Reversible Photocontrol of Biological Systems by the Incorporation of Molecular Photoswitches

Rational design, production and in vitro analysis of photoxenoproteins

In synthetic biology, the artificial control of proteins by light is of growing interest since it enables the spatio-temporal regulation of downstream molecular processes. This precise photocontrol can be established by the site-directed incorporation of photo-sensitive non-canonical amino acids (ncAAs) into proteins, which generates so-called photoxenoproteins. Photoxenoproteins can be engineered using ncAAs that facilitate the irreversible activation or reversible regulation of their activity upon irradiation. In this chapter, we provide a general outline of the engineering process based on the current methodological state-of-the-art to obtain artificial photocontrol in proteins using the ncAAs o-nitrobenzyl-O-tyrosine as example for photocaged ncAAs (irreversible), and phenylalanine-4'-azobenzene as example for photoswitchable ncAAs (reversible). We thereby focus on the initial design as well as the production and characterization of photoxenoproteins in vitro. Finally, we outline the analysis of photocontrol under steady-state and non-steady-state conditions using the allosteric enzyme complexes imidazole glycerol phosphate synthase and tryptophan synthase as examples.

Research progress of stimulus-responsive antibacterial materials for bone infection

Infection is one of the most serious complications harmful to human health, which brings a huge burden to human health. Bone infection is one of the most common and serious complications of fracture and orthopaedic surgery. Antibacterial treatment is the premise of bone defect healing. Among all the antibacterial strategies, irritant antibacterial materials have unique advantages and the ability of targeted therapy. In this review, we focus on the research progress of irritating materials, the development of antibacterial materials and their advantages and disadvantages potential applications in bone infection.

Current perspective on retinal remodeling: Implications for therapeutics

The retinal degenerative diseases retinitis pigmentosa and age-related macular degeneration are a leading cause of irreversible vision loss. Both present with progressive photoreceptor degeneration that is further complicated by processes of retinal remodeling. In this perspective, we discuss the current state of the field of retinal remodeling and its implications for vision-restoring therapeutics currently in development. Here, we discuss the challenges and pitfalls retinal remodeling poses for each therapeutic strategy under the premise that understanding the features of retinal remodeling in totality will provide a basic framework with which therapeutics can interface. Additionally, we discuss the potential for approaching therapeutics using a combined strategy of using diffusible molecules in tandem with other vision-restoring therapeutics. We end by discussing the potential of the retina and retinal remodeling as a model system for more broadly understanding the progression of neurodegeneration across the central nervous system.

Biomimetic Photo-Switches Softening Model Lipid Membranes

We report the synthesis and characterization of a novel photo-switch based on biomimetic cyclocurcumin analogous and interacting with the lipid bilayer, which can be used in the framework of oxygen-independent light-induced therapy. More specifically, by using molecular dynamics simulations and free energy techniques, we show that the inclusion of hydrophobic substituents is needed to allow insertion in the lipid membrane. After having confirmed experimentally that the substituents do not preclude the efficient photoisomerization, we show through UV-vis and dynamic light scattering measurements together with compression isotherms that the chromophore is internalized in both lipid vesicles and monomolecular film, respectively, inducing their fluidification. The irradiation of the chromophore-loaded lipid aggregates modifies their properties due to the different organization of the two diastereoisomers, E and Z. In particular, a competition between a fast structural reorganization and a slower expulsion of the chromophore after isomerization can be observed in the kinetic profiles recorded during E to Z photoisomerization. This report paves the way for future investigations in the optimization of biomimetic photoswitches potentially useful in modern light-induced therapeutic strategies.

Spatiotemporal Control of Biology: Synthetic Photochemistry Toolbox with Far-Red and Near-Infrared Light

The complex network of naturally occurring biological pathways motivates the development of new synthetic molecules to perturb and/or detect these processes for fundamental research and clinical applications. In this context, photochemical tools have emerged as an approach to control the activity of drug or probe molecules at high temporal and spatial resolutions. Traditional photochemical tools, particularly photolabile protecting groups (photocages) and photoswitches, rely on high-energy UV light that is only applicable to cells or transparent model animals. More recently, such designs have evolved into the visible and near-infrared regions with deeper tissue penetration, enabling photocontrol to study biology in tissue and model animal contexts. This Review highlights recent developments in synthetic far-red and near-infrared photocages and photoswitches and their current and potential applications at the interface of chemistry and biology.

