Channelrhodopsins are photosensitive proteins that trigger flagella motion in single cell algae and have been successfully utilized in optogenetic applications. In optogenetics light is used to activate neural cells in living organisms, which can be achieved by exploiting the ion channel signaling of channelrhodopsins. Tailoring channelrhodopsins for such applications includes the tuning of the absorption maximum. In order to establish rational design and to obtain a desired spectral shift, a

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... sic understanding of the absorption spectrum is required. We have studied the chimera C1C2 as a representative of this protein family and the first member with an available crystal structure. For this purpose, we sampled the conformations of C1C2 using QM/MM molecular dynamics, and subjected the resulting snapshots of the trajectory to excitation energy calculations using ADC(2) and simplified TD-DFT. In contrast to previous reports, we found that different hydrogen-bonding networks—involving the retinal protonated Schiff base, the putative counterions E162 and D292 as well as water molecules—had only a small impact on the absorption spectrum. However, in case of deprotonated E162 increasing the distance to the Schiff base hydrogen-bonding partner led to a systematic blue shift. The β-ionone ring rotation was identified as another important contributor. Yet the most important factor was found to be the bond length alternation and bond order alternation that were linearly correlated to the absorption maximum by up to 62 % and 82 %, respectively. We ascribe this novel insight into the structural basis of the absorption spectrum to our enhanced protein setup that includes membrane embedding as well as long and extensive sampling.