Channelrhodopsins (ChRs) are light-gated ion channels that are widely used in optogenetics. They allow precise control of neuronal activity with light, but a detailed understanding of how the channel is gated and the ions are conducted is still lacking. The recent determination of the X-ray structural model in the closed state marks an important milestone. Herein the open state structure is presented and the early formation of the ion conducting pore is elucidated in atomic detail using time-resolved FTIR spectroscopy. Photo-isomerization of the retinal-chromophore causes a downward movement of the highly conserved E90, which opens the pore. Molecular dynamic (MD) simulations show that water molecules invade through this opened pore, Helix 2 tilts and the channel fully opens within ms. Since E90 is a highly conserved residue, the proposed E90-Helix2-tilt (EHT) model might describe a general activation mechanism and provides a new avenue for further mechanistic studies and engineering.

Background: Channelrhodopsin-2 is a light-gated ion channel extensively used in optogenetics.Results: Glu-90 is deprotonated in the open state and is crucial for ion selectivity.Conclusion: Protonation change of Glu-90 is part of the opening/closing of the conductive pore, and the functional protein unit is assumed to be the monomer.Significance: Understanding the gating mechanism is necessary for optimizing this optogenetic tool.

Although channelrhodopsin (ChR) is a widely applied light-activated ion channel, important properties such as light adaptation, photocurrent inactivation, and alteration of the ion selectivity during continuous illumination are not well understood from a molecular perspective. Herein, we address these open questions using single-turnover electrophysiology, time-resolved step-scan FTIR, and Raman spectroscopy of fully dark-adapted ChR2. This yields a unifying parallel photocycle model integrating now all so far controversial discussed data. In dark-adapted ChR2, the protonated retinal Schiff base chromophore (RSBH+) adopts an all-trans,C=N-anti conformation only. Upon light activation, a branching reaction into either a 13-cis,C=N-anti or a 13-cis,C=N-syn retinal conformation occurs. The anti-cycle features sequential H+ and Na+ conductance in a late M-like state and an N-like open-channel state. In contrast, the 13-cis,C=N-syn isomer represents a second closed-channel state identical to the long-lived P480 state, which has been previously assigned to a late intermediate in a single-photocycle model. Light excitation of P480 induces a parallel syn-photocycle with an open-channel state of small conductance and high proton selectivity. E90 becomes deprotonated in P480 and stays deprotonated in the C=N-syn cycle. Deprotonation of E90 and successive pore hydration are crucial for late proton conductance following light adaptation. Parallel anti- and syn-photocycles now explain inactivation and ion selectivity changes of ChR2 during continuous illumination, fostering the future rational design of optogenetic tools.

Integration of experimental and computational approaches to investigate chemical reactions in proteins has proven to be very successful. Experimentally, time-resolved FTIR difference-spectroscopy monitors chemical reactions at atomic detail. To decode detailed structural information encoded in IR spectra, QM/MM calculations are performed. Here, we present a novel method which we call local mode analysis (LMA) for calculating IR spectra and assigning spectral IR-bands on the basis of movements of nuclei and partial charges from just a single QM/MM trajectory. Through LMA the decoding of IR spectra no longer requires several simulations or optimizations. The novel approach correlates the motions of atoms of a single simulation with the corresponding IR bands and provides direct access to the structural information encoded in IR spectra. Either the contributions of a particular atom or atom group to the complete IR spectrum of the molecule are visualized, or an IR-band is selected to visualize the corresponding structural motions. Thus, LMA decodes the detailed information contained in IR spectra and provides an intuitive approach for structural biologists and biochemists. The unique feature of LMA is the bidirectional analysis connecting structural details to spectral features and vice versa spectral features to molecular motions.

