Exploiting protein symmetry to design light-controllable enzyme inhibitors

2,3-Diarylmaleate salts as a versatile class of diarylethenes with a full spectrum of photoactivity in water

There is incessant interest in the transfer of common chemical processes from organic solvents to water, which is vital for the development of bioinspired and green chemical technologies. Diarylethenes feature a rich photochemistry, including both irreversible and reversible reactions that are in demand in organic synthesis, materials chemistry, and photopharmacology. Herein, we introduce the first versatile class of diarylethenes, namely, potassium 2,3-diarylmaleates (DAMs), that show excellent solubility in water. DAMs obtained from highly available precursors feature a full spectrum of photoactivity in water and undergo irreversible reactions (oxidative cyclization or rearrangement) or reversible photocyclization (switching), depending on their structure. This finding paves a way towards wider application of diarylethenes in photopharmacology and bioinspired technologies that require aqueous media for photochemical reactions.

Investigation on Novel E/Z 2-Benzylideneindan-1-One-Based Photoswitches with AChE and MAO-B Dual Inhibitory Activity

The multitarget therapeutic strategy, as opposed to the more traditional 'one disease-one target-one drug', may hold promise in treating multifactorial neurodegenerative syndromes, such as Alzheimer's disease (AD) and related dementias. Recently, combining a photopharmacology approach with the multitarget-directed ligand (MTDL) design strategy, we disclosed a novel donepezil-like compound, namely 2-(4-((diethylamino)methyl)benzylidene)-5-methoxy-2,3-dihydro-1H-inden-1-one (1a), which in the E isomeric form (and about tenfold less in the UV-B photo-induced isomer Z) showed the best activity as dual inhibitor of the AD-related targets acetylcholinesterase (AChE) and monoamine oxidase B (MAO-B). Herein, we investigated further photoisomerizable 2-benzylideneindan-1-one analogs 1b-h with the unconjugated tertiary amino moiety bearing alkyls of different bulkiness and lipophilicity. For each compound, the thermal stable E geometric isomer, along with the E/Z mixture as produced by UV-B light irradiation in the photostationary state (PSS, 75% Z), was investigated for the inhibition of human ChEs and MAOs. The pure E-isomer of the N-benzyl(ethyl)amino analog 1h achieved low nanomolar AChE and high nanomolar MAO-B inhibition potencies (IC50s 39 and 355 nM, respectively), whereas photoisomerization to the Z isomer (75% Z in the PSS mixture) resulted in a decrease (about 30%) of AChE inhibitory potency, and not in the MAO-B one. Molecular docking studies were performed to rationalize the different E/Z selectivity of 1h toward the two target enzymes.

Design, synthesis and biological evaluation of light-driven on-off multitarget AChE and MAO-B inhibitors

Neurodegenerative diseases are multifactorial disorders characterized by protein misfolding, oxidative stress, and neuroinflammation, finally resulting in neuronal loss and cognitive dysfunctions. Nowadays, an attractive strategy to improve the classical treatments is the development of multitarget-directed molecules able to synergistically interact with different enzymes and/or receptors. In addition, an interesting tool to refine personalized therapies may arise from the use of bioactive species able to modify their activity as a result of light irradiation. To this aim, we designed and synthesized a small library of cinnamic acid-inspired isomeric compounds with light modulated activity able to inhibit acetylcholinesterase (AChE) and monoamine oxidase B (MAO-B), with remarkable selectivity over butyrylcholinesterase (BChE) and MAO-A, which have been investigated as the enzyme targets related to Alzheimer's disease (AD). The inhibitory activities were evaluated for the pure E-diastereomers and the E/Z-diastereomer mixtures, obtained upon UV irradiation. Molecular docking studies were carried out to rationalize the differences in the inhibition potency of the E and Z diastereomers of the best performing analogue 1c. Our preliminary findings may open-up the way for developing innovative multitarget photo-switch drugs against neurodegenerative diseases.