Light-Switchable Membrane Permeability in Giant Unilamellar Vesicles

In this work, giant unilamellar vesicles (GUVs) were synthesized by blending the natural phospholipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) with a photoswitchable amphiphile (1) that undergoes photoisomerization upon irradiation with UV-A (E to Z) and blue (Z to E) light. The mixed vesicles showed marked changes in behavior in response to UV light, including changes in morphology and the opening of pores. The fine control of membrane permeability with consequent cargo release could be attained by modulating either the UV irradiation intensity or the membrane composition. As a proof of concept, the photocontrolled release of sucrose from mixed GUVs is demonstrated using microscopy (phase contrast) and confocal studies. The permeability of the GUVs to sucrose could be increased to ~4 × 10-2 μm/s when the system was illuminated by UV light. With respect to previously reported systems (entirely composed of synthetic amphiphiles), our findings demonstrate the potential of photosensitive GUVs that are mainly composed of natural lipids to be used in medical and biomedical applications, such as targeted drug delivery and localized topical treatments.

Photostability and Antiproliferative Activity of Furan Analogues of Combretastatin A-4

Cancer is one of the most serious health problems that usually require heavy medical treatment. It is important to ensure that no additional burden is placed on patients due to the modes of administration and/or poor quality of pharmaceuticals. In this regard, understanding, quantifying, and improving the photostability (resistance to UV light or sunlight) of drugs is among the important elements that can improve the patient's quality of life. In this work, the photochemical properties of a wide range of furanone analogues of combretastatin A-4 and their antiproliferative activity against A-431 epidermoid carcinoma cells were studied in a search for compounds with improved photostability and antiproliferative activity. It was found that the incorporation of an arylidene moiety led to a significant improvement in photostability, while the antiproliferative activity strongly depends on the nature of the aryl residue in the arylidene moiety. The high photostability of arylidenes was achieved due to the delocalization of the central double bond of the 1,3,5-hexatriene system, which limited the 6π-electrocyclization. The best results in terms of antiproliferative activity were obtained for thiophene arylidene (IC₅₀ = 0.6 μM) and 3,4-diarylfuran (IC₅₀ = 0.047 μM). The obtained results address the lack of data available now in scientific literature on the photodegradation of combretastatin A-4 analogues and should be taken into account in studies of the side effects of pharmaceuticals based on them.

Photochromic Fentanyl Derivatives for Controlled μ-Opioid Receptor Activation

Photoswitchable ligands as biological tools provide an opportunity to explore the kinetics and dynamics of the clinically relevant μ-opioid receptor. These ligands can potentially activate or deactivate the receptor when desired by using light. Spatial and temporal control of biological activity allows for application in a diverse range of biological investigations. Photoswitchable ligands have been developed in this work, modelled on the known agonist fentanyl, with the aim of expanding the current "toolbox" of fentanyl photoswitchable ligands. In doing so, ligands have been developed that change geometry (isomerize) upon exposure to light, with varying photophysical and biochemical properties. This variation in properties could be valuable in further studying the functional significance of the μ-opioid receptor.

Design and Synthesis of Visible-Light-Responsive Azobenzene Building Blocks for Chemical Biology

Tetra-ortho-fluoro-azobenzenes are a class of photoswitches useful for the construction of visible-light-controlled molecular systems. They can be used to achieve spatio-temporal control over the properties of a chosen bioactive molecule. However, the introduction of different substituents to the tetra-fluoro-azobenzene core can significantly affect the photochemical properties of the switch and compromise biocompatibility. Herein, we explored the effect of useful substituents, such as functionalization points, attachment handles, and water-solubilizing groups, on the photochemical properties of this photochromic system. In general, all the tested fluorinated azobenzenes exhibited favorable photochemical properties, such as high photostationary state distribution and long half-lives, both in organic solvents and in water. One of the azobenzene building blocks was functionalized with a trehalose group to enable the uptake of the photoswitch into mycobacteria. Following metabolic uptake and incorporation of the trehalose-based azobenzene in the mycobacterial cell wall, we demonstrated photoswitching of the azobenzene in the isolated total lipid extract.

Light-Controlled Modulation and Analysis of Neuronal Functions

Light is an extraordinary tool allowing us to read out and control neuronal functions thanks to its unique properties: it has a great degree of bioorthogonality and is minimally invasive; it can be precisely delivered with high spatial and temporal precision; and it can be used simultaneously or consequently at multiple wavelengths and locations [...].

Phototunable Absorption and Nonlinear Optical Properties of Thermally Stable Dihydroazulene-Vinylheptafulvene Photochrome Pair

The UV-vis absorption characteristics and nonlinear optical properties of a series of substituted dihydroazulene (DHA)/vinylheptafulvene (VHF) photoswitches are investigated by applying quantum calculations. Introduction of substituents at the seven-membered ring resulted in significant changes in their absorption properties depending on the nature and position of the substituent. Electron-donating groups at positions 5, 6, 7, and 8 generally exhibited red shifts with respect to the parent compound. However, the steric effect at positions 8a and 4 is responsible for the loss of planarity and conjugation, which generally leads to blue shifts. In contrast, any electron-withdrawing group, particularly at positions 8a and 4, would cause a blue shift. The presence of bulky groups at position 8a results in a loss of planarity and, as a result, a decrease in electronic conjugation within the molecule, resulting in a blue shift in the maximum absorption. When it comes to halogens, the red shift is directly correlated to the nucleophilicity; the higher the nucleophilicity, the larger the red shift. Regarding hyperpolarizability, the charge separation induces higher hyperpolarizabilities for all substituted VHFs compared to the corresponding DHAs, resulting in a much higher NLO response. In addition, for all DHA and VHF, the highest values of hyperpolarizabilities are calculated for 6-substituted systems. Finally, the objective of this detailed theoretical investigation is to continue exploring the photophysical properties of DHA-VHF through structural modifications.

