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Evidence for tautomerisation of glutamine in BLUF blue light receptors by vibrational spectroscopy and computational chemistry

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Domratcheva,  Tatiana
Department of Biomolecular Mechanisms, Max Planck Institute for Medical Research, Max Planck Society;

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Hartmann,  Elisabeth
Department of Biomolecular Mechanisms, Max Planck Institute for Medical Research, Max Planck Society;

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Schlichting,  Ilme
Department of Biomolecular Mechanisms, Max Planck Institute for Medical Research, Max Planck Society;

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Citation

Domratcheva, T., Hartmann, E., Schlichting, I., & Kottke, T. (2016). Evidence for tautomerisation of glutamine in BLUF blue light receptors by vibrational spectroscopy and computational chemistry. Scientific Reports, 6: 22669, pp. 1-14. doi:10.1038/srep22669.


Cite as: http://hdl.handle.net/11858/00-001M-0000-002A-1B9C-6
Abstract
BLUF (blue light sensor using flavin) domains regulate the activity of various enzymatic effector domains in bacteria and euglenids. BLUF features a unique photoactivation through restructuring of the hydrogen-bonding network as opposed to a redox reaction or an isomerization of the chromophore. A conserved glutamine residue close to the flavin chromophore plays a central role in the light response, but the underlying modification is still unclear. We labelled this glutamine with 15N in two representative BLUF domains and performed time-resolved infrared double difference spectroscopy. The assignment of the signals was conducted by extensive quantum chemical calculations on large models with 187 atoms reproducing the UV-vis and infrared signatures of BLUF photoactivation. In the dark state, the comparatively low frequency of 1,667 cm−1 is assigned to the glutamine C=O accepting a hydrogen bond from tyrosine. In the light state, the signature of a tautomerised glutamine was extracted with the C=N stretch at ~1,691 cm−1 exhibiting the characteristic strong downshift by 15N labelling. Moreover, an indirect isotope effect on the flavin C4=O stretch was found. We conclude that photoactivation of the BLUF receptor does not only involve a rearrangement of hydrogen bonds but includes a change in covalent bonds of the protein.