Reaction dynamics of the chimeric channelrhodopsin C1C2

Abstract

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 (S1) state through two conical intersections CI1 and CI2, that are reached via clockwise and counter-clockwise rotations: (i) the C13=C14 isomerization path with 450 fs via CI1 and (ii) a relaxation path to the initial ground state with 2.0 ps and 11 ps via CI2, depending on the hydrogen-bonding network, hence indicating active-site structural heterogeneity. The presence of the additional conical intersection CI2 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.

QM/MM Photodynamics of Retinal in the Channelrhodopsin Chimera C1C2 with OM3/MRCI

“…An exponential fit of the population indicates an S 1 lifetime beyond ca. 6 ps, which is rather comparable to the longer reaction time components in the experiment (2 ps and 11 ps) . A subset of those trajectories that hop to the ground state is further analyzed in the SI.…”

“…This observation agrees with the results obtained for S 1 torsion scans (see SI), where we note overall small barriers for both torsions with a slightly smaller activation energy for counterclockwise torsion. This slower model is, however, not entirely unproductive as suggested by Hontani et al …”

“…The study suggests that the fast 450 fs component leads to the 13‐ cis photoproduct with 0.3 quantum yield, while the slower components are not productive. These findings are supported by quantum mechanical/molecular mechanical (QM/MM) calculations of the complete active space with second‐order perturbational corrections//complete active space selfconsistent field with 12 active electrons in 12 active orbitals (CASSCF(12,12)//CASPT2) profiles for torsion about the C13=C14 bond and a CASSCF trajectory without initial velocities (denoted as 0K‐trajectory) . This single trajectory can, however, only provide qualitative insight into the photodynamics of retinal as it does not account for the statistical effects occuring in a molecular ensemble.…”

“…First insights into the photodynamics of C1C2 were given in a recent, combined experimental and computational study by Hontani et al . The authors report three major components in the decay of excited retinal with time constants of 450 fs, 2 ps and 11 ps, estimated by transient absorption and pump‐dump‐probe experiments.…”

Charge‐Transfer versus Charge‐Separated Triplet Excited States of [ReI(dmp)(CO)3(His124)(Trp122)]+ in Water and in Modified Pseudomonas aeruginosa Azurin Protein

“…We also note that, upon comparing Figure and Figure S2 in the Supporting Information, the QM/MM optimization of all selected snapshots does not result in a unique global minimum on the respective 3 MLCT and 3 CS, but different low‐lying local minima are found. This agrees with the description of a relatively flexible environment, inherent of a complex system, as already shown in other proteins …”