Channelrhodopsin (ChR) is a key protein of the optogenetic toolkit. C1C2, a functional chimeric protein of Chlamydomonas reinhardtii ChR1 and ChR2, is the only ChR whose crystal structure has been solved, and thus uniquely suitable for structure-based analysis. We report C1C2 photoreaction dynamics with ultrafast transient absorption and multi-pulse spectroscopy combined with target analysis and structure-based hybrid quantum mechanics/molecular mechanics calculations. Two relaxation pathways exist on the excited (S-1) state through two conical intersections Cl-1 and Cl-2, that are reached via clockwise and counter-clockwise rotations: (i) the C13=C14 isomerization path with 450 fs via Cl-1 and (ii) a relaxation path to the initial ground state with 2.0 ps and 11 ps via Cl-2, depending on the hydrogen-bonding network, hence indicating active-site structural heterogeneity. The presence of the additional conical intersection Cl-2 rationalizes the relatively low quantum yield of photoisomerization (30 +/- 3%), reported here. Furthermore, we show the photoreaction dynamics from picoseconds to seconds, characterizing the complete photocycle of C1C2.
Hontani, Y., Marazzi, M., Stehfest, K., Mathes, T., van Stokkum, I. H. M., Elstner, M., ... Kennis, J. T. M. (2017). Reaction dynamics of the chimeric channelrhodopsin C1C2. Scientific Reports, 7(1), . https://doi.org/10.1038/s41598-017-07363-w