Structural factors determining the absorption spectrum of the channelrhodopsin chimaera C1C2

Channelrhodopsins (ChR) are light-activated ion channels with a retinal chromophore covalently attached to a lysine amino acid residue via a protonated Schiff base.^1 After absorbing a photon the retinal isomerises, which starts a photocycle that leads to cations entering the cell, thereby causing a depolarization of the plasma membrane.² ChRs have found application in optogenetics, where cells or whole organisms are controlled by light-sensitive ion channels.^2-3

We have investigated factors that determine the absorption maximum of the retinal chromophore inside the ChR chimaera C1C2.^4 Our aim is to derive an understanding at the molecular level in order to be able to tailor the absorption wavelength by mutations. We have sampled the geometries of membrane-embedded C1C2 and computed absorption spectra for 3000 snapshots. Our calculated absorption maximum of 524 nm is within 0.3 eV of the experimental value of 470 nm.⁴ Dissection of our spectra according to different structural and electronic determinants reveals that protonation of the counterion E162 causes a red shift of ~20 nm. Moreover, the absorption maximum is strongly correlated with the bond order alternation of the retinal (r = 0.8). Lastly, we conclude that differences in the hydrogen-bonding networks involving the retinal Schiff base have a negligible effect on the absorption spectrum.

References

1. Nagel, G.; Ollig, D.; Fuhrmann, M.; Kateriya, S.; Musti, A. M.; Bamberg, E.; Hegemann, P., Channelrhodopsin-1: a light-gated proton channel in green algae. Science 2002, 296 (5577), 2395-8.

2. Berthold, P.; Tsunoda, S. P.; Ernst, O. P.; Mages, W.; Gradmann, D.; Hegemann, P., Channelrhodopsin-1 initiates phototaxis and photophobic responses in chlamydomonas by immediate light-induced depolarization. The Plant cell 2008, 20 (6), 1665-77.

3. Yizhar, O.; Fenno, L. E.; Davidson, T. J.; Mogri, M.; Deisseroth, K., Optogenetics in neural systems. Neuron 2011, 71 (1), 9-34.

4. Kato, H. E.; Zhang, F.; Yizhar, O.; Ramakrishnan, C.; Nishizawa, T.; Hirata, K.; Ito, J.; Aita, Y.; Tsukazaki, T.; Hayashi, S.; Hegemann, P.; Maturana, A. D.; Ishitani, R.; Deisseroth, K.; Nureki, O., Crystal structure of the channelrhodopsin light-gated cation channel. Nature 2012, 482 (7385), 369-74.