Design, Synthesis, and Photo-Responsive Properties of a Collagen Model Peptide Bearing an Azobenzene

Collagen is a vital component of the extracellular matrix in animals. Collagen forms a characteristic triple helical structure and plays a key role in supporting connective tissues and cell adhesion. The ability to control the collagen triple helix structure is useful for medical and conformational studies because the physicochemical properties of the collagen rely on its conformation. Although some photo-controllable collagen model peptides (CMPs) have been reported, satisfactory photo-control has not yet been achieved. To achieve this objective, detailed investigation of the isomerization behavior of the azobenzene moiety in CMPs is required. Herein, two CMPs were attached via an azobenzene linker to control collagen triple helix formation by light irradiation. Azo-(PPG)10 with two (Pro-Pro-Gly)10 CMPs linked via a photo-responsive azobenzene moiety was designed and synthesized. Conformational changes were evaluated by circular dichroism and the cis-to-trans isomerization rate calculated from the absorption of the azobenzene moiety indicated that the collagen triple helix structure was partially disrupted by isomerization of the internal azobenzene.

Photochromic spiro-indoline naphthoxazines and naphthopyrans in dye-sensitized solar cells

Photochromic dyes possess unique properties that can be exploited in different domains, including optics, biomedicine and optoelectronics. Herein, we explore the potential of photochromic spiro-indoline naphthoxazine (SINO) and naphthopyran (NIPS) for application in photovoltaics. We designed and synthesized four new photosensitizers with a donor-pi-acceptor structure embedding SINO and NIPS units as photochromic cores. Their optical, photochromic and acidochromic properties were thoroughly studied to establish structure-properties relationships. Then, after unravelling the possible forms adopted depending on the stimuli, their photovoltaic properties were evaluated in DSSCs. Although the photochromic behavior is not always preserved, we elucidate the interplay between photochromic, acidochromic and photovoltaic properties and we demonstrate that these dyes can act as photosensitizers in DSSCs. We report a maximum power conversion efficiency of 2.7% with a NIPS-based dye, a tenfold improvement in comparison to previous works on similar class of compounds. This work opens new perspectives of developments for SINO and NIPS in optical and photovoltaic devices, and it provides novel research directions to design photochromic materials with improved characteristics.

Deactivatable Bisubstrate Inhibitors of Protein Kinases

Bivalent ligands, including bisubstrate inhibitors, are conjugates of pharmacophores, which simultaneously target two binding sites of the biomolecule. Such structures offer attainable means for the development of compounds whose ability to bind to the biological target could be modulated by an external trigger. In the present work, two deactivatable bisubstrate inhibitors of basophilic protein kinases (PKs) were constructed by conjugating the pharmacophores via linkers that could be cleaved in response to external stimuli. The inhibitor ARC-2121 incorporated a photocleavable nitrodibenzofuran-comprising β-amino acid residue in the structure of the linker. The pharmacophores of the other deactivatable inhibitor ARC-2194 were conjugated via reduction-cleavable disulfide bond. The disassembly of the inhibitors was monitored by HPLC-MS. The affinity and inhibitory potency of the inhibitors toward cAMP-dependent PK (PKAcα) were established by an equilibrium competitive displacement assay and enzyme activity assay, respectively. The deactivatable inhibitors possessed remarkably high 1-2-picomolar affinity toward PKAcα. Irradiation of ARC-2121 with 365 nm UV radiation led to reaction products possessing a 30-fold reduced affinity. The chemical reduction of ARC-2194 resulted in the decrease of affinity of over four orders of magnitude. The deactivatable inhibitors of PKs are valuable tools for the temporal inhibition or capture of these pharmacologically important enzymes.