Dithienylethene-Bridged Fluoroquinolone Derivatives for Imaging-Guided Reversible Control of Antibacterial Activity

The emerging field of photopharmacology has offered a promising alternative to guard against the bacterial resistance by effectively avoiding antibiotic accumulation in the body or environment. However, the degradation, toxicity, and thermal reversibility have always been an ongoing concern for potential applications of azobenzene-based photopharmacology. Developing novel photopharmacological agents based on a more matched switch is highly in demand and remains a major challenge. Herein, two novel dithienylethene-bridged dual-fluoroquinolone derivatives have been developed by introducing two fluoroquinolone drugs into both ends of the dithienylethene (DTE) switch, in which the fluoroquinolone acts as a fluorophore except for the pharmacodynamic component. For comparison, two monofluoroquinolone-DTE hybrids were also prepared by a similar strategy. As expected, these resultant DTE-based antibacterial agents displayed efficient photochromism and fluorescence switching behavior in dimethyl sulfoxide. Moreover, improved antibacterial activities compared to those of monofluoroquinolone derivatives and a maximum fourfold active difference against Escherichia coli (E. coli) for open and closed isomers and photoswitchable bacterial imaging for Staphylococcus aureus and E. coli were observed. The molecular docking to DNA gyrase gave a rationale for the discrepancies in antibacterial activity for both isomers. Therefore, these fluoroquinolone derivatives can act as interesting imaging-guided photopharmacological agents for further in vivo studies.

Molecular photoswitches in aqueous environments

Molecular photoswitches enable dynamic control of processes with high spatiotemporal precision, using light as external stimulus, and hence are ideal tools for different research areas spanning from chemical biology to smart materials. Photoswitches are typically organic molecules that feature extended aromatic systems to make them responsive to (visible) light. However, this renders them inherently lipophilic, while water-solubility is of crucial importance to apply photoswitchable organic molecules in biological systems, like in the rapidly emerging field of photopharmacology. Several strategies for solubilizing organic molecules in water are known, but there are not yet clear rules for applying them to photoswitchable molecules. Importantly, rendering photoswitches water-soluble has a serious impact on both their photophysical and biological properties, which must be taken into consideration when designing new systems. Altogether, these aspects pose considerable challenges for successfully applying molecular photoswitches in aqueous systems, and in particular in biologically relevant media. In this review, we focus on fully water-soluble photoswitches, such as those used in biological environments, in both in vitro and in vivo studies. We discuss the design principles and prospects for water-soluble photoswitches to inspire and enable their future applications.

Diarylethene-Based Photoswitchable Inhibitors of Serine Proteases

A bicyclic peptide scaffold was chemically adapted to generate diarylethene-based photoswitchable inhibitors of serine protease Bos taurus trypsin 1 (T1). Starting from a prototype molecule-sunflower trypsin inhibitor-1 (SFTI-1)-we obtained light-controllable inhibitors of T1 with Ki in the low nanomolar range, whose activity could be modulated over 20-fold by irradiation. The inhibitory potency as well as resistance to proteolytic degradation were systematically studied on a series of 17 SFTI-1 analogues. The hydrogen bond network that stabilizes the structure of inhibitors and possibly the enzyme-inhibitor binding dynamics were affected by isomerization of the photoswitch. The feasibility of manipulating enzyme activity in time and space was demonstrated by controlled digestion of gelatin-based hydrogel and an antimicrobial peptide BP100-RW. Finally, our design principles of diarylethene photoswitches are shown to apply also for the development of other serine protease inhibitors.

Diarylethene moiety as an enthalpy-entropy switch: photoisomerizable stapled peptides for modulating p53/MDM2 interaction

Analogs of the known inhibitor (peptide pDI) of the p53/MDM2 protein-protein interaction are reported, which are stapled by linkers bearing a photoisomerizable diarylethene moiety. The corresponding photoisomers possess significantly different affinities to the p53-interacting domain of the human MDM2. Apparent dissociation constants are in the picomolar-to-low nanomolar range for those isomers with diarylethene in the "open" configuration, but up to eight times larger for the corresponding "closed" isomers. Spectroscopic, structural, and computational studies showed that the stapling linkers of the peptides contribute to their binding. Calorimetry revealed that the binding of the "closed" isomers is mostly enthalpy-driven, whereas the "open" photoforms bind to the protein stronger due to their increased binding entropy. The results suggest that conformational dynamics of the protein-peptide complexes may explain the differences in the thermodynamic profiles of the binding.

Charge-Sensitive Optical Detection of Small Molecule Binding Kinetics in Normal Ionic Strength Buffer

Most label-free detection technologies detect the masses of molecules, and their sensitivities thus decrease with molecular weight, making it challenging to detect small molecules. To address this need, we have developed a charge-sensitive optical detection (CSOD) technique, which detects the charge rather than the mass of a molecule with an optical fiber. However, the effective charge of a molecule decreases with the buffer ionic strength. For this reason, the previous CSOD works with diluted buffers, which could affect the measured molecular binding kinetics. Here, we show a technique capable of detecting molecular binding kinetics in normal ionic strength buffers. An H-shaped sample well was developed to increase the current density at the sensing area to compensate the signal loss due to ionic screening at normal ionic strength buffer, while keeping the current density low at the electrodes to minimize the electrode reaction. In addition, agarose gels were used to cover the electrodes to prevent electrode reaction generated bubbles from entering the sensing area. With this new design, we have measured the binding kinetics between G-protein-coupled receptors (GPCRs) and their small molecule ligands in normal buffer. We found that the affinities measured in normal buffer are stronger than those measured in diluted buffer, likely due to the stronger electrostatic repulsion force between the same charged ligands and receptors in the diluted buffer.