Channelrhodopsins (ChRs) are retinal binding membrane proteins found in single-cell algae. Photoisomerization of ChRs leads to formation of an ion channel. The resulting change in membrane voltage modulates flagellate motions allowing phototaxis and photophobic responses. Heterologously expressed in host cells, ChRs allow the evocation or suppression of changes in membrane potential with high spatio-temporal resolution -this method has become known as optogenetics. Functional studies have raised questions concerning the molecular determinants for absorption, formation and closing of the ion channel and ion selectivity. It was the scope of this thesis to address these questions based on a comparison of the three different ChR variants C1C2, ReaChR (Red-activatable ChR) and Chrimson. C1C2 (λmax ≈ 470 nm) is a chimera of the Chlamydomonas reinhardtii ChRs CrChR1 and CrChR2. ReaChR (λmax ≈ 520 nm) is a variant of Volvox carteri ChR1 whose red-shifted absorption allows its use in deeper layers of organic tissue in optogenetic experiments. The even further red-shifted (Cs)Chrimson (λmax ≈ 590 nm) is a more distantly related ChR from Chlamydomonas noctigama with the N-terminal sequence from Chloromonas subdivisa ChR that is significantly more proton-selective. The photoreaction mechanism was investigated using FTIR (Fourier Transform Infrared) spectroscopy at room temperature and at cryostatic conditions. The results were complemented by UV-Vis spectroscopy and retinal extraction and subsequent HPLC (High Performance Liquid Chromatography) analysis.As in most microbial rhodopsins, the retinal cofactor in ChRs is predominantly in 13-trans,15anti conformation and bound to the protein by a retinal Schiff base (RSB) linkage to a lysine.Usually, the RSB is protonated in the dark (RSBH + ) stabilized by the counter-ion complex formed by a glutamate (counter-ion 1, Ci1) and an aspartate (counter-ion 2, Ci2).Photoreceptors are optimized to use photon energy to drive conformational changes of the protein backbone. Therefore, a fraction of the photon energy is stored by a transient distortion of the chromophore and separation of the charges in the active site by increased distance between the RSBH + and its counter-ions. In this thesis, it is shown that in ReaChR the transfer of the stored energy to the protein is largely affected by the Ci1 (Glu163) protonation state, being decelerated by protonated Ci1 due to an enhanced rigidity of the active site that stabilizes the distorted chromophore conformation. Instead, in Chrimson the chromophore

The light-gated ion channel … …c hannelrhodopsin-2 (ChR 2 )i st he key protein in optogenetics.I nt heir Communication on page 4953 ff., K. Gerwert et al. elucidate channel opening at amolecular level by ac ombination of time-resolved FTIR spectroscopy and molecular dynamics simulations.

Background The World Health Organization (WHO) is currently assessing the potential health effects of exposure to radiofrequency electromagnetic fields (RF-EMFs) in the general and working population. Related to one such health effect, there is a concern that RF-EMFs may affect cognitive performance in humans. The systematic review (SR) aims to identify, summarize and synthesize the evidence base related to this question. Here, we present the protocol for the planned SR. Objectives The main objective is to present a protocol for a SR which will evaluate the associations between short-term exposure to RF-EMFs and cognitive performance in human experimental studies. Data sources We will search the following databases: PubMed, Embase, Web of Science, Scopus, and the EMF-Portal. The reference lists of included studies and retrieved review articles will be manually searched. Study eligibility and criteria We will include randomized human experimental studies that assess the effects of RF-EMFs on cognitive performance compared to no exposure or lower exposure. We will include peer-reviewed articles of any publication date in any language that report primary data. Data extraction and analysis Data will be extracted according to a pre-defined set of forms developed and piloted by the review author team. To assess the risk of bias, we will apply the Rating Tool for Human and Animal Studies developed by NTP/OHAT, supplemented with additional questions relevant for cross-over studies. Where sufficiently similar studies are identified (e.g. the heterogeneity concerning population, exposure and outcome is low and the studies can be combined), we will conduct random-effects meta-analysis; otherwise, we will conduct a narrative synthesis. Assessment of certainty of evidence The certainty of evidence for each identified outcome will be assessed according to Grading of Recommendations Assessment, Development, and Evaluation (GRADE). Performing the review according to this protocol will allow the identification of possible effects of RF-EMFs on cognitive performance in humans. The protocol has been registered in PROSPERO, an open-source protocol registration system, to foster transparency.

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