Optical Modulation of Antibiotic Resistance by Photoswitchable Cystobactamids

The rise of antibiotic resistance causes a serious health care problem, and its counterfeit demands novel, innovative concepts. The combination of photopharmacology, enabling a light-controlled reversible modulation of drug activity, with antibiotic drug design has led to first photoswitchable antibiotic compounds derived from established scaffolds. In this study, we converted cystobactamids, gyrase-inhibiting natural products with an oligoaryl scaffold and highly potent antibacterial activities, into photoswitchable agents by inserting azobenzene in the N-terminal part and/or an acylhydrazone moiety near the C-terminus, yielding twenty analogs that contain mono- as well as double-switches. Antibiotic and gyrase inhibition properties could be modulated 3.4-fold and 5-fold by light, respectively. Notably, the sensitivity of photoswitchable cystobactamids towards two known resistance factors, the peptidase AlbD and the scavenger protein AlbA, was light-dependent. While irradiation of an analog with an N-terminal azobenzene with 365 nm light led to less degradation by AlbD, the AlbA-mediated inactivation was induced. This provides a proof-of-principle that resistance towards photoswitchable antibiotics can be optically controlled.     

Photoinduced Reversible Solid‐to‐Liquid Transitions for Photoswitchable Materials

Heating and cooling can induce reversible solid‐to‐liquid transitions of matter. In contrast, athermal photochemical processes can induce reversible solid‐to‐liquid transitions of some newly developed azobenzene compounds. Azobenzene is photoswitchable. UV light induces trans‐to‐cis isomerization; visible light or heat induces cis‐to‐trans isomerization. Trans and cis isomers usually have different melting points (T_m) or glass transition temperatures (T_g). If T_m or T_g of an azobenzene compound in trans and cis forms are above and below room temperature, respectively, light may induce reversible solid‐to‐liquid transitions. In this Review, we introduce azobenzene compounds that exhibit photoinduced reversible solid‐to‐liquid transitions, discuss the mechanisms and design principles, and show their potential applications in healable coatings, adhesives, transfer printing, lithography, actuators, fuels, and gas separation. Finally, we discuss remaining challenges in this field.     

Repeat Module‐Based Rational Design of a Photoswitchable Protein for Light‐Driven Control of Biological Processes

Light‐driven control of biological processes using photoswitchable proteins allows high spatiotemporal interrogation or manipulation of such processes, assisting in understanding their functions. Despite considerable advances, however, the wide spread use of optical control has been hampered by a limited repertoire of photoswitchable proteins and a lack of generalized design strategy. Herein, we present a repeat module‐based rational design of a photoswitchable protein composed of LRR (Leucine‐rich repeat) modules using azobenzene as a photochromic ligand. Our design approach involves the rational selection of a C_β pair between two nearby modules within a convex region and subsequent cross‐linking with a photochromic ligand. We demonstrate the general utility and potential of our strategy by showing the design of three target‐specific photoswitchable proteins and a light‐driven modulation of the cell signaling. With an abundance of LRR proteins in nature, our approach can expand the repertoire of photoswitchable proteins for light‐driven control of biological processes.     

Photoswitchable Adhesives Using Azobenzene‐Containing Materials

Photoinduced reversible solid‐to‐liquid transitions of azobenzene‐containing materials can control adhesion. Photoswitchable adhesives based on azobenzene‐containing small molecules and polymers are under intense investigation. The melting points or glass transition temperatures of such azobenzene‐containing materials in trans and cis forms are above and below room temperature, respectively. Photoswitching of these materials results in reversible trans‐cis isomerization and solid‐to‐liquid transitions. The solid trans azobenzene‐containing materials have strong adhesion and the liquid cis azobenzene‐containing materials have weaker adhesion. In this Minireview, we introduce adhesives based on azobenzene‐containing small molecules and polymers. The remaining challenges and perspectives in the field of photoswitchable adhesives using azobenzene‐containing materials are also discussed.     

