Reversible Photocontrol of Biological Systems by the Incorporation of Molecular Photoswitches

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.

A Two-Photon-Photocleavable Linker for Triggering Light-Induced Strand Breaks in Oligonucleotides

We synthesized a two-photon-sensitive photocleavable linker based on the 7-diethylaminocoumarin structure and introduced it successfully into DNA strands. First, we demonstrated the inducibility of strand scissions upon irradiation at 365 nm. To verify and visualize the two-photon activity, we used a fluorescence assay based on a DNA strand displacement immobilized in a hydrogel. Additionally, we investigated its use in a new class of DNA decoys that are able to catch and release nuclear factor κB (NF-κB) by using light as an external trigger signal. In cell culture we were able to show the regulation of NF-κB-controlled transcription of green fluorescent protein.

Smart Biointerface with Photoswitched Functions between Bactericidal Activity and Bacteria-Releasing Ability

Smart biointerfaces with capability to regulate cell-surface interactions in response to external stimuli are of great interest for both fundamental research and practical applications. Smart surfaces with "ON/OFF" switchability for a single function such as cell attachment/detachment are well-known and useful, but the ability to switch between two different functions may be seen as the next level of "smart". In this work reported, a smart supramolecular surface capable of switching functions reversibly between bactericidal activity and bacteria-releasing ability in response to UV-visible light is developed. This platform is composed of surface-containing azobenzene (Azo) groups and a biocidal β-cyclodextrin derivative conjugated with seven quaternary ammonium salt groups (CD-QAS). The surface-immobilized Azo groups in trans form can specially incorporate CD-QAS to achieve a strongly bactericidal surface that kill more than 90% attached bacteria. On irradiation with UV light, the Azo groups switch to cis form, resulting in the dissociation of the Azo/CD-QAS inclusion complex and release of dead bacteria from the surface. After the kill-and-release cycle, the surface can be easily regenerated for reuse by irradiation with visible light and reincorporation of fresh CD-QAS. The use of supramolecular chemistry represents a promising approach to the realization of smart, multifunctional surfaces, and has the potential to be applied to diverse materials and devices in the biomedical field.

Isomer-Specific Hydrogen Bonding as a Design Principle for Bidirectionally Quantitative and Redshifted Hemithioindigo Photoswitches

A new class of bidirectionally quantitative photoswitches based on the hemithioindigo (HTI) scaffold is reported. Incorporation of a pyrrole hydrogen-bond donor leads to a bathochromic shift allowing for quantitative bidirectional isomerization. Additionally, extending conjugation from the electron-rich pyrrole results in quantitative visible-light photoswitches, as well as photoswitches that isomerize with red and near-infrared light. The presence of the hydrogen bond leading to the observed redshift is supported by computational and spectroscopic evidence.

Photo-enhanced Aqueous Solubilization of an Azo-compound

We previously showed that disruption of intermolecular interactions, e.g., by lowering the molecular planarity and/or introducing bent structures, improves the aqueous solubility of compounds, and based upon that work, we hypothesized that azobenzene trans-to-cis photoswitching could also be utilized to enhance the aqueous solubility of compounds. Here, we demonstrate that UV/visible light irradiation can reversibly switch the aqueous solubilization of an anti-cancer candidate drug, a low-molecular-weight kinase inhibitor bearing an azobenzene moiety. The increase of solubilization associated with UV-induced trans-to-cis conversion may have clinical relevance, because the time-scale of thermal cis-to-trans reversion at 37 °C is longer than that of oral absorption.

The Art of Building Small: From Molecular Switches to Motors (Nobel Lecture)

A journey into the nano-world: The ability to design, use and control motor-like functions at the molecular level sets the stage for numerous dynamic molecular systems. In his Nobel Lecture, B. L. Feringa describes the evolution of the field of molecular motors and explains how to program and control molecules by incorporating responsive and adaptive properties.

