A photochromic ATP analogue driving a motor protein with reversible light-controlled motility: controlling velocity and binding manner of a kinesin-microtubule system in an in vitro motility assay

Click Chemoselective Probe with a Photoswitchable Handle for Highly Sensitive Determination of Steroid Hormones in Food Samples

Residues of endocrine disrupting steroid hormones in food might cause various diseases like cardiovascular diseases and breast and prostate cancers. Monitoring steroid hormone levels plays a vital role in ensuring food safety and exploring the pathogenic mechanism of steroid hormone-related diseases. Based on the Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) click reaction, a novel chemoselective probe, Azo-N3, which contains a reactive site N3, an imidazolium salt-based MS tag, and an azobenzene-based photoswitchable handle, was designed and synthesized to label ethynyl-bearing steroid hormones. The probe Azo-N3 was applied for the highly selective and sensitive detection of four ethynyl-bearing steroid hormones in food samples (milk, egg, and pork) by using ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The ionization efficiency of the labeled analytes could be increased by 6-105-fold, and such a labeled method exhibited satisfactory detection limits (0.04-0.2 μg/L), recovery (80.6-122.4%), and precision (RSDs% lower than 6.9%). Interestingly, the efficient immobilization of the probe Azo-N3 onto α-cyclodextrin (α-CD)-modified magnetic particles to construct a solid supported chemoselective probe Fe3O4-CD-Azo-N3 and UV light-controlled release of the labeled analytes from a magnetic support can be achieved by taking advantage of the photoswitched host-guest inclusion between the azobenzene unit and α-CD. The potential applications of Fe3O4-CD-Azo-N3 for labeling, capturing, and the photocontrolled release of the labeled steroid hormones were fully investigated by mass spectrometry imaging analysis. This work not only provides a sensitive and accurate method to detect steroid hormones in food but also opens a new avenue in designing solid supported chemoselective probes.

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.

Fast Photochromism of the Imidazole Dimers Bridged by Group 14 Atoms

We developed fast photochromic imidazole dimers bridged by group 14 atoms. These compounds reversibly break the C-N bond to generate the colored open-ring biradical form. The colored form thermally reproduces the initial colorless form in the microsecond time scales. Furthermore, the color of the biradical can be easily controlled by the introduction of two different types of the imidazolyl radicals. These results give attractive insights for the further development of fast photochromic imidazole dimers.

Photo- and Redox-Driven Artificial Molecular Motors

Directed motion at the nanoscale is a central attribute of life, and chemically driven motor proteins are nature's choice to accomplish it. Motivated and inspired by such bionanodevices, in the past few decades chemists have developed artificial prototypes of molecular motors, namely, multicomponent synthetic species that exhibit directionally controlled, stimuli-induced movements of their parts. In this context, photonic and redox stimuli represent highly appealing modes of activation, particularly from a technological viewpoint. Here we describe the evolution of the field of photo- and redox-driven artificial molecular motors, and we provide a comprehensive review of the work published in the past 5 years. After an analysis of the general principles that govern controlled and directed movement at the molecular scale, we describe the fundamental photochemical and redox processes that can enable its realization. The main classes of light- and redox-driven molecular motors are illustrated, with a particular focus on recent designs, and a thorough description of the functions performed by these kinds of devices according to literature reports is presented. Limitations, challenges, and future perspectives of the field are critically discussed.

Selective Single-Step Oxidation of Amine to Cross-Azo Compounds with an Unhampered Primary Benzyl Alcohol Functionality

This is the first report of a single-step synthesis of primary benzyl alcohol containing different cross-azo compounds (14 examples) by Cu(II) in the presence of a newly synthesized amino-ether heteroditopic macrobicycle cage. Interestingly, even with extreme conditions, the benzyl alcohol remains unoxidized by the Cu(II) catalyst due to the protective etherial pocket of the cage.

Simultaneous complementary photoswitching of hemithioindigo tweezers for dynamic guest relocalization

Remote control of complex molecular behavior and function is one key problem in modern chemistry. Using light signaling for this purpose has many advantages, however the integration of different photo processes into a wholesome yet complex system is highly challenging. Here we report an alternative approach to increase complexity of light control-simultaneous complementary photoswitching-in which spectral overlap is used as an advantage to drastically reduce the signaling needed for controlling multipart supramolecular assemblies. Two photoswitchable molecular tweezers respond to the same light signals with opposite changes in their binding affinities. In this way the configuration of two host tweezers and ultimately the dynamic relocation of a guest molecule can be trigged by only one signal reversibly in the same solution. This approach should provide a powerful tool for the construction of sophisticated, integrated, and multi-responsive smart molecular systems in any application driven field of chemistry.

