The recently discovered Neorhodopsin (NeoR) exhibits absorption and emission maxima in the near-infrared spectral region, which together with the high fluorescence quantum yield makes it an attractive retinal protein for optogenetic applications. The unique optical properties can be rationalized by a theoretical model that predicts a high charge transfer character in the electronic ground state (S0) which is otherwise typical of the excited state S1 in canonical retinal proteins. The present study sets out to assess the electronic structure of the NeoR chromophore by resonance Raman (RR) spectroscopy since frequencies and relative intensities of RR bands are controlled by the ground and excited state's properties. The RR spectra of NeoR differ dramatically from those of canonical rhodopsins but can be reliably reproduced by the calculations carried out within two different structural models. The remarkable agreement between the experimental and calculated spectra confirms the consistency and robustness of the theoretical approach.

Department of Chemistry Bowling Green State University Overman Hall Bowling Green Ohio 43403 United States

Department of Chemistry Wroclaw University of Science and Technology Wybrzeże Wyspiańskiego 27 50 370 Wrocław Poland

Department of Physics and Astronomy Faculty of Science Vrije Universiteit Amsterdam De Boelelaan 1081 1081 HV Amsterdam The Netherlands

Dipartimento di Biotecnologie Chimica e Farmacia Universitâ di Siena via A Moro 2 53100 Siena Italy

ELI Beamlines Facility The Extreme Light Infrastructure ERIC Za Radnicí 835 25241 Dolní Břežany Czech Republic

Institut für Biologie Experimentelle Biophysik Humboldt Universität zu Berlin Invalidenstr 42 D 10115 Berlin Germany

Institut für Chemie Technische Universität Berlin Sekr PC14 Straße des 17 Juni 135 D 10623 Berlin Germany

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