Understanding and improving photo-control of ion channels in nociceptors with azobenzene photo-switches

BACKGROUND AND PURPOSE

The photo-isomerizable local anaesthetic, quaternary ammonium-azobenzene-quaternary ammonium (QAQ), provides rapid, optical control over pain signalling without involving genetic modification. In darkness or in green light, trans-QAQ blocks voltage-gated K⁺ and Na⁺ channels and silences action potentials in pain-sensing neurons. Upon photo-isomerization to cis with near UV light, QAQ blockade is rapidly relieved, restoring neuronal activity. However, the molecular mechanism of cis and trans QAQ blockade is not known. Moreover, the absorption spectrum of QAQ requires UV light for photo-control, precluding use deep inside neural tissue.

EXPERIMENTAL APPROACH

Electrophysiology and molecular modelling were used to characterize the binding of cis and trans QAQ to voltage-gated K⁺ channels and to develop quaternary ammonium-ethylamine-azobenzene-quaternary ammonium (QENAQ), a red-shifted QAQ derivative controlled with visible light.

KEY RESULTS

trans QAQ was sixfold more potent than cis QAQ, in blocking current through Shaker K⁺ channels. Both isomers were use-dependent, open channel blockers, binding from the cytoplasmic side, but only trans QAQ block was slightly voltage dependent. QENAQ also blocked native K⁺ and Na⁺ channels preferentially in the trans state. QENAQ was photo-isomerized to cis with blue light and spontaneously reverted to trans within seconds in darkness, enabling rapid photo-control of action potentials in sensory neurons.

CONCLUSIONS AND IMPLICATIONS

Light-switchable local anaesthetics provide a means to non-invasively photo-control pain signalling with high selectivity and fast kinetics. Understanding the mode of action of QAQ and related compounds will help to design of drugs with improved photo-pharmacological properties.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Targeting the N terminus for site-selective protein modification

The formation of well-defined protein bioconjugates is critical for many studies and technologies in chemical biology. Tried-and-true methods for accomplishing this typically involve the targeting of cysteine residues, but the rapid growth of contemporary bioconjugate applications has required an expanded repertoire of modification techniques. One very powerful set of strategies involves the modification of proteins at their N termini, as these positions are typically solvent exposed and provide chemically distinct sites for many protein targets. Several chemical techniques can be used to modify N-terminal amino acids directly or convert them into unique functional groups for further ligations. A growing number of N-terminus-specific enzymatic ligation strategies have provided additional possibilities. This Perspective provides an overview of N-terminal modification techniques and the chemical rationale governing each. Examples of specific N-terminal protein conjugates are provided, along with their uses in a number of diverse biological applications.

Anticancer activity of hydroxy- and sulfonamide-azobenzene platinum(II) complexes in cisplatin-resistant ovarian cancer cells

The syntheses of three platinum(II) complexes bearing sulfonamide- ( (E)-2-(4-methylphenylsulfonamido)-2',6'-difluoroazobenzene, HL1) and hydroxy-azo-2,6-difluorobenzene ((E)-2-((2,6-difluorophenyl)diazenyl)phenol, HL2) bidentate ligands is described. These complexes, [Pt(L1)(DMSO)Cl] (1), [Pt(L2)(DMSO)Cl] (2), and [Pt(L2)₂] (3), were characterized by multinuclear NMR spectroscopy, mass spectrometry, and X-ray crystallography. Despite bearing azobenzene functional groups, none of the three complexes undergo photoisomerization. The anticancer activities of these complexes were evaluated in wild-type (A2780) and cisplatin-resistant (A2780CP70) ovarian cancer cells. All three complexes exhibited IC₅₀ values below 10μM and displayed similar activity in both A2780 and A2780CP70 cell lines, indicating that they are not cross-resistant with cisplatin. The DNA-binding properties of 1-3 were investigated by circular dichroism spectroscopy and by agarose gel electrophoresis. Both studies suggest that 1 and 2 form monofunctional DNA adducts.