Controlling complex photoresponsive systems while minimizing light input is highly challenging. Here, the authors report two photoswitchable molecular tweezers responding to the same light signals with opposite changes in their binding affinities towards a guest molecule allowing for its “light-economic” relocation.

Driving and photo-regulation of myosin-actin motors at molecular and macroscopic levels by photo-responsive high energy molecules

We employed an azobenzene based non-nucleoside triphosphate, AzoTP, in a myosin-actin motile system and demonstrated its efficiency as an energy molecule to drive and photo-regulate the myosin-actin motile function at the macroscopic level along with an in vitro motility assay. The AzoTP in its trans state induced shortening of a glycerinated muscle fibre whilst a cis isomer had no significant effect. Direct photoirradiation of a cis-AzoTP infused muscle fibre induced shortening triggered by a locally photo-generated trans-AzoTP in the muscle fibre. Furthermore, we designed and synthesized three new derivatives of AzoTPs that served as substrates for myosin by driving and photo-regulating the myosin-actin motile function at the molecular as well as the macroscopic level with varied efficiencies.

Non-equilibrium assembly of microtubules: from molecules to autonomous chemical robots

Biological systems have evolved to harness non-equilibrium processes from the molecular to the macro scale. It is currently a grand challenge of chemistry, materials science, and engineering to understand and mimic biological systems that have the ability to autonomously sense stimuli, process these inputs, and respond by performing mechanical work. New chemical systems are responding to the challenge and form the basis for future responsive, adaptive, and active materials. In this article, we describe a particular biochemical-biomechanical network based on the microtubule cytoskeletal filament - itself a non-equilibrium chemical system. We trace the non-equilibrium aspects of the system from molecules to networks and describe how the cell uses this system to perform active work in essential processes. Finally, we discuss how microtubule-based engineered systems can serve as testbeds for autonomous chemical robots composed of biological and synthetic components.

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.

Exploiting molecular motors as nanomachines: the mechanisms of de novo and re-engineered cytoskeletal motors

Cytoskeletal molecular motors provide exciting proof that nanoscale transporters can be highly efficient, moving for microns along filamentous tracks by hydrolyzing ATP to fuel nanometer-size steps. For nanotechnology, such conversion of chemical energy into productive work serves as an enticing platform for re-purposing and re-engineering. It also provides a roadmap for successful molecular mechanisms that can be mimicked to create de novo molecular motors for nanotechnology applications. Here we focus specifically on how the mechanisms of molecular motors are being re-engineered for greater control over their transport parameters. We then discuss mechanistic work to create fully synthetic motors de novo and conclude with future directions in creating novel motor systems.

Photoswitchable azobenzene-appended iridium(iii) complexes

The use of aliphatic-bridging units to append azobenzene fragments on triscyclometalated Ir(iii) complexes permits the construction of photoswitchable organometallics.

Right Time, Right Place: Probing the Functions of Organelle Positioning

The proper spatial arrangement of organelles underlies many cellular processes including signaling, polarization, and growth. Despite the importance of local positioning, the precise connection between subcellular localization and organelle function is often not fully understood. To address this, recent studies have developed and employed different strategies to directly manipulate organelle distributions, such as the use of (light-sensitive) heterodimerization to control the interaction between selected organelles and specific motor proteins, adaptor molecules, or anchoring factors. We review here the importance of subcellular localization as well as tools to control local organelle positioning. Because these approaches allow spatiotemporal control of organelle distribution, they will be invaluable tools to unravel local functioning and the mechanisms that control positioning.

Torque generation mechanism of F1-ATPase upon NTP binding

Molecular machines fueled by NTP play pivotal roles in a wide range of cellular activities. One common feature among NTP-driven molecular machines is that NTP binding is a major force-generating step among the elementary reaction steps comprising NTP hydrolysis. To understand the mechanism in detail,in this study, we conducted a single-molecule rotation assay of the ATP-driven rotary motor protein F1-ATPase using uridine triphosphate (UTP) and a base-free nucleotide (ribose triphosphate) to investigate the impact of a pyrimidine base or base depletion on kinetics and force generation. Although the binding rates of UTP and ribose triphosphate were 10(3) and 10(6) times, respectively, slower than that of ATP, they supported rotation, generating torque comparable to that generated by ATP. Affinity change of F1 to UTP coupled with rotation was determined, and the results again were comparable to those for ATP, suggesting that F1 exerts torque upon the affinity change to UTP via rotation similar to ATP-driven rotation. Thus, the adenine-ring