Channelrhodopsin-1 from Chlamydomonas augustae (CaChR1) is a light-activated cation channel, which is a promising optogenetic tool. We show by resonance Raman spectroscopy and retinal extraction followed by high pressure liquid chromatography (HPLC) that the isomeric ratio of all-trans to 13-cis of solubilized channelrhodopsin-1 is with 70:30 identical to channelrhodopsin-2 from Chlamydomonas reinhardtii (CrChR2). Critical frequency shifts in the retinal vibrations are identified in the Raman spectrum upon transition to the open (conductive P2380) state. Fourier transform infrared spectroscopy (FTIR) spectra indicate different structures of the open states in the two channelrhodopsins as reflected by the amide I bands and the protonation pattern of acidic amino acids.