A Novel pH/Light‐Triggered Surface for DNA Adsorption and Release

A simple strategy for the immobilization of Cy3‐labeled single strand DNA (Cy3‐ssDNA) on a Si(001) surface and its release under control of both light and pH stimuli is presented. In order to prepare a dual pH/light‐triggered surface, positively chargeable azobenzene molecules are self‐assembled on the Si(001) surface. The surface wettability of this substrate can be changed under influence of both light and pH conditions. The substrates can be positively charged under mildly acidic conditions. The pH‐sensitive behavior of the film allows binding of Cy3‐ssDNA on the functionalized Si(001) surface through e?ective electrostatic interactions with the negatively charged polynucleotide backbone. Moreover, irradiation of the film with UVA light induces trans–cis isomerization of the azobenzene units on the surface. As a result, the binding a?nity for DNA decreases due to the changing surface hydrophilicity. In order to understand and control the reversible photoswitchable mechanism of this surface, water contact angles are measured after UVA and visible light irradiation. The release of DNA from a dual pH/light‐sensitive sample is performed using fluorescence microscopy. The results show that irradiation of the film with UVA light induces trans–cis isomerization of the photoresponsive azobenzene units; this leads to significant changes in the surface hydrophilicity and reduces the binding affinity for DNA.     

Synthesis and Surface‐Spectroscopic Characterization of Photoisomerizable glyco‐SAMs on Au(111)

Photoisomerizable glyco‐SAMs (self‐assembled monolayers), utilizing synthetic azobenzene glycoside derivatives were fabricated. The ultimate goal of this project is to assay the influence of the 3D arrangement of sugar ligands on cell adhesion, and eventually make cell adhesion photoswitchable. However, it is a prerequisite for any biological study on the spatial conditions of carbohydrate recognition, that photoisomerization of the surface molecules can be verified. Here, we employed IRRAS and XPS to spectroscopically characterize glyco‐SAMs. In particular and unprecedented to date, we prove reversible E→Z→E isomerization of azobenzene glycoside‐terminated SAMs.     

Photoswitchable Phase Separation and Oligonucleotide Trafficking in DNA Coacervate Microdroplets

Coacervate microdroplets produced by liquid–liquid phase separation have been used as synthetic protocells that mimic the dynamical organization of membrane‐free organelles in living systems. Achieving spatiotemporal control over droplet condensation and disassembly remains challenging. Herein, we describe the formation and photoswitchable behavior of light‐responsive coacervate droplets prepared from mixtures of double‐stranded DNA and an azobenzene cation. The droplets disassemble and reassemble under UV and blue light, respectively, due to azobenzene trans/cis photoisomerisation. Sequestration and release of captured oligonucleotides follow the dynamics of phase separation such that light‐activated transfer, mixing, hybridization, and trafficking of the oligonucleotides can be controlled in binary populations of the droplets. Our results open perspectives for the spatiotemporal control of DNA coacervates and provide a step towards the dynamic regulation of synthetic protocells.     

Photoswitchable Chiral Organocatalysts: Photocontrol of Enantioselective Reactions

This study presents recent advances in photoswitchable chiral organocatalysts and their applications in the photomodulation of enantioselective reactions. Under irradiation with an appropriate wavelength of light, the E/Z-photoisomerization of the photoresponsive units on the catalysts leads to the control of the catalytic activity and/or selectivity of the enantioselective reactions. Additionally, this study elucidates the design, synthesis, and catalytic application of the fabricated azobenzene BINOL-based photoswitchable chiral phase-transfer catalysts. This account will provide insights into the appropriate design of a photoswitchable chiral organocatalyst that can achieve both good enantioselectivity and photocontrol.     

Synthesis and photoresponsive behavior of azobenzene‐containing side‐chain liquid crystalline diblock polymers with polypeptide block

