer excited states, which require the use of a high-level method. However, in the case of luciferin, keto-enol tautomerization and charge-transfer excited states are not present (10,23).…”

Section: Introductionmentioning

confidence: 99%

Theoretical Study of Fluorescence Spectra Utilizing the pK_a Values of Acids in Their Excited States

Hiyama

Akiyama

Yamada

et al. 2013

Photochem & Photobiology

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Assignment of the fluorescence spectrum of firefly luciferin in aqueous solutions was achieved by utilizing not only emission energies but also theoretical absorption spectra and relative concentrations as estimated by pKa values. Calculated Gibbs free energies were utilized to estimate pKa values. These pKa values were then corrected by employing the experimental results. It was previously thought that the main peak near 550 nm observed in the experimental fluorescence spectra at all pH values corresponds to emission from the first excited state of the luciferin dianion [Ando et al. (2010) Jpn. J. Appl. Phys. 49, 117002-117008]. However, we found that the peak near 550 nm at low pH corresponds to emission from the first excited state of the phenolate monoanion of luciferin. Furthermore, we found that the causes of the red fluorescence at pH 1-2 are not only the emission from phenol monoanion but also the emission from the protonated species at nitrogen atom in the thiazoline ring of dianion.

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“…If the theoretical method which is useful to analyze the spectra of luciferin is established, it can be adopted to understand the spectra of oxyluciferin, because the photochemical properties of luciferin and oxyluciferin are considered to be similar (1,6,7). We have already succeeded in understanding the absorption spectra of oxyluciferin in aqueous solutions at various pH by adopting the theoretical formula which was used to analyze the absorption spectra of luciferin in aqueous solutions (8). Our results for absorption spectra of oxyluciferin showed very good agreement with the experimental study (9).…”

Section: Introductionmentioning

confidence: 99%

Analysis of Photoexcitation Energy Dependence in the Photoluminescence of Firefly Luciferin

Hiyama

Akiyama

Mochizuki

et al. 2014

Photochem & Photobiology

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The whole pathways for photoluminescence, which include absorption, relaxation and emission, of firefly luciferin in aqueous solutions of different pH values with different photoexcitation energies were theoretically investigated by considering protonation/deprotonation. It is experimentally known that the color of fluorescence changes from green to red with a decrease in the photoexcitation energy. We confirmed with the theoretical analysis that the peak energy shift in the fluorescence spectra with varying photoenergies is due to a change in photoluminescence pathway. When the photoexcitation energy is decreased, the red emission from a monoanion form of firefly luciferin with carboxylate and phenolate groups and N-protonated thiazoline ring occurs irrespective of the pH values. However, because the species abundant in the solution and those excited by the photon depend on the solution pH, the pathway leading to the monoanion form changes with the solution pH.

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Theoretical study for absorption spectra of oxyluciferin in aqueous solutions

Cited by 21 publications

References 39 publications

Distinguishing between keto-enol and acid-base forms of firefly oxyluciferin through calculation of excited-state equilibrium constants

Distinguishing between keto-enol and acid-base forms of firefly oxyluciferin through calculation of excited-state equilibrium constants

Theoretical Study of Fluorescence Spectra Utilizing the pK_a Values of Acids in Their Excited States

Analysis of Photoexcitation Energy Dependence in the Photoluminescence of Firefly Luciferin

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Theoretical study for absorption spectra of oxyluciferin in aqueous solutions

Cited by 21 publications

References 39 publications

Distinguishing between keto-enol and acid-base forms of firefly oxyluciferin through calculation of excited-state equilibrium constants

Distinguishing between keto-enol and acid-base forms of firefly oxyluciferin through calculation of excited-state equilibrium constants

Theoretical Study of Fluorescence Spectra Utilizing the pKa Values of Acids in Their Excited States

Analysis of Photoexcitation Energy Dependence in the Photoluminescence of Firefly Luciferin

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Theoretical Study of Fluorescence Spectra Utilizing the pK_a Values of Acids in Their Excited States