Azobenzene-containing photoswitchable proteasome inhibitors with selective activity and cellular toxicity

A series of azobenzene-containing peptidic boronate esters was prepared and the activity of the thermally adapted states (TAS), enriched in trans isomer, and the photostationary states (PSS), enriched in cis isomer, for each compound were evaluated against β5 and β1 proteasome subunits. Compounds with a sterically demanding phenyl-substituted azobenzene at P2 (4c), and a less sterically demanding unsubstituted azobenzene at the N-terminus (5a), showed the greatest difference in activity between the two states. In both cases, the more active trans-enriched TAS had activity comparable to bortezomib and delanzomib. Furthermore, cis-enriched 4c inhibited tumor growth in both breast and colorectal carcinoma cell lines. Significantly, the initial trans-enriched TAS of 4c was not cytotoxic against the non-malignant MCF-10A cells.

Color-Change Photoswitching of an Alkynylpyrene Excimer Dye

We describe a photoswitchable DNA-based dimeric dye that visibly changes fluorescence from green to blue upon UV irradiation. A novel bis-alkyne-dependent [2+2+2] cycloaddition is proposed as a mechanism for the color change in air. The photoinduced structural switching results in spatial separation of stacked pyrene units, thereby causing selective loss of the excimer emission. We demonstrate and suggest several applications for this novel photoswitch.

Illuminating developmental biology through photochemistry

Developmental biology has been continually shaped by technological advances, evolving from a descriptive science into one immersed in molecular and cellular mechanisms. Most recently, genome sequencing and 'omics' profiling have provided developmental biologists with a wealth of genetic and biochemical information; however, fully translating this knowledge into functional understanding will require new experimental capabilities. Photoactivatable probes have emerged as particularly valuable tools for investigating developmental mechanisms, as they can enable rapid, specific manipulations of DNA, RNA, proteins, and cells with spatiotemporal precision. In this Perspective, we describe optochemical and optogenetic systems that have been applied in multicellular organisms, insights gained through the use of these probes, and their current limitations. We also suggest how chemical biologists can expand the reach of photoactivatable technologies and bring new depth to our understanding of organismal development.

Development of Green/Red-Absorbing Chromophores Based on a Coumarin Scaffold That Are Useful as Caging Groups

We report the design, synthesis, and spectroscopic characterization of a series of push-pull chromophores based on a novel coumarin scaffold in which the carbonyl of the lactone function of the original coumarin dyes has been replaced by the cyano(4-nitrophenyl)methylene moiety. The skeleton of the compounds was synthesized by condensation of a thiocoumarin precursor with the corresponding arylacetonitrile derivatives, and their photophysical properties were fine-tuned through the incorporation of electron-withdrawing groups (EWGs) like nitro and cyano at the phenyl ring, leading to absorption in the green to red region. Although fluorescence emission was weakened or even canceled upon introduction of two or three strong EWGs, the emission of the mononitro-containing coumarin derivatives in the red region upon excitation with green light is noticeable, as are their significantly large Stokes shifts. The new coumarin derivatives can be useful as photocleavable protecting groups, as demonstrated through the synthesis and characterization of a series of coumarin-based photocages of benzoic acid. Preliminary photolysis studies with green light have demonstrated that the structure of the coumarin chromophore influences the rate of the uncaging process, opening the way to exploiting these new coumarin scaffolds as caging groups that can be removed with visible light.

Synapses in the spotlight with synthetic optogenetics

Membrane receptors and ion channels respond to various stimuli and relay that information across the plasma membrane by triggering specific and timed processes. These include activation of second messengers, allowing ion permeation, and changing cellular excitability, to name a few. Gaining control over equivalent processes is essential to understand neuronal physiology and pathophysiology. Recently, new optical techniques have emerged proffering new remote means to control various functions of defined neuronal populations by light, dubbed optogenetics. Still, optogenetic tools do not typically address the activity of receptors and channels native to neurons (or of neuronal origin), nor gain access to their signaling mechanisms. A related method-synthetic optogenetics-bridges this gap by endowing light sensitivity to endogenous neuronal receptors and channels by the appending of synthetic, light-receptive molecules, or photoswitches. This provides the means to photoregulate neuronal receptors and channels and tap into their native signaling mechanisms in select regions of the neurons, such as the synapse. This review discusses the development of synthetic optogenetics as a means to study neuronal receptors and channels remotely, in their natural environment, with unprecedented spatial and temporal precision, and provides an overview of tool design, mode of action, potential clinical applications and insights and achievements gained.

