Time-Resolved Spectroscopy of Ultrafast Photoisomerization of Octopus Rhodopsin under Photoexcitation

A primary process in vision is the cis-trans photoisomerization of a chromophore of rhodopsin, called retinal. In the present work, we have performed ultrafast time-resolved spectroscopy of octopus rhodopsin using a sub-5-fs pulse laser. In comparison with our previous study on bacteriorhodopsin, we found that octopus rhodopsin follows similar dynamics after photoexcitation but with different time constants. Spectrogram analysis showed that a C═N stretching mode appeared directly after photoexcitation. After reaching the conical intersection region at 80 fs, the overlapping hydrogen out-of-plane and in-plane C═C-H modes emerged as distinct peaks at ~200 fs, finishing a rapid relaxation along the coordinate related with these modes. The intensities of these peaks and a C═C stretching mode were found to be modulated at a period of ∼500 fs, reflecting torsional motion around the C═C double bond before thermalization with the distribution of structural variations in the all-trans structure in configuration space.