The retinal structure of channelrhodopsin‐2 assessed by resonance Raman spectroscopy

Channelrhodopsin‐2 mediates phototaxis in green algae by acting as a light‐gated cation channel. As a result of this property, it is used as a novel optogenetic tool in neurophysiological applications. Structural information is still scant and we present here the first resonance Raman spectra of channelrhodopsin‐2. Spectra of detergent solubilized and lipid‐reconstituted protein were recorded under pre‐resonant conditions to exclusively probe retinal in its electronic ground state. All‐trans retinal was identified to be the favoured configuration of the chromophore but significant contributions of 13‐cis were detected. Pre‐illumination hardly changed the isomeric composition but small amounts of presumably 9‐cis retinal were found in the light‐adapted state. Spectral analysis suggested that the Schiff base proton is strongly hydrogen‐bonded to a nearby water molecule.

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