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Distinguishing Between Keto-Enol and Acid-Base Forms of Firefly Oxyluciferin Through Calculation of Excited-State Equilibrium Constants
Linköping University, Department of Physics, Chemistry and Biology, Computational Physics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Computational Physics. Linköping University, The Institute of Technology.
2014 (English)In: Journal of Computational Chemistry, ISSN 0192-8651, E-ISSN 1096-987X, Vol. 35, no 30, 2184-2194 p.Article in journal (Refereed) Published
Abstract [en]

Although 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 (OxyLH(2)) 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, monoanionic and dianionic forms of OxyLH(2). Particularly, rather than relying on the standard Forster 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 OxyLH(2) 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(-) monoanion is intrinsically the preferred form of OxyLH(2) in the excited state, which suggests a potential key role for this species in the bioluminescence of fireflies.

Place, publisher, year, edition, pages
Wiley: 12 months , 2014. Vol. 35, no 30, 2184-2194 p.
Keyword [en]
light emission; tautomerism; protonation state; Born-Haber cycle; density functional theory
National Category
Physical Sciences
URN: urn:nbn:se:liu:diva-112610DOI: 10.1002/jcc.23735ISI: 000344173700003PubMedID: 25226816OAI: diva2:770194

Funding Agencies|Linkoping University; Swedish Research Council; Olle Engkvist Foundation; Wenner-Gren Foundations

Available from: 2014-12-10 Created: 2014-12-05 Last updated: 2015-11-19
In thesis
1. Computational Studies of Photobiological Keto-Enol Reactions and Chromophores
Open this publication in new window or tab >>Computational Studies of Photobiological Keto-Enol Reactions and Chromophores
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents computational chemistry studies of keto-enol reactions and chromophores of photobiological signicance.

The rst part of the thesis is concerned with two protein-bound chromophores that, depending on the chemical conditions, can exist in a number of dierent ketonic and enolic forms. The rst chromophore is astaxanthin, which occurs in the protein complex responsible for the deep-blue color of lobster carapace. By investigating how dierent forms of astaxanthin absorb UV-vis radiation of dierent wavelengths, a model is presented that explains the origin of the dramatic color change from deep-blue to red upon cooking of live lobsters.

The second chromophore is the oxyluciferin light emitter of fireflies, which is formed in the catalytic center of the enzyme firefly luciferase. To date, there is no consensus regarding which of the possible ketonic and enolic forms is the key contributor to the light emission. In the thesis, the intrinsic tendency of oxyluciferin to prefer one particular form over other possible forms is established through calculation of keto-enol and acid-base excited-state equilibrium constants in aqueous solution.

The second part of the thesis is concerned with two families of biological photoreceptors: the blue-light-absorbing LOV-domain proteins and the red-light-absorbing phytochromes. Based on the ambient light environment, these proteins regulate physiological and developmental processes by switching between inactive and active conformations. In both families, the conversion of the inactive into the active conformation is triggered by a chemical reaction of the respective chromophore.

The LOV-domain proteins bind a LOV-domain proteins bidn in flavin chromophore and regulate processes such as chloroplast relocation and phototropism in plants. An important step in the activation of these photoreceptors is a singlet-triplet transition between two electronically excited states of the flavin chromophore. In the thesis, this transition is used as a prototype example for illustrating, for the rst time, the ability of rst-principles methods to calculate rate constants of inter-excited state phosphorescence events.

Phytochromes, in turn, bind bilin chromophores and are active in the regulation of processes like seed germination and  flowering time in plants. Following two systematic studies identifying the best way to model the UV-vis absorption and fluorescence spectra of these photoreceptors, it is demonstrated that steric interactions between the chromophore and the apoprotein play a decisive role for how phytochromes are activated by light.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. 76 p.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1713
National Category
Theoretical Chemistry
urn:nbn:se:liu:diva-122614 (URN)10.3384/diss.diva-122614 (DOI)978-⁠91-⁠7685-⁠922-⁠3 (print) (ISBN)
Public defence
2015-12-18, Nobel (BL32), B-huset, Campus Valla, Linköping, 13:15 (English)
Available from: 2015-11-19 Created: 2015-11-11 Last updated: 2015-11-26Bibliographically approved

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