Microwaves as "Co-Catalysts" or as Substitute for Catalysts in Organophosphorus Chemistry

The purpose of this review is to summarize the importance of microwave (MW) irradiation as a kind of catalyst in organophosphorus chemistry. Slow or reluctant reactions, such as the Diels-Alder cycloaddition or an inverse-Wittig type reaction, may be performed efficiently under MW irradiation. The direct esterification of phosphinic and phosphonic acids, which is practically impossible on conventional heating, may be realized under MW conditions. Ionic liquid additives may promote further esterifications. The opposite reaction, the hydrolysis of P-esters, has also relevance among the MW-assisted transformations. A typical case is when the catalysts are substituted by MWs, which is exemplified by the reduction of phosphine oxides, and by the Kabachnik-Fields condensation affording α-aminophosphonic derivatives. Finally, the Hirao P-C coupling reaction may serve as an example, when the catalyst may be simplified under MW conditions. All of the examples discussed fulfill the expectations of green chemistry.

Conductance switching at the nanoscale of diarylethene derivative self-assembled monolayers on La0.7Sr0.3MnO3

We report on the phosphonic acid route for the grafting of functional molecules, optical switch (dithienylethene diphosphonic acid, DDA), on La_0.7Sr_0.3MnO₃ (LSMO). Compact self-assembled monolayers (SAMs) of DDA are formed on LSMO as studied by topographic atomic force microscopy (AFM), ellipsometry, water contact angle measurements and X-ray photoemission spectroscopy (XPS). The conducting AFM measurements show that the electrical conductance of LSMO/DDA is about 3 decades below that of a bare LSMO substrate. Moreover, the presence of the DDA SAM suppresses the known conductance switching of the LSMO substrate that is induced by mechanical and/or bias constraints during C-AFM measurements. A partial light-induced conductance switching between the open and closed forms of the DDA is observed for the LSMO/DDA/C-AFM tip molecular junctions (closed/open conductance ratio of about 8). We show that, in the case of long-time exposure to UV light, this feature can be masked by a non-reversible decrease (a factor of about 15) of the conductance of the LSMO electrode.

Quantifying Ligand-Protein Binding Kinetics with Self-Assembled Nano-oscillators

Measuring ligand-protein interactions is critical for unveiling molecular-scale biological processes in living systems and for screening drugs. Various detection technologies have been developed, but quantifying the binding kinetics of small molecules to the proteins remains challenging because the sensitivities of the mainstream technologies decrease with the size of the ligand. Here, we report a method to measure and quantify the binding kinetics of both large and small molecules with self-assembled nano-oscillators, each consisting of a nanoparticle tethered to a surface via long polymer molecules. By applying an oscillating electric field normal to the surface, the nanoparticle oscillates, and the oscillation amplitude is proportional to the number of charges on the nano-oscillator. Upon the binding of ligands onto the nano-oscillator, the oscillation amplitude will change. Using a plasmonic imaging approach, the oscillation amplitude is measured with subnanometer precision, allowing us to accurately quantify the binding kinetics of ligands, including small molecules, to their protein receptors. This work demonstrates the capability of nano-oscillators as an useful tool for measuring the binding kinetics of both large and small molecules.

A Novel Optical Method To Reversibly Control Enzymatic Activity Based On Photoacids

Most biochemical reactions depend on the pH value of the aqueous environment and some are strongly favoured to occur in an acidic environment. A non-invasive control of pH to tightly regulate such reactions with defined start and end points is a highly desirable feature in certain applications, but has proven difficult to achieve so far. We report a novel optical approach to reversibly control a typical biochemical reaction by changing the pH and using acid phosphatase as a model enzyme. The reversible photoacid G-acid functions as a proton donor, changing the pH rapidly and reversibly by using high power UV LEDs as an illumination source in our experimental setup. The reaction can be tightly controlled by simply switching the light on and off and should be applicable to a wide range of other enzymatic reactions, thus enabling miniaturization and parallelization through non-invasive optical means.