Supramolecular Platform with Switchable Multivalent Affinity: Photo-Reversible Capture and Release of Bacteria

Surfaces having dynamic control of interactions at the biological system-material interface are of great scientific and technological interest. In this work, a supramolecular platform with switchable multivalent affinity was developed to efficiently capture bacteria and on-demand release captured bacteria in response to irradiation with light of different wavelengths. The system consists of a photoresponsive self-assembled monolayer containing azobenzene (Azo) groups as guest and β-cyclodextrin (β-CD)-mannose (CD-M) conjugates as host with each CD-M containing seven mannose units to display localized multivalent carbohydrates. Taking the advantage of multivalent effect of CD-M, this system exhibited high capacity and specificity for the capture of mannose-specific type 1-fimbriated bacteria. Moreover, ultraviolet (UV) light irradiation caused isomerization of the Azo groups from trans-form to cis-form, resulting in the dissociation of the host-guest Azo/CD-M inclusion complexes and localized release of the captured bacteria. The capture and release process could be repeated for multiple cycles, suggesting good reproducibility. This platform provides the basis for development of reusable biosensors and diagnostic devices for the detection and measurement of bacteria and exhibits great potential for use as a standard protocol for the on-demand switching of surface functionalities.

Invention of stimulus-responsive peptide-bond-cleaving residue (Spr) and its application to chemical biology tools

Elucidation of biological functions of peptides and proteins is essential for understanding peptide/protein-related biological events and developing drugs. Caged peptides and proteins that release a parent active peptide/protein by photo-irradiation have successfully been employed to elucidate the functions. Whereas the usual caged peptide/protein enables conversion of an inactive form to an active form (OFF-to-ON conversion) by photo-induced deprotection, photo-triggered main chain cleavage is reported to be applicable to ON-to-OFF conversion. These peptides and proteins are photo-responsive; however, if peptides and proteins could respond to other stimuli such as disease-related environment or enzymes, their range of application should be widened. To convert the photo-responsive peptide/protein into other stimulus-responsive peptide/protein, quite laborious de novo design and synthesis of the stimulus-responsive unit are required. In this context, we designed a stimulus-responsive peptide-bond-cleaving residue (Spr) in which the stimuli available for the main chain cleavage vary according to the choice of protecting groups on the residue. In this review, design and synthesis of Spr are introduced, and challenges to apply Spr to other fields to enable, for example, functional control, localization control, delivery of cargos, labeling of a protein of interest in living cells, and identification of target proteins of bioactive ligands are discussed. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

High-Frequency Mechanostimulation of Cell Adhesion

Cell adhesion is regulated by molecularly defined protein interactions and by mechanical forces, which can activate a dynamic restructuring of adhesion sites. Previous attempts to explore the response of cell adhesion to forces have been limited to applying mechanical stimuli that involve the cytoskeleton. In contrast, we here apply a new, oscillatory type of stimulus through push-pull azobenzenes. Push-pull azobenzenes perform a high-frequency, molecular oscillation upon irradiation with visible light that has frequently been applied in polymer surface relief grating. We here use these oscillations to address single adhesion receptors. The effect of molecular oscillatory forces on cell adhesion has been analyzed using single-cell force spectroscopy and gene expression studies. Our experiments demonstrate a reinforcement of cell adhesion as well as upregulated expression levels of adhesion-associated genes as a result of the nanoscale "tickling" of integrins. This novel type of mechanical stimulus provides a previously unprecedented molecular control of cellular mechanosensing.

Structural stability of the photo-responsive DNA duplexes containing one azobenzene via a confined pore

Herein, the structural stability of single azobenzene modified DNA duplexes, including the trans form and cis form, has been examined separately based on their distinguishable unzipping kinetics from the mixture by an α-hemolysin nanopore.