Wide-range IR spectra of diarylethene derivatives and their simulation using the density functional theory

Diarylethenes (DAEs), promising photochromic molecular switches, undergo pericyclic reactions upon ultraviolet or visible light illumination. For this reason, most studies on DAEs employ UV–vis spectroscopies. However, also their infrared (IR) spectra are valuable, in particular, for understanding the vibrational dynamics which accompanies the relevant photoreactions. An accurate assignment of IR bands to molecular modes can be achieved through a comparison between experimental and computed theoretical spectra. Even though more sophisticated computational methods are available, the density functional theory (DFT) is usually employed for this task, because of its modest cost and versatility. Here, we have tested the ability of several DFT functionals to reproduce the wide-range, 400–3200 cm⁻¹, IR spectra of open and closed isomers of four representative DAE molecules. We find that global and range-separated, corrected for anharmonicity by scaling factors, hybrid DFT functionals are able to reproduce the IR spectra of DAEs, however, instead of the popular B3LYP functional we propose the use of the dispersion-corrected PBE0 functional. The paper also proposes a semi-automatic method of band assignment.

Photoswitchable molecular tweezers: isomerization to control substrate binding, and what about vice versa?

The linkage of two identical binding motifs by a molecular photoswitch has proven to be a straightforward and versatile strategy to control substrate binding affinity by light. Stimulus control of binding properties in artificial receptors is partly inspired by the dynamic behavior of proteins and is highly attractive as it could, for example, improve extraction processes and allow (de)activation of membrane transport on demand. This feature article summarizes the development and design principles of molecular tweezers containing a molecular photoswitch as the core unit. Besides the control of binding affinity by isomerization, the effect of substrate binding on the isomerization behavior is discussed where data is available. While the latter often receives less attention, it could be of benefit in the future creation of multi-stimuli-controlled molecular switching and machine-like systems.

Synthesis of Non-Symmetric Azoarenes by Palladium-Catalyzed Cross-Coupling of Silicon-Masked Diazenyl Anions and (Hetero)Aryl Halides

The photoswitchable motif of azobenzenes is of great importance across the life and materials sciences. This maintains a constant demand for their efficient synthesis, especially that of non-symmetric derivatives. We disclose here a general strategy for their synthesis through an unprecedented C(sp2 )-N(sp2 ) cross-coupling where functionalized aryl-substituted diazenes masked with a silyl group are employed as diazenyl pronucleophiles. These equivalents of fragile diazenyl anions couple with a diverse set of (hetero)aryl bromides under palladium catalysis with no loss of dinitrogen. The competing denitrogenative biaryl formation is fully suppressed. The reaction requires only a minimal excess, that is 1.2 equivalents, of the diazenyl component. By this, a broad range of azoarenes decorated with two electron-rich/deficient aryl groups can be accessed in a predictable way with superb functional-group tolerance.

Optically Switchable NIR Photoluminescence of PbS Semiconducting Nanocrystals using Diarylethene Photoswitches

Precisely modulated photoluminescence (PL) with external control is highly demanded in material and biological sciences. However, it is challenging to switch the PL on and off in the NIR region with a high modulation contrast. Here, we demonstrate that reversible on and off switching of the PL in the NIR region can be achieved in a bicomponent system comprised of PbS semiconducting nanocrystals (NCs) and diarylethene (DAE) photoswitches. Photoisomerization of DAE to the ring-closed form upon UV light irradiation causes substantial quenching of the NIR PL of PbS NCs due to efficient triplet energy transfer. The NIR PL fully recovers to an on state upon reversing the photoisomerization of DAE to the ring-open form with green light irradiation. Importantly, fully reversible switching occurs without obvious fatigue, and the high PL on/off ratio (>100) outperforms all previously reported assemblies of NCs and photoswitches.

The Development and Application of Opto-Chemical Tools in the Zebrafish

The zebrafish is one of the most widely adopted animal models in both basic and translational research. This popularity of the zebrafish results from several advantages such as a high degree of similarity to the human genome, the ease of genetic and chemical perturbations, external fertilization with high fecundity, transparent and fast-developing embryos, and relatively low cost-effective maintenance. In particular, body translucency is a unique feature of zebrafish that is not adequately obtained with other vertebrate organisms. The animal's distinctive optical clarity and small size therefore make it a successful model for optical modulation and observation. Furthermore, the convenience of microinjection and high embryonic permeability readily allow for efficient delivery of large and small molecules into live animals. Finally, the numerous number of siblings obtained from a single pair of animals offers large replicates and improved statistical analysis of the results. In this review, we describe the development of opto-chemical tools based on various strategies that control biological activities with unprecedented spatiotemporal resolution. We also discuss the reported applications of these tools in zebrafish and highlight the current challenges and future possibilities of opto-chemical approaches, particularly at the single cell level.

Highly Twisted Azobenzene Ligand Causes Crystals to Continuously Roll in Sunlight

Direct conversion of solar energy to mechanical work promises higher efficiency than multistep processes, adding a key tool to the arsenal of energy solutions necessary for our global future. The ideal photomechanical material would convert sunlight into mechanical motion rapidly, without attrition, and proportionally to the stimulus. We describe crystals of a tetrahedral isocyanoazobenzene-copper complex that roll continuously when irradiated with broad spectrum white light, including sunlight. The rolling results from bending and straightening of the crystal due to blue light-driven isomerization of a highly twisted azobenzene ligand. These findings introduce geometrically constrained crystal packing as a strategy for manipulating the electronic properties of chromophores. Furthermore, the continuous, solar-driven motion of the crystals demonstrates direct conversion of solar energy to continuous physical motion using easily accessed molecular systems.