A building-block design for enhanced visible-light switching of diarylethenes

Current development of light-responsive materials and technologies imposes an urgent demand on visible-light photoswitching on account of its mild excitation with high penetration ability and low photo-toxicity. However, complicated molecular design and laborious synthesis are often required for visible-light photoswitch, especially for diarylethenes. Worse still, a dilemma is encountered as the visible-light excitation of the diarylethene is often achieved at the expense of photochromic performances. To tackle these setbacks, we introduce a building-block design strategy to achieve all-visible-light photochromism with the triplet-sensitization mechanism. The simply designed diarylethene system is constructed by employing a sensitizer building-block with narrow singlet-triplet energy gap (ΔE_ST) to a diarylethene building-block. A significant improvement on the photochromic efficiency is obtained as well as an enhanced photo-fatigue resistance over those under UV irradiation. The balance between the visible-light excitation and decent photochromism is thus realized, promoting a guiding principle for the visible-light photochromism.

Sign Inversion in Photopharmacology: Incorporation of Cyclic Azobenzenes in Photoswitchable Potassium Channel Blockers and Openers

Photopharmacology relies on ligands that change their pharmacodynamics upon photoisomerization. Many of these ligands are azobenzenes that are thermodynamically more stable in their elongated trans-configuration. Often, they are biologically active in this form and lose activity upon irradiation and photoisomerization to their cis-isomer. Recently, cyclic azobenzenes, so-called diazocines, have emerged, which are thermodynamically more stable in their bent cis-form. Incorporation of these switches into a variety of photopharmaceuticals could convert dark-active ligands into dark-inactive ligands, which is preferred in most biological applications. This "pharmacological sign-inversion" is demonstrated for a photochromic blocker of voltage-gated potassium channels, termed CAL, and a photochromic opener of G protein-coupled inwardly rectifying potassium (GIRK) channels, termed CLOGO.

Ultrafast ring closing of a diarylethene-based photoswitchable nucleoside

Deoxyuridine nucleosides embodied into diarylethenes form an especial class of photoswitchable compounds that are designed to stack and pair with DNA bases. The molecular geometry can be switched between "open" and "closed" isomers by a pericyclic reaction that affects the stability of the surrounding double helix. This potentially enables light-induced control of DNA hybridization at microscopic resolution. Despite its importance for the optimization of DNA photoswitches, the ultrafast photoisomerization mechanism of these diarylethenes is still not well understood. In this work, femtosecond transient absorption spectroscopy is applied to study the ring closing reaction upon UV excitation with 45 fs pulses. Excited-state absorption decays rapidly and gives rise to the UV-Vis difference spectrum of the "closed" form within ≈15 ps. Time constants of 0.09, 0.49 and 6.6 ps characterize the multimodal dynamics, where a swift recurrence in the signal anisotropy indicates transient population of the intermediate 21A-like state.

Stimulus-Responsive Regulation of Enzyme Activity for One-Step and Multi-Step Syntheses

Multi-step biocatalytic reactions have gained increasing importance in recent years because the combination of different enzymes enables the synthesis of a broad variety of industrially relevant products. However, the more enzymes combined, the more crucial it is to avoid cross-reactivity in these cascade reactions and thus achieve high product yields and high purities. The selective control of enzyme activity, i.e., remote on-/off-switching of enzymes, might be a suitable tool to avoid the formation of unwanted by-products in multi-enzyme reactions. This review compiles a range of methods that are known to modulate enzyme activity in a stimulus-responsive manner. It focuses predominantly on in vitro systems and is subdivided into reversible and irreversible enzyme activity control. Furthermore, a discussion section provides indications as to which factors should be considered when designing and choosing activity control systems for biocatalysis. Finally, an outlook is given regarding the future prospects of the field.

Synthesis and properties of dithienylethene-functionalized switchable antibacterial agents

Photopharmacology involving azobenzene has offered a viable alternative for combating bacterial resistance. However, the degradation and potential toxicity of azobenzene limit its further study in vivo. Therefore, searching for an appropriate photoswitch for further clinical application is highly desirable. Herein a series of dithienylethene-functionalized switchable antibacterial agents have been designed and prepared by the introduction of the dithienylethene scaffold into fluoroquinolones. And it was found that these switchable antibacterial agents displayed good photochromism and fluorescence switching behaviors upon irradiation with UV/Vis light in DMSO. Surprisingly, methoxy-substituted dithienylethenes 3a and 3b exhibited fluorescence turn-on behavior. Furthermore, it was found that all of the open-isomers showed partial antibacterial activity on E. coli and S. aureus compared with the native drugs. Apart from 2a and 2b, the other switchable antibacterial agents showed a large difference in antibacterial activity on Gram-negative E. coli between the open and closed forms, in which the antimicrobial activity of the ring-closed isomers for 1b and 3b was 16 times that of the corresponding ring-open isomers. DFT calculations showed that the ring-closed isomers of 1b and 3b presented a rigid "S-type" conformation, which may be conducive to forming more stable complexes with the DNA gyrase of E. coli.

