Ultrafast Infrared Spectroscopy on Channelrhodopsin‑2 Reveals Efficient Energy Transfer from the Retinal Chromophore to the Protein

The primary reaction dynamics of channelrhodopsin-2 was investigated using femtosecond vis-pump/mid-IR probe spectroscopy. Due to the fast deactivation of the excited state in channelrhodopsin-2, it is possible to observe the direct impact of retinal isomerization on the protein surrounding. We show that the dominant negative band at 1665 cm⁻¹ tentatively assigned to an amide I vibration is developed with a time constant of 0.5 ps. Also a variety of side-chain vibrations are formed or intensified on this time scale. The comparison of the light-induced FT-IR spectra of channelrhodopsin-2 in H₂O and D₂O at 80 K enabled us to tentatively identify the contribution of Arg side chain(s). The subsequently observed decay of nearly the whole difference pattern has a particularly high impact on the C=C and C=N stretching vibrations of the retinal. This suggests that the underlying mechanism describes a cooling process in which the excess energy is redirected toward the retinal surrounding, e.g., the protein and functional water molecules. The pronounced protein contributions in comparison to other rhodopsins point to a very efficient energy redistribution in channelrhodopsin-2