Process intensification in continuous flow organic synthesis with enabling and hybrid technologies

Industrial organic synthesis is time and energy consuming, and generates substantial waste. Traditional conductive heating and mixing in batch reactors is no longer competitive with continuous-flow synthetic methods and enabling technologies that can strongly promote reaction kinetics. These advances lead to faster and simplified downstream processes with easier workup, purification and process scale-up. In the current Industry 4.0 revolution, new advances that are based on cyber-physical systems and artificial intelligence will be able to optimize and invigorate synthetic processes by connecting cascade reactors with continuous in-line monitoring and even predict solutions in case of unforeseen events. Alternative energy sources, such as dielectric and ohmic heating, ultrasound, hydrodynamic cavitation, reactive extruders and plasma have revolutionized standard procedures. So-called hybrid or hyphenated techniques, where the combination of two different energy sources often generates synergistic effects, are also worthy of mention. Herein, we report our consolidated experience of all of these alternative techniques.

Reversible Photocontrol of Dopaminergic Transmission in Wild-Type Animals

Understanding the dopaminergic system is a priority in neurobiology and neuropharmacology. Dopamine receptors are involved in the modulation of fundamental physiological functions, and dysregulation of dopaminergic transmission is associated with major neurological disorders. However, the available tools to dissect the endogenous dopaminergic circuits have limited specificity, reversibility, resolution, or require genetic manipulation. Here, we introduce azodopa, a novel photoswitchable ligand that enables reversible spatiotemporal control of dopaminergic transmission. We demonstrate that azodopa activates D₁-like receptors in vitro in a light-dependent manner. Moreover, it enables reversibly photocontrolling zebrafish motility on a timescale of seconds and allows separating the retinal component of dopaminergic neurotransmission. Azodopa increases the overall neural activity in the cortex of anesthetized mice and displays illumination-dependent activity in individual cells. Azodopa is the first photoswitchable dopamine agonist with demonstrated efficacy in wild-type animals and opens the way to remotely controlling dopaminergic neurotransmission for fundamental and therapeutic purposes.

Linear and Nonlinear Optical Properties of Azobenzene Derivatives Modified with an (Amino)naphthalene Moiety

The design of two-photon absorbing azobenzene (AB) derivatives has received much attention; however, the two-photon absorption (2PA) properties of bis-conjugated azobenzene systems are relatively less explored. Here, we present the synthesis of six azobenzene derivatives and three bis-azobenzenes substituted (or not) at para position(s) with one or two amino group(s). Their linear and nonlinear absorption properties are studied experimentally and theoretically. The switching behavior and thermal stability of the Z-isomer are studied for unsubstituted mono- (1a, 2a) and bis-azobenzene (3a) compounds, showing that when the length of the π system increases, the half-life of the Z-isomer decreases. Moreover, along with the increase of π-conjugation, the photochromic characteristics are impaired and the photostationary state (PSS) related to E-Z photoisomerization is composed of 89% of the Z-isomer for 2a and 26% of the Z-isomer for 3a. Importantly, the 2PA cross-section increases almost five-fold on extending the π-conjugation (2a vs 3a) and by about one order of magnitude when comparing two systems: the unsubstituted π-electron one (2a, 3a) with D-π-D (2c, 3c). This work clarifies the contribution of π-conjugation and substituent effects to the linear and nonlinear optical properties of mono- and bis-azobenzene compounds based on the experimental and theoretical approaches.

Chemistries of bifunctional PROTAC degraders

Proteolysis targeting chimeras (PROTACs) technology is a novel and promising therapeutic strategy using small molecules to induce ubiquitin-dependent degradation of proteins. It has received extensive attention from both academia and industry as it can potentially access previously inaccessible targets. However, the design and optimization of PROTACs present big challenges for researchers, and the general strategy for its development and optimization is a lot of trial and error based on experience. This review highlights the important advances in this rapidly growing field and critical limitations of the traditional trial-and-error approach to developing PROTACs by analyzing numerous representative examples of PROTACs development. We summarize and analyze the general principles and strategies for PROTACs design and optimization from the perspective of chemical structure design, and propose potential future pathways to facilitate the development of PROTACs.

