Volume 53, Issue 37 pp. 9870-9875
Communication

A Conical Intersection Controls the Deactivation of the Bacterial Luciferase Fluorophore

Dr. Samer Gozem

Dr. Samer Gozem

Chemistry Department, Bowling Green State University, Bowling Green, OH 43403 (USA) http://www.bgsu.lcpp.com/

These authors contributed equally to this work.

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Dr. Ekaterina Mirzakulova

Dr. Ekaterina Mirzakulova

Chemistry Department, Bowling Green State University, Bowling Green, OH 43403 (USA) http://www.bgsu.lcpp.com/

These authors contributed equally to this work.

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Dr. Igor Schapiro

Dr. Igor Schapiro

Chemistry Department, Bowling Green State University, Bowling Green, OH 43403 (USA) http://www.bgsu.lcpp.com/

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Dr. Federico Melaccio

Dr. Federico Melaccio

Department of Biotechnology, Chemistry and Pharmacy, Università di Siena, via A. Moro 2, 53100 Siena (Italy)

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Prof. Dr. Ksenija D. Glusac

Corresponding Author

Prof. Dr. Ksenija D. Glusac

Chemistry Department, Bowling Green State University, Bowling Green, OH 43403 (USA) http://www.bgsu.lcpp.com/

Chemistry Department, Bowling Green State University, Bowling Green, OH 43403 (USA) http://www.bgsu.lcpp.com/Search for more papers by this author
Prof. Dr. Massimo Olivucci

Corresponding Author

Prof. Dr. Massimo Olivucci

Chemistry Department, Bowling Green State University, Bowling Green, OH 43403 (USA) http://www.bgsu.lcpp.com/

Department of Biotechnology, Chemistry and Pharmacy, Università di Siena, via A. Moro 2, 53100 Siena (Italy)

Chemistry Department, Bowling Green State University, Bowling Green, OH 43403 (USA) http://www.bgsu.lcpp.com/Search for more papers by this author
First published: 14 July 2014
Citations: 24

We thank Dr. T. Domratcheva for helpful discussion. K.D.G. acknowledges support from the National Science Foundation (CHE-1055397 CAREER award). M.O. acknowledges support from the National Science Foundation (grant no. CHE-1152070), the Human Frontier Science Program Organization (grant RGP0049/2012CHE09-56776), and the EU-FP7 (Marie-Curie PIOF-GA-2012-332233). The authors are indebted to NSF-XSEDE and OSC for granted computer time. The European Cooperation in Science and Technology Action CM1002 is also acknowledged.

Graphical Abstract

How nature lights up flavins: 4a-hydroxy flavins display weak fluorescence and ultrafast excited-state decay in solution, but exhibit strong fluorescence when produced in a protein cavity. A joint experimental and theoretical study explains the fluorescence properties of these flavin adducts in terms of a deactivation pathway mediated by a conical intersection that becomes inaccessible in sterically constrained environments.

Abstract

The photophysics of flavins is highly dependent on their environment. For example, 4a-hydroxy flavins display weak fluorescence in solution, but exhibit strong fluorescence when bound to a protein. To understand this behavior, we performed temperature-dependent fluorescent studies on an N(5)-alkylated 4a-hydroxy flavin: the putative bacterial luciferase fluorophore. We find an increase in fluorescence quantum yield upon reaching the glass transition temperature of the solvent. We then employ multiconfigurational quantum chemical methods to map the excited-state deactivation path of the system. The result reveals a shallow but barrierless excited state deactivation path that leads to a conical intersection displaying an orthogonal out-of-plane distortion of the terminal pyrimidine ring. The intersection structure readily explains the observed spectroscopic behavior in terms of an excited-state barrier imposed by the rigid glass cavity.

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