Well‐defined azobenzene‐containing side‐chain liquid crystalline diblock copolymers composed of poly[6‐(4‐methoxy‐azobenzene‐4′‐oxy) hexyl methacrylate] (PMMAZO) and poly(γ‐benzyl‐L ‐glutamate) (PBLG) were synthesized by click reaction from alkyne‐ and azide‐functionalized homopolymers. The alkyne‐terminated PMMAZO homopolymers were synthesized by copper‐mediated atom transfer radical polymerization with a bromine‐containing alkyne bifunctional initiator, and the azido‐terminated PBLG homopolymers were synthesized by ring‐opening polymerization of γ‐benzyl‐L ‐glutamate‐N‐carboxyanhydride in DMF at room temperature using an amine‐containing azide initiator. The thermotropic phase behavior of PMMAZO‐b‐PBLG diblock copolymers in bulk were investigated using differential scanning calorimetry and polarized light microscopy. The PMMAZO‐b‐PBLG diblock copolymers exhibited a smectic phase and a nematic phase when the weight fraction of PMMAZO block was more than 50%. Photoisomerization behavior of PMMAZO‐b‐PBLG diblock copolymers and the corresponding PMMAZO homopolymers in solid film and in solution were investigated using UV–vis. In solution, trans–cis isomerization of diblock copolymers was slower than that of the corresponding PMMAZO homopolymers. These results may provide guidelines for the design of effective photoresponsive anisotropic materials. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Click chemistry‐assisted triazole‐substituted azobenzene and fulgimide units in the pendant‐based copoly(decyloxymethacrylate)s for dual‐mode optical switches

Copolymer containing new thermally reversible click chemistry‐assisted triazole‐substituted azobenzene and fulgimide units in the pendant F‐co‐A was prepared by free‐radical solution addition polymerization technique. The F and A were also prepared for comparison. The DSC analysis of F indicates that the polymer possessing the C‐form of fulgimide unit exhibited higher T_m than that of E‐form of the same polymer and revealed that the C‐form of fulgimide unit in F is highly ordered. The cis‐trans back isomerization behavior of the click chemistry‐assisted triazole‐substituted azobenzene unit in film A has thermal irreversibility, while in F‐co‐A it exhibited thermal reversibility. The UV‐exposed film of F‐co‐A heated around T_g leads to cis‐trans back isomerization of azobenzene unit and thermally stable C‐form of fulgimide which retains its conjugated structure where both the photochromic units are converted into planar conformations and exhibit high fluorescence properties. The fluorescence maxima of C‐form in F‐co‐A red shifted compared with F , because the substituted triazole ring in the azobenzene unit stabilized the C‐form of fulgimide unit. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7843–7860, 2008     

A Fast,Visible‐Light‐Sensitive Azobenzene for Bioorthogonal Ligation

Azobenzenes have been used as photoresponsive units for the control of numerous biological processes. Primary prerequisites for such applications are site‐selective incorporation of photoswitchable units into biomolecules and the possibility of using non‐destructive and deep‐tissue‐penetrating visible light for the photoisomerization. Here we report a push–pull azobenzene that readily undergoes a Staudinger–Bertozzi ligation with azide groups, that can be addressed with visible light (>440 nm) and exhibits the solvato‐ and acidochromism typical for push–pull systems. The thermal relaxation in aqueous environment proceeds on the low‐millisecond timescale, thus enabling control over biological processes on similar timescales. The approach is demonstrated in the modification of a quartz surface and in the incorporation of an azobenzene unit into a functional peptide, the third zinc finger in the mammalian factor Sp1.     

Ultrafast Light‐Driven Substrate Expulsion from the Active Site of a Photoswitchable Catalyst

The photoswitchable piperidine general base catalyst is a prototype structure for light control of catalysis. Its azobenzene moiety moves sterically shielding groups to either protect or expose the active site, thereby changing the basicity and hydrogen‐bonding affinity of the compound. The reversible switching dynamics of the catalyst is probed in the infrared spectral range by monitoring hydrogen bond (HB) formation between its active site and methanol (MeOH) as HB donor. Steady‐state infrared (IR) and ultrafast IR and UV/Vis spectroscopies are used to uncover ultrafast expulsion of MeOH from the active site within a few picoseconds. Thus, the force generated by the azobenzene moiety even in the final phase of its isomerization is sufficient to break a strong HB within 3 ps and to shut down access to the active site.     