Photoswitchable Diazocine-Based Estrogen Receptor Agonists: Stabilization of the Active Form inside the Receptor

Photopharmacology is an emerging approach in drug design and pharmacological therapy. Light is used to switch a pharmacophore between a biologically inactive and an active isomer with high spatiotemporal resolution at the site of illness, thus potentially avoiding side effects in neighboring healthy tissue. The most frequently used strategy to design a photoswitchable drug is to replace a suitable functional group in a known bioactive molecule with azobenzene. Our strategy is different in that the photoswitch moiety is closer to the drug's scaffold. Docking studies reveal a very high structural similarity of natural 17β-estradiol and the E isomers of dihydroxy diazocines, but not their Z isomers, respectively. Seven dihydroxy diazocines were synthesized and subjected to a biological estrogen reporter gene assay. Four derivatives exhibit distinct estrogenic activity after irradiation with violet light, which can be shut off with green light. Most remarkably, the photogenerated, active E form of one of the active compounds isomerizes back to the inactive Z form with a half-life of merely several milliseconds in water, but nevertheless is active for more than 3 h in the presence of the estrogen receptor. The results suggest a significant local impact of the ligand-receptor complex toward back-isomerization. Thus, drugs that are active when bound but lose their activity immediately after leaving the receptor could be of great pharmacological value because they strongly increase target specificity. Moreover, the drugs are released into the environment in their inactive form. The latter argument is particularly important for drugs that act as endocrine disruptors.

Design and Validation of the First Family of Photo-Activatable Ligands for Melatonin Receptors

Melatonin is a neurohormone released in a circadian manner with peak levels at night. Melatonin mediates its effects mainly through G protein-coupled MT₁ and MT₂ receptors. Drugs acting on melatonin receptors are indicated for circadian rhythm- and sleep-related disorders. Tools to study the activation of these receptors with high temporal resolution are lacking. Here, we synthesized a family of light-activatable caged compounds by attaching o-nitrobenzyl (o-NB) or coumarin photocleavable groups to melatonin indolic nitrogen. All caged compounds showed the expected decrease in binding affinity for MT₁ and MT₂. The o-NB derivative MCS-0382 showed the best uncaging and biological properties, with 250-fold increase in affinity and potency upon illumination. Generation of melatonin from MCS-0382 was further demonstrated by its ability to modulate the excitation of SCN neurons in rat brain slices. MCS-0382 is available to study melatonin effects in a temporally controlled manner in cellular and physiological settings.

On the Computational Design of Azobenzene-Based Multi-State Photoswitches

In order to theoretically design multi-state photoswitches with specific properties, an exhaustive computational study is first carried out for an azobenzene dimer that has been recently synthesized and experimentally studied. This study allows for a full comprehension of the factors that govern the photoactivated isomerization processes of these molecules so to provide a conceptual/computational protocol that can be applied to generic multi-state photoswitches. From this knowledge a new dimer with a similar chemical design is designed and also fully characterized. Our theoretical calculations predict that the new dimer proposed is one step further in the quest for a double photoswitch, where the four metastable isomers could be selectively interconverted through the use of different irradiation sequences.

Reversible, Red-Shifted Photoisomerization in Protonated Azobenzenes

Azobenzenes are among the best-studied molecular photoswitches and play a key role in the search for red-shifted photoresponsive materials for extended applications. Currently, most approaches deal with aromatic substitution patterns to achieve visible light application, on occasion paired with protonation to yield red-shifted absorption of the azonium species. Appropriate substitution patterns are essential to stabilize the latter approach, as conventional acids are known to induce a fast Z- to E-conversion. Here, we show that steady-state protonation of the azo-bridge instead is possible in simple azobenzenes when the pK_a of the acid is low enough, yielding both the Z- and E-azonium as supported by UV–vis- and ¹H NMR spectroscopy as well as density functional theory calculations. Moreover, the steady-state protonation of para-methoxyazobenzene, specifically, yields photoisomerizable azonium ions in which the direction of switching is essentially reversed, that is, visible light produces the out-of-equilibrium Z-azonium. Although the current conditions render the visible light photoswitch unsuitable for in vivo and material application, the demonstrated understanding of simple azobenzenes paves the way for a great range of further work on this already widely studied photoswitch.

Towards in vivo photomediated delivery of anticancer peptides: Insights from pharmacokinetic and -dynamic data

An in vivo study of a photoswitchable cytotoxic peptide LMB040 has been undertaken on a chemically induced hepatocellular carcinoma model in immunocompetent rats. We analysed the pharmacokinetic profile of the less toxic photoform ("ring-closed" dithienylethene) of the compound in tumors, plasma, and healthy liver. Accordingly, the peptide can reach a tumor concentration sufficiently high to exert a cytotoxic effect upon photoconversion into the more active ("ring-open") photoform. Tissue morphology, histology, redox state of the liver, and hepatic biochemical parameters in blood serum were analysed upon treatment with (i) the less active photoform, (ii) the in vivo light-activated alternative photoform, and (iii) compared with a reference chemotherapeutic 5-fluorouracil. We found that application of the less toxic form followed by a delayed in vivo photoconversion into the more toxic ring-open form of LMB040 led to a higher overall survival of the animals, and signs of enhanced immune response were observed compared to the untreated animals.