New Developments on the Hirao Reactions, Especially from "Green" Point of View

BACKGROUND

The Hirao reaction discovered ca. 35 years ago is an important P-C coupling protocol between dialkyl phosphites and aryl halides in the presence of Pd(PPh3)4 as the catalyst and a base to provide aryl phosphonates. Then, the reaction was extended to other Preagents, such as secondary phosphine oxides and H-phosphinates and to other aryl and hetaryl derivatives to afford also phosphinic esters and tertiary phosphine oxides. Instead of the Pd(PPh3)4 catalyst, Pd(OAc)2 and Ni-salts were also applied as catalyst precursors together with a number of mono- and bidentate P-ligands.

OBJECTIVE

In our review, we undertook to summarize the target reaction with a special stress on the developments attained in the last 6 years, hence this paper is an update of our earlier reviews in a similar topic.

CONCLUSIONS

"Greener" syntheses aimed at utilizing phase transfer catalytic and microwave-assisted approaches, even under "P-ligand-free. or even solvent-free conditions are the up-to date versions of the classical Hirao reaction. The mechanism of the reaction is also in the focus these days.

Molecular Motors in Aqueous Environment

Molecular motors are Nature's solution for (supra)molecular transport and muscle functioning and are involved in most forms of directional motion at the cellular level. Their synthetic counterparts have also found a myriad of applications, ranging from molecular machines and smart materials to catalysis and anion transport. Although light-driven rotary molecular motors are likely to be suitable for use in an artificial cell, as well as in bionanotechnology, thus far they are not readily applied under physiological conditions. This results mainly from their inherently aromatic core structure, which makes them insoluble in aqueous solution. Here, the study of the dynamic behavior of these motors in biologically relevant media is described. Two molecular motors were equipped with solubilizing substituents and studied in aqueous solutions. Additionally, the behavior of a previously reported molecular motor was studied in micelles, as a model system for the biologically relevant confined environment. Design principles were established for molecular motors in these media, and insights are given into pH-dependent behavior. The work presented herein may provide a basis for the application of the remarkable properties of molecular motors in more advanced biohybrid systems.

Efficiently Photocontrollable or Not? Biological Activity of Photoisomerizable Diarylethenes

Diarylethene derivatives, the biological activity of which can be reversibly changed by irradiation with light of different wavelengths, have shown promise as scientific tools and as candidates for photocontrollable drugs. However, examples demonstrating efficient photocontrol of their biological activity are still relatively rare. This concept article discusses the possible reasons for this situation and presents a critical analysis of existing data and hypotheses in this field, in order to extract the design principles enabling the construction of efficient photocontrollable diarylethene-based molecules. Papers addressing biologically relevant interactions between diarylethenes and biomolecules are analyzed; however, in most published cases, the efficiency of photocontrol in living systems remains to be demonstrated. We hope that this article will encourage further discussion of design principles, primarily among pharmacologists, synthetic and medicinal chemists.

Artificial Light Regulation of an Allosteric Bienzyme Complex by a Photosensitive Ligand

The artificial regulation of proteins by light is an emerging subdiscipline of synthetic biology. Here, we used this concept to photocontrol both catalysis and allostery within the heterodimeric enzyme complex imidazole glycerol phosphate synthase (ImGP-S). ImGP-S consists of the cyclase subunit HisF and the glutaminase subunit HisH, which is allosterically stimulated by substrate binding to HisF. We show that a light-sensitive diarylethene (1,2-dithienylethene, DTE)-based competitive inhibitor in its ring-open state binds with low micromolar affinity to the cyclase subunit and displaces its substrate from the active site. As a consequence, catalysis by HisF and allosteric stimulation of HisH are impaired. Following UV-light irradiation, the DTE ligand adopts its ring-closed state and loses affinity for HisF, restoring activity and allostery. Our approach allows for the switching of ImGP-S activity and allostery during catalysis and appears to be generally applicable for the light regulation of other multienzyme complexes.