Modification of Nucleic Acids by Azobenzene Derivatives and Their Applications in Biotechnology and Nanotechnology

Azobenzene has been widely used as a photoregulator due to its reversible photoisomerization, large structural change between E and Z isomers, high photoisomerization yield, and high chemical stability. On the other hand, some azobenzene derivatives can be used as universal quenchers for many fluorophores. Nucleic acid is a good candidate to be modified because it is not only the template of gene expression but also widely used for building well‐organized nanostructures and nanodevices. Because the size and polarity distribution of the azobenzene molecule is similar to a nucleobase pair, the introduction of azobenzene into nucleic acids has been shown to be an ingenious molecular design for constructing light‐switching biosystems or light‐driven nanomachines. Here we review recent advances in azobenzene‐modified nucleic acids and their applications for artificial regulation of gene expression and enzymatic reactions, construction of photoresponsive nanostructures and nanodevices, molecular beacons, as well as obtaining structural information using the introduced azobenzene as an internal probe. In particular, nucleic acids bearing multiple azobenzenes can be used as a novel artificial nanomaterial with merits of high sequence specificity, regular duplex structure, and high photoregulation efficiency. The combination of functional groups with biomolecules may further advance the development of chemical biotechnology and biomolecular engineering.     

Dynamic Control of Microbial Movement by Photoswitchable ATP Antagonists

Adenosine triphosphate (ATP) is the energy source for various biochemical processes and biomolecular motors in living things. Development of ATP antagonists and their stimuli-controlled actions offer a novel approach to regulate biological processes. Herein, we developed azobenzene-based photoswitchable ATP antagonists for controlling the activity of motor proteins; cytoplasmic and axonemal dyneins. The new ATP antagonists showed reversible photoswitching of cytoplasmic dynein activity in an in vitro dynein-microtubule system due to the trans and cis photoisomerization of their azobenzene segment. Importantly, our ATP antagonists reversibly regulated the axonemal dynein motor activity for the force generation in a demembranated model of Chlamydomonas reinhardtii. We found that the trans and cis isomers of ATP antagonists significantly differ in their affinity to the ATP binding site.     

A visible light responsive azobenzene‐functionalized polymer: Synthesis,self‐assembly,and photoresponsive properties

A novel visible light responsive random copolymer consisting of hydrophobic azobenzene‐containing acrylate units and hydrophilic acrylic acid units has been prepared. The azobenzene molecule bearing methoxy groups at all four ortho positions is readily synthesized by one‐step conversion of diazotization. The as‐prepared polymer can self‐assemble into nanoparticles in water due to its amphiphilic nature. The tetra‐o‐methoxy‐substituted azobenzene‐functionalized polymer can exhibit the trans‐to‐cis photoswitching under the irradiation with green light of 520 nm and the cis‐to‐trans photoswitching under the irradiation with blue light of 420 nm in both solution and aggregate state. The morphologies of the self‐assembled nanoparticles are revealed by TEM and DLS. The controlled release of loaded molecules from the nanoparticles can be realized by adjusting pH value since the copolymer possesses pH responsive acrylic acid groups. The fluorescence of loaded Nile Red in the nanoparticles can be tuned upon the visible light irradiation. The reversible photoswitching of the azobenzene‐functionalized polymer under visible light may endow the polymer with wide applications without using ultraviolet light at all. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2768–2775     

Synthesis of methyl‐substituted azobenzene–carbazole conjugated copolymers with photoinduced structural changes

Azobenzene switches its structure instantaneously by reversible trans‐to‐cis and cis‐to‐trans photoisomerization with light irradiations. Dynamic change in polymer structure is expected via introducing an azobenzene unit into the main chain. In this study, a set of methyl‐substituted azobenzene–carbazole conjugated copolymers is synthesized by the Suzuki–Miyaura coupling method. Introduction of methyl substituents to the azobenzene unit of the monomer, and polymerization in a high‐boiling solvent improve the molecular weight of the polymer. Decrease of effective conjugation length due to the twisted structure of the main chain allows progress of photoisomerization. The microstructure of the polymer was determined with grazing incidence X‐ray diffraction (GIXD) measurements using synchrotron radiation. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1756–1764     

Optical Control of Insulin Secretion Using an Incretin Switch

Incretin mimetics are set to become a mainstay of type 2 diabetes treatment. By acting on the pancreas and brain, they potentiate insulin secretion and induce weight loss to preserve normoglycemia. Despite this, incretin therapy has been associated with off‐target effects, including nausea and gastrointestinal disturbance. A novel photoswitchable incretin mimetic based upon the specific glucagon‐like peptide‐1 receptor (GLP‐1R) agonist liraglutide was designed, synthesized, and tested. This peptidic compound, termed LirAzo , possesses an azobenzene photoresponsive element, affording isomer‐biased GLP‐1R signaling as a result of differential activation of second messenger pathways in response to light. While the trans isomer primarily engages calcium influx, the cis isomer favors cAMP generation. LirAzo thus allows optical control of insulin secretion and cell survival.     