Stepping Out of the Blue: From Visible to Near-IR Triggered Photoswitches

Light offers unique opportunities for controlling the activity of materials and biosystems with high spatiotemporal resolution. Molecular photoswitches are chromophores that undergo reversible isomerization between different states upon irradiation with light, allowing a convenient means to control their influence over the system of interest. However, a significant limitation of classical photoswitches is the requirement to initiate the switching in one or both directions using deleterious UV light with poor tissue penetration. Red-shifted photoswitches are hence in high demand and have attracted keen recent research interest. In this Review, we highlight recent progress towards the development of visible- and NIR-activated photoswitches characterized by distinct photochromic reaction mechanisms. We hope to inspire further endeavors in this field, allowing the full potential of these tools in biotechnology and materials chemistry applications to be realized.

Advanced Strategies in Enzyme Activity Regulation for Biomedical Applications

Enzymes are important macromolecular biocatalysts that accelerate chemical and biochemical reactions in living organisms. Most human diseases are related to alterations in enzyme activity. Moreover, enzymes are potential therapeutic tools for treating different diseases, such as cancer, infections, and cardiovascular and cerebrovascular diseases. Precise remote enzyme activity regulation provides new opportunities to combat diseases. This review summarizes recent advances in the field of enzyme activity regulation, including reversible and irreversible regulation. It also discusses the mechanisms and approaches for on-demand control of these activities. Furthermore, a range of stimulus-responsive inhibitors, polymers, and nanoparticles for regulating enzyme activity and their prospective biomedical applications are summarized. Finally, the current challenges and future perspectives on enzyme activity regulation are discussed.

A Photoswitchable Ligand Targeting the β 1 ‐Adrenoceptor Enables Light‐Control of the Cardiac Rhythm**

Catecholamine‐triggered β‐adrenoceptor (β‐AR) signaling is essential for the correct functioning of the heart. Although both β₁‐ and β₂‐AR subtypes are expressed in cardiomyocytes, drugs selectively targeting β₁‐AR have proven this receptor as the main target for the therapeutic effects of beta blockers in the heart. Here, we report a new strategy for the light‐control of β₁‐AR activation by means of photoswitchable drugs with a high level of β₁‐/β₂‐AR selectivity. All reported molecules allow for an efficient real‐time optical control of receptor function in vitro. Moreover, using confocal microscopy we demonstrate that the binding of our best hit, pAzo‐2, can be reversibly photocontrolled. Strikingly, pAzo‐2 also enables a dynamic cardiac rhythm management on living zebrafish larvae using light, thus highlighting the therapeutic and research potential of the developed photoswitches. Overall, this work provides the first proof of precise control of the therapeutic target β₁‐AR in native environments using light.

Photoswitchable allosteric modulators for metabotropic glutamate receptors

Metabotropic glutamate receptors (mGlu) are a family of class C G protein-coupled receptors (GPCRs) with important biological functions and widespread expression. The mechanisms of mGlu activation and the development of allosteric modulators for these dimeric proteins have attracted singular attention including the use of light regulated ligands. Photopharmacology involves the integration of a photoactive moiety into the ligand structure that following specific illumination undergoes a structural rearrangement and changes its biological activity. The use of light-regulated allosteric ligands offers the opportunity to manipulate mGlu signalling with spatiotemporal precision, unattainable with classical pharmacological approaches. In this review, we will discuss some of the innovations that have been made in the allosteric photopharmacology of mGlu receptors to date. We discuss the prospects of these molecular tools in the control of mGluRs and the new perspectives in understanding mGlu mechanisms, pharmacology and (patho)physiology that can ultimately result in innovative drug discovery concepts.

Sulfur and Azobenzenes, a Profitable Liaison: Straightforward Synthesis of Photoswitchable Thioglycosides with Tunable Properties

Azobenzene photoswitches are valuable tools for controlling properties of molecular systems with light. We have been investigating azobenzene glycoconjugates to probe carbohydrate-protein interactions and to design glycoazobenzene macrocycles with chiroptical and physicochemical properties modulated by light irradiation. To date, direct conjugation of glycosides to azobenzenes was performed by reactions providing target compounds in limited yields. We therefore sought a more effective and reliable coupling method. In this paper, we report on a straightforward thioarylation of azobenzene derivatives with glycosyl thiols as well as other thiols, thereby increasing the scope of azobenzene conjugation. Even challenging unsymmetrical conjugates can be achieved in good yields via sequential or one-pot procedures. Importantly, red-shifted azoswitches, which are addressed with visible light, were easily functionalized. Additionally, by oxidation of the sulfide bridge to the respective sulfones, both the photochromic and the thermal relaxation properties of the core azobenzene can be tuned. Utilizing this option, we realized orthogonal three-state photoswitching in mixtures containing two distinct azobenzene thioglycosides.

Multiscale simulation unravels the light-regulated reversible inhibition of dihydrofolate reductase by phototrexate

Molecular photoswitches are widely used in photopharmacology, where the biomolecular functions are photo-controlled reversibly with high spatiotemporal precision. Despite the success of this field, it remains elusive how the protein environment modulates the photochemical properties of photoswitches. Understanding this fundamental question is critical for designing more effective light-regulated drugs with mitigated side effects. In our recent work, we employed first-principles non-adiabatic dynamics simulations to probe the effects of protein on the trans to cis photoisomerization of phototrexate (PTX), a photochromic analog of the anticancer therapeutic methotrexate that inhibits the target enzyme dihydrofolate reductase (DHFR). Building upon this study, in this work, we employ multiscale simulations to unravel the full photocycle underlying the light-regulated reversible inhibition of DHFR by PTX, which remains elusive until now. First-principles non-adiabatic dynamics simulations reveal that the cis to trans photoisomerization quantum yield is hindered in the protein due to backward isomerization on the ground-state following non-adiabatic transition, which arises from the favorable binding of the cis isomer with the protein. However, free energy simulations indicate that cis to trans photoisomerization significantly decreases the binding affinity of the PTX. Thus, the cis to trans photoisomerization most likely precedes the ligand unbinding from the protein. We propose the most probable photocycle of the PTX-DHFR system. Our comprehensive simulations highlight the trade-offs among the binding affinity, photoisomerization quantum yield, and the thermal stability of the ligand's different isomeric forms. As such, our work reveals new design principles of light-regulated drugs in photopharmacology.

Inactivation of Competitive Decay Channels Leads to Enhanced Coumarin Photochemistry

In the development of photolabile protecting groups, it is of high interest to selectively modify photochemical properties with structural changes as simple as possible. In this work, knowledge of fluorophore optimization was adopted and used to design new coumarin‐ based photocages. Photolysis efficiency was selectively modulated by inactivating competitive decay channels, such as twisted intramolecular charge transfer (TICT) or hydrogen‐bonding, and the photolytic release of the neurotransmitter serotonin was demonstrated. Structural modifications inspired by the fluorophore ATTO 390 led to a significant increase in the uncaging cross section that can be further improved by the simple addition of a double bond. Ultrafast transient absorption spectroscopy gave insights into the underlying solvent‐dependent photophysical dynamics. The chromophores presented here are excellently suited as new photocages in the visible wavelength range due to their simple synthesis and their superior photochemical properties.

Photopharmacology for vision restoration

Blinding diseases that are caused by degeneration of rod and cone photoreceptor cells often spare the rest of the retinal circuit, from bipolar cells, which are directly innervated by photoreceptor cells, to the output ganglion cells that project axons to the brain. A strategy for restoring vision is to introduce light sensitivity to the surviving cells of the retina. One approach is optogenetics, in which surviving cells are virally transfected with a gene encoding a signaling protein that becomes sensitive to light by binding to the biologically available chromophore retinal, the same chromophore that is used by the opsin photo-detectors of rods and cones. A second approach uses photopharmacology, in which a synthetic photoswitch associates with a native or engineered ion channel or receptor. We review these approaches and look ahead to the next generation of advances that could reconstitute core aspects of natural vision.

A photochemical-responsive nanoparticle boosts doxorubicin uptake to suppress breast cancer cell proliferation by apoptosis

In the course of chemotherapy for breast cancer, doxorubicin (DOX) is one of the most commonly prescribed agents. However, it has been recognized as clinically circumscribed on account of its poor selectivity and toxic reactions to normal tissues. Fortunately, the distinct merit of photochemical-responsive nanoparticle delivery systems to enhance cellular drugs uptake through localized concentration, adequate selective and minimizing systemic toxicity has aroused substantial interest recently. In this study, we synthesized photochemical-responsive nanoparticle by incorporating DOX, curcumin (CUR), and perfluorooctyl bromide (PFOB) into poly(lactic-co-glycolic acid) (PLGA) via double emulsification (DOX-CUR-PFOB-PLGA). The synthesized composite nanoparticles, which featured good ultrasound imaging, engendered photochemical activation for drug release when given laser irradiation. Cumulative release rates for DOX were 76.34%, and for CUR were 83.64%, respectively. Also, MCF-7 cells displayed significant intracellular DOX uptake and reactive oxygen species (ROS) levels, degraded cytoskeleton, and decreased cell growth and migration capacity. At the molecular level, cellular pAKT levels decreased, which resulted in downregulated HIF-1α and BAX/BCl-2 levels, leading to Caspase-3 activation and thus induction of apoptosis. Therefore, the photochemical-responsive nanoparticles possess the potential to elicit apoptosis in MCF-7 cells via enhanced DOX uptake.