Sub-5-fs Real-time Spectroscopy of Transition States in Bacteriorhodopsin During Retinal Isomerization

Authors

  • Takayoshi Kobayashi,

    Corresponding author
    1. Department of Physics, Graduate School of Science, University of Tokyo, Bunkyo, Tokyo, Japan
    2. Department of Applied Physics and Chemistry and Institute of Laser Research, The University of Electro-Communications, Chofu-shi, Tokyo, Japan
    3. Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan, China
    4. Institute of Laser Engineering, Osaka University, Yamada-oka, Suita, Osaka, Japan
      *Corresponding author email: kobayashi@ils.uec.ac.jp (Takayoshi Kobayashi)
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  • Atsushi Yabushita,

    1. Department of Physics, Graduate School of Science, University of Tokyo, Bunkyo, Tokyo, Japan
    2. Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan, China
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  • Takashi Saito,

    1. Department of Physics, Graduate School of Science, University of Tokyo, Bunkyo, Tokyo, Japan
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  • Hiroyuki Ohtani,

    1. Department of Physics, Graduate School of Science, University of Tokyo, Bunkyo, Tokyo, Japan
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  • Motoyuki Tsuda

    1. Department of Biomolecular Engineering, Graduate School of Life Science, University of Hyogo, Hyogo, Japan
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  • This paper is part of the Proceedings of the 12th International Conference on Retinal Proteins held at Awaji Island, Hyogo, Japan on 4–8 June 2006.

*Corresponding author email: kobayashi@ils.uec.ac.jp (Takayoshi Kobayashi)

Abstract

By using a sub-5-fs visible laser pulse, we have made the first observation of the vibrational spectra of the transition state during trans-cis isomerization in the retinal chromophore of bacteriorhodopsin (bRS68). No instant isomerization of the retinal occurs in spite of electron promotion from the bonding π-orbital to the anti-bonding π*-orbital. The difference between the in-plane and out-of-plane vibrational frequencies (about 1150–1250 and 900–1000 cm−1, respectively) is reduced during the first time period. The vibrational spectra after this period became very broad and weak and are ascribed to a “silent state.” The silent state lasts for 700–900 fs until the chromophore isomerizes to the cis-C13=C14 conformation. The frequency of the C=C stretching mode was modulated by the torsion mode of the C13=C14 double bond with a period of 200 fs. The modulation was clearly observed for four to five periods. Using the empirical equation for the relation between bond length and stretching frequency, we determined the transitional C=C bond length with about 0.01 Å accuracy during the torsion motion around the double bond with 1-fs time resolution.

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