Reversible Stereoselective Folding/Unfolding Fueled by the Interplay of Photoisomerism and Hydrogen Bonding

A linear molecular architecture equipped with complementary three‐fold hydrogen‐bonding units embedded with a photoswitchable trans ‐tetrafluoroazobenzene moiety was synthesized. The trans to cis photoisomerism of the azobenzene unit induced drastic changes in the molecular architecture as a result of intramolecular hydrogen bonding as evidenced by NMR spectroscopy and size exclusion chromatography. A minute stereogenic element in the linear trans state enabled stereoselective folding into the cis state, thus producing a globular architecture with enhanced chiroptical property.     

The effect of light penetration depth on the LCST phase behavior of a thermo‐ and photoresponsive statistical copolymer in an ionic liquid

A reflection cloud point technique allows for rapid screening of light‐dependent phase separation temperatures of thermo‐ and photoresponsive polymer/ionic liquid solutions as a function of sample thickness, molecular weight, and copolymer composition. We systematically investigate the lower critical solution temperature (LCST) phase behavior of poly(benzyl methacrylate‐stat‐(4‐phenylazophenyl methacrylate)). Under UV light, the photoresponsive azobenzene‐based repeat unit becomes more polar as the cis form dominates, increasing its solubility in the ionic liquids 1‐ethyl‐3‐methyl imidazolium and 1‐butyl‐3‐methyl imidazolium bis(trifluoromethanesulfonyl)imide. This light‐dependent polarity change leads to two phase separation temperatures, depending on the illumination wavelength. Under visible light, which drives the azobenzene moiety into the trans ground state, the LCST shows no sample thickness dependence. Under UV light, however, sample thickness plays a significant role. Samples of around 1 mm thickness show no apparent difference under UV and visible light, whereas thinner samples show an increasing difference between the phase separation temperatures with decreasing sample thickness. Neither phase separation temperature exhibits a significant dependence on molecular weight. Increasing the photoresponsive monomer content did not lead to an increase in the difference between the phase separation temperatures at fixed thickness, due to a concomitant increase in UV light absorbed at the sample surface. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 281–287     

Light and pH dual‐sensitive biodegradable polymeric nanoparticles for controlled release of cargos

In this article, a light and pH dual‐sensitive block copolymer PEG‐b‐poly(MPC‐Azo/DEA) was facilely prepared for the first time by azide‐alkyne click chemistry between amphiphilic block copolymer bearing pendant alkynyl group poly(ethylene glycol)‐poly(5‐methyl‐5‐propargylxycarbonyl‐1,3‐dioxane‐2‐one) (PEG‐b‐poly(MPC)) and two azide‐containing compounds azobenzene derivative (Azo‐N₃) and 2‐azido‐1‐ethyl‐diethylamine (DEA‐N₃). Light response of the polymeric nanoparticles benefits from the azobenzene segments and pH responsiveness is attributed to DEA moieties. The prepared copolymer could self‐assemble into spherical micelle particles. The morphological changes of these particles in response to dual stimuli were investigated by UV/vis spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM). Nile Red (NR) was utilized as probe, and fluorescence spectroscopy was served as an evidence for the enhanced release of cargos from polymeric nanoparticles under combined stimulation. Anticancer drug, DOX was loaded into the nanoparticles and the loaded‐DOX could be released from these nanoparticles under dual stimuli. MTT assays further demonstrated that PEG‐b‐poly(MPC) and PEG‐b‐poly(MPC‐Azo/DEA) were of biocompatibility and low toxicity against HepG2 cells as well as SMCC‐7721 cells. More importantly, the prepared DOX‐loaded nanoparticles exhibited good anticancer ability for the two cells. The synthesized light and pH dual‐sensitive biodegradable polymeric nanoparticles were expected to be platforms for precisely controlled release of encapsulated molecules. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1773–1783