Channelrhodopsin-2 (ChR2) is a microbial type rhodopsin and a light-gated cation channel that controls phototaxis in Chlamydomonas. We expressed ChR2 in COS-cells, purified it, and subsequently investigated this unusual photoreceptor by flash photolysis and UV-visible and Fourier transform infrared difference spectroscopy. Several transient photoproducts of the wild type ChR2 were identified, and their kinetics and molecular properties were compared with those of the ChR2 mutant E90Q. Based on the spectroscopic data we developed a model of the photocycle comprising six distinguishable intermediates. This photocycle shows similarities to the photocycle of the ChR2-related Channelrhodopsin of Volvox but also displays significant differences. We show that molecular changes include retinal isomerization, changes in hydrogen bonding of carboxylic acids, and large alterations of the protein backbone structure. These alterations are stronger than those observed in the photocycle of other microbial rhodopsins like bacteriorhodopsin and are related to those occurring in animal rhodopsins. UV-visible and Fourier transform infrared difference spectroscopy revealed two late intermediates with different time constants of ‫؍‬ 6 and 40 s that exist during the recovery of the dark state. The carboxylic side chain of Glu 90 is involved in the slow transition. The molecular changes during the ChR2 photocycle are discussed with respect to other members of the rhodopsin family. Channelrhodopsins (ChRs)3 are light-gated cation channels (1, 2) that share homology with other microbial rhodopsins such as bacteriorhodopsin (BR), halorhodopsin (HR), and sensory rhodopsin (SR). In nature they serve as sensory photoreceptors for phobic responses and phototaxis in green algae (3-5). The light-induced ion conductance leads to depolarization of the cell membrane within milliseconds. Because of this property, in recent years ChRs have been widely used in the neuroscience field as a tool for depolarization of selected cell types or cell ensembles (6, 7). Furthermore, channelrhodopsins were used to control neuronal activity in Caenorhabditis elegans, Drosophila, zebrafish, chicken embryos, and mice (8 -13).As is typical for rhodopsins, light absorption induces isomerization of the ChR-chromophore with subsequent conformational changes in the protein (photocycle). Based on UV-visible spectroscopic and electrophysiological measurements, several schemes for this photocycle have been presented. A recent model for recombinant Volvox channelrhodopsin (VChR), purified from green monkey COS cells, comprises the two dark states D470 and D480, characterized by a fine structured UVvisible absorption spectrum with maxima at 470 and 480 nm, respectively (14). These two states, which exist in a pH-dependent equilibrium, are both converted by light via retinal isomerization and transient Schiff base deprotonation to the conducting state P510 or, under acidic conditions, to P530. These intermediates thermally relax back to the dark state equilibrium in a biphas...

Phytochromes are bimodal photoswitches composed of a photosensor and an output module. Photoactivation of the sensor is initiated by a double bond isomerization of the tetrapyrrole chromophore and eventually leads to protein conformational changes. Recently determined structural models of phytochromes identify differences between the inactive and the signalling state but do not reveal the mechanism of photosensor activation or deactivation. Here, we report a vibrational spectroscopic study on bathy phytochromes that demonstrates that the formation of the photoactivated state and thus (de)activation of the output module is based on proton translocations in the chromophore pocket coupling chromophore and protein structural changes. These proton transfer steps, involving the tetrapyrrole and a nearby histidine, also enable thermal back-isomerization of the chromophore via keto-enol tautomerization to afford the initial dark state. Thus, the same proton re-arrangements inducing the (de)activation of the output module simultaneously initiate the reversal of this process, corresponding to a negative feedback mechanism.

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.

Aza-derivatives of phenols such as Schiff bases and hydrazones are an important and interesting group of chemical compounds. They exhibit a number of biological activities and play an important role in the regulation of many biochemical processes. Due to these properties, these compounds are potentialy useful for the design and production of novel anticancer, antimalaria, antivirus and antimicrobial drugs.The primary aim of the present review is to compare selected groups of phenol-derivatives containing azo-, heterocyclic, aromatic, alkyl and sugar moieties in view of their potential use and undesired effects, since some of them are widely used as dyes in the industry.Despite the vast number of azo derivatives of phenols which were synthesized and tested with respect to their biological activities, there is a continuous search for compounds of this group which might exhibit interesting, new biological effects.The main contribution to the development of Schiff base chemistry undoubtedly came from pharmaceutical research that began with the early studies of Hugo Schiff in the 19th century. Indeed, a vast number of publications on this matter underline the relevance of Schiff bases and related compounds in chemistry, pharmacy, biochemistry and industry. GOSSYPOLGossypol (Scheme 1) is a bisesquiterpene with interesting physico-chemical properties and pronounced biological activities. Despite many useful biological and physiological characteristics the main obstacle in using this compound as a drug is its relatively high toxicity and some other side effects [1][2][3][4][5]. The toxicity of gossypol is directed mainly towards the reproductive system, liver, heart and lipid membranes [6,7]. In the last thirty years biological and medical applications of gossypol and its derivatives mainly of its Schiff bases were on the focus of research. More than two thousand publications about gossypol and its derivates in medical therapy show the importance of this compound today. Scheme 1.