Photochemical Control of Protein Arginine Deiminase (PAD) Activity

Protein arginine deiminases (PADs) play an important role in the pathogenesis of various diseases, including rheumatoid arthritis, multiple sclerosis, lupus, ulcerative colitis, and breast cancer. Therefore, the development of PAD inhibitors has drawn significant research interest in recent years. Herein, we describe the development of the first photoswitchable PAD inhibitors. These compounds possess an azobenzene photoswitch to optically control PAD activity. Screening of a series of inhibitors structurally similar to BB-Cl-amidine afforded compounds 1 and 2 as the most promising candidates for the light-controlled inhibition of PAD2; the cis isomer of 1 is 10-fold more potent than its trans isomer, whereas the trans isomer of 2 is 45-fold more potent than the corresponding cis isomer. The altered inhibitory potency upon photoisomerization has been confirmed in a competitive activity-based protein profiling (ABPP) assay. Further investigations indicate that the trans isomer of 2 is an irreversible inhibitor, whereas the cis isomer acts as a competitive inhibitor. In cells, the trans isomer of compound 1 is completely inactive, whereas the cis isomer inhibits histone H3-citrullination in a dose-dependent manner. Taken together, 1 serves as the foundation for developing photopharmaceuticals that can be activated at the desired tissue, using light, to treat diseases where PAD activity is dysregulated.

Modular design of optically controlled protein affinity reagents

Reversible, optical control of a generalizable protein affinity reagent.

Photochromic coenzyme Q derivatives: switching redox potentials with light

Coenzyme Q is an important redox cofactor involved in a variety of cellular processes, and is thus found in several cell compartments. We report a photochromic derivative of coenzyme Q that combines the molecular structures of the redox active cofactor and a photochromic dye. Light irradiation triggers an electronic rearrangement reversibly changing the redox potential. We used this effect to control the intermolecular redox reaction of the photochromic coenzyme Q derivative with dihydropyridine in solution by light irradiation. On mitochondria, the altered redox properties showed an effect on the respiratory chain. The experiments demonstrate that the redox reactions can be initiated inside the system of interest through irradiation with light and the accompanied photoisomerization.

Photochromic Dopamine Receptor Ligands Based on Dithienylethenes and Fulgides

We describe the incorporation of the well-investigated class of photochromic dithienylethenes (DTEs) and fulgides into known dopamine receptor ligands such as 1,4-disubstituted aromatic and hydroxybenzoxazinone piperazines as well as aminoindanes. Subtype and functional selective photochromic ligands were obtained and characterized by NMR and UV/VIS spectroscopic measurements. The photophysical properties of the DTE based dopamine ligands revealed a high fatigue resistance for the diarylmaleimides, but the ringclosure could not be accomplished in polar solvents due to a known twisted intramolecular charge transfer (TICT). Several cyclopentene-DTEs showed high PSS, but a fast degradation by forming an irreversible byproduct. Focusing on the fulgides, high photostationary states and switching in polar solvents were possible. The compounds 43, 45 and 46 containing the isopropyl group showed only isomerization between the open E-form and the closed C-form. At a concentration of 1 nm, the cyclopentene-DTE 29-open showed a more than 11-fold higher activation of D_2S , a pharmacologically important G protein-coupled receptor, than its photochromic congener 29-closed. Interestingly, the fulgimide-based pair 52-(E)-open/52-closed could be discovered as an alternative photoswitch with inverse activation properties exhibiting four-fold higher activity in the closed state.

Light-Controlled Reversible Modulation of Frontier Molecular Orbital Energy Levels in Trifluoromethylated Diarylethenes

Among bistable photochromic molecules, diarylethenes (DAEs) possess the distinct feature that upon photoisomerization they undergo a large modulation of their π-electronic system, accompanied by a marked shift of the HOMO/LUMO energies and hence oxidation/reduction potentials. The electronic modulation can be utilized to remote-control charge- as well as energy-transfer processes and it can be transduced to functional entities adjacent to the DAE core, thereby regulating their properties. In order to exploit such photoswitchable systems it is important to precisely adjust the absolute position of their HOMO and LUMO levels and to maximize the extent of the photoinduced shifts of these energy levels. Here, we present a comprehensive study detailing how variation of the substitution pattern of DAE compounds, in particular using strongly electron-accepting and chemically stable trifluoromethyl groups either in the periphery or at the reactive carbon atoms, allows for the precise tuning of frontier molecular orbital levels over a broad energy range and the generation of photoinduced shifts of more than 1 eV. Furthermore, the effect of different DAE architectures on the transduction of these shifts to an adjacent functional group is discussed. Whereas substitution in the periphery of the DAE motif has only minor implications on the photochemistry, trifluoromethylation at the reactive carbon atoms strongly disturbs the isomerization efficiency. However, this can be overcome by using a nonsymmetrical substitution pattern or by combination with donor groups, rendering the resulting photoswitches attractive candidates for the construction of remote-controlled functional systems.

Photochromic histone deacetylase inhibitors based on dithienylethenes and fulgimides

The synthesis, photochromic properties, inhibition of different HDACs and corresponding molecular dockings of photochromic inhibitors are described.

Optical control of AMPA receptors using a photoswitchable quinoxaline-2,3-dione antagonist

We have developed the first photoswitchable AMPA receptor antagonist, termed ShuBQX-3. It permits the precise optical control of AMPA receptors and exhibits a remarkable red-shifting of its photoswitching properties when bound to the receptor.

AMPA receptors respond to the neurotransmitter glutamate and play a critical role in excitatory neurotransmission. They have been implicated in several psychiatric disorders and have rich pharmacology. Antagonists of AMPA receptors have been explored as drugs and one has even reached the clinic. We now introduce a freely diffusible photoswitchable antagonist that is selective for AMPA receptors and endows them with light-sensitivity. Our photoswitch, ShuBQX-3, is active in its dark-adapted trans-isoform but is significantly less active as its cis-isoform. ShuBQX-3 exhibits a remarkable red-shifting of its photoswitching properties through interactions with the AMPA receptor ligand binding site. Since it can be used to control action potential firing with light, it could emerge as a powerful tool for studying synaptic transmission with high spatial and temporal precision.

The role of alkyl substituents in deazaadenine-based diarylethene photoswitches

Diarylethenes are an important class of reversible photoswitches and often claimed to require two alkyl substituents at the carbon atoms between which the bond is formed or broken in the electrocyclic rearrangement. Here we probe this claim by the synthesis and characterization of four pairs of deazaadenine-based diarylethene photoswitches with either one or two methyl groups at these positions. Depending on the substitution pattern, diarylethenes with one alkyl group can exhibit significant photochromism, but they generally show poor stability towards extended UV irradiation, low thermal stability, and decreased fatigue resistance. The results obtained provide an important direction for the design of new efficient DNA photoswitches for the application in bionanotechnology and synthetic biology.

Ciprofloxacin-Photoswitch Conjugates: A Facile Strategy for Photopharmacology

Photopharmacology aims to locally treat diseases and study biological processes with photoresponsive drugs. Herein, easy access to photoswitchable drugs is crucial, which is supported by simple and robust drug modifications. We investigated the possibility of creating drugs that can undergo remote activation and deactivation with light, by conjugating molecular photoswitches to the exterior of an existing drug in a single chemical step. This facile strategy allows the convenient introduction of various photochromic systems into a drug molecule, rendering it photoresponsive. To demonstrate the feasibility of this approach, two photoswitch-modified ciprofloxacin antibiotics were synthesized. Remarkably, for one of them a 50-fold increase in activity compared to the original ciprofloxacin was observed. Their antimicrobial activity could be spatiotemporally controlled with light, which was exemplified by bacterial patterning studies.

Ultrafast Time-Resolved Spectroscopy of Diarylethene-Based Photoswitchable Deoxyuridine Nucleosides

Photoswitches based on the diarylethene architecture have been attracting considerable attention for the investigation and control of a variety of biological processes. The reversible photoisomerization reaction between their open- and closed-ring forms can be selectively addressed by irradiation with light of two markedly different wavelengths. In this work, the dynamics of the photochromic ring-opening reaction of four novel diarylethene-based photoswitchable deoxyuridine nucleosides is investigated by femtosecond transient absorption. Upon photoexcitation with sub-20 fs pulses in the first absorption band (500 nm), all four photoswitches showed a fast ballistic excited-state deactivation within less than 500 fs toward the S1/S0 conical intersection. Transient data was globally analyzed, and a relaxation kinetic model with a branching between open and closed ring forms without any loss channels was derived. Ring-opening quantum yields, Φr-o, were found to strongly depend on the substituents (R), ranging from 0.64 (dU(PSI): R = 2-naphthyl) to 0.30 (dU(PSIV): R = 2-pyridyl).

Optical control of neuronal activity using a light-operated GIRK channel opener (LOGO)

G-protein coupled inwardly rectifying potassium channels (GIRKs) are ubiquitously expressed throughout the human body and are an integral part of inhibitory signal transduction pathways. Upon binding of G_βγ subunits released from G-protein coupled receptors (GPCRs), GIRK channels open and reduce the activity of excitable cells via hyperpolarization. As such, they play a role in cardiac output, the coordination of movement and cognition. Due to their involvement in a multitude of pathways, the precision control of GIRK channels is an important endeavour. Here, we describe the development of the photoswitchable agonist LOGO (the Light Operated GIRK-channel Opener), which activates GIRK channels in the dark and is rapidly deactivated upon exposure to long wavelength UV irradiation. LOGO is the first K⁺ channel opener and selectively targets channels that contain the GIRK1 subunit. It can be used to optically silence action potential firing in dissociated hippocampal neurons and LOGO exhibits activity in vivo, controlling the motility of zebrafish larvae in a light dependent fashion. We envisage that LOGO will be a valuable research tool to dissect the function of GIRK channels from other GPCR dependent signalling pathways.

Light-Controlled Histone Deacetylase (HDAC) Inhibitors: Towards Photopharmacological Chemotherapy

Cancer treatment suffers from limitations that have a major impact on the patient's quality of life and survival. In the case of chemotherapy, the systemic distribution of cytotoxic drugs reduces their efficacy and causes severe side effects due to nonselective toxicity. Photopharmacology allows a novel approach to address these problems because it employs external, local activation of chemotherapeutic agents by using light. The development of photoswitchable histone deacetylase (HDAC) inhibitors as potential antitumor agents is reported herein. Analogues of the clinically used chemotherapeutic agents vorinostat, panobinostat, and belinostat were designed with a photoswitchable azobenzene moiety incorporated into their structure. The most promising compound exhibits high inhibitory potency in the thermodynamically less stable cis form and a significantly lower activity for the trans form, both in terms of HDAC activity and proliferation of HeLa cells. This approach offers a clear prospect towards local photoactivation of HDAC inhibition to avoid severe side effects in chemotherapy.

Reversible photoswitching specifically responds to mercury(II) ions: the gated photochromism of bis(dithiazole)ethene

Photoswitching of bis(dithiazole)ethene can be regulated by Hg(II) ions and EDTA in a "lock-and-unlock" manner. The molecular photoswitch provides an enzyme-like binding pocket that selectively binds specifically to mercury ions, thus modulating the degree of photoswitching in its presence.

The near-symmetry of proteins

The majority of protein oligomers form clusters which are nearly symmetric. Understanding of that imperfection, its origins, and perhaps also its advantages requires the conversion of the currently used vague qualitative descriptive language of the near-symmetry into an accurate quantitative measure that will allow to answer questions such as: "What is the degree of symmetry deviation of the protein?," "how do these deviations compare within a family of proteins?," and so on. We developed quantitative methods to answer this type of questions, which are capable of analyzing the whole protein, its backbone or selected portions of it, down to comparison of symmetry-related specific amino-acids, and which are capable of visualizing the various levels of symmetry deviations in the form of symmetry maps. We have applied these methods on an extensive list of homomers and heteromers and found that apparently all proteins never reach perfect symmetry. Strikingly, even homomeric protein clusters are never ideally symmetric. We also found that the main burden of symmetry distortion is on the amino-acids near the symmetry axis; that it is mainly the more hydrophilic amino-acids that take place in symmetry-distortive interactions; and more. The remarkable ability of heteromers to preserve near-symmetry, despite the different sequences, was also shown and analyzed. The comprehensive literature on the suggested advantages symmetric oligomerizations raises a yet-unsolved key question: If symmetry is so advantageous, why do proteins stop shy of perfect symmetry? Some tentative answers to be tested in further studies are suggested in a concluding outlook.

Functionalization of photochromic dithienylmaleimides

Synthetic routes for the functionalization of photochromic dithienylmaleimides at three different positions are reported.

A straightforward approach towards cyclic photoactivatable tubulysin derivatives

The development of a new photolabile protecting group containing an additional allyl functionality allows the synthesis of cyclic photoactivatable natural products. Cyclization occurs between the allyl moiety in the protecting group and a second double bond in the target molecule by means of ring-closing metathesis. Cyclization should increase the metabolic stability towards proteases. On the other hand, the conformational change should cause diminished biological activity. As illustrated for tubulysin derivatives, cyclic and photoactivatable drug candidates can easily be obtained in only two steps from simple building blocks through Ugi reaction and ring-closing metathesis. The photolabile protecting group is introduced by means of the isocyanide component during the Ugi reaction.

A photoresponsive glycosidase mimic

Azobenzene-3,3'-dicarboxylic acid exists in photoisomerizable (E) and (Z)-forms. Deprotonation of the carboxylic acid groups from the (E)-form occurs simultaneously, whereas in the (Z)-form it occurs in a stepwise fashion. The mono anionic form of the (Z)-isomer acts as a glycosidase mimic that proceeds through a general acid-general base catalytic mechanism. This is the first example of a photoresponsive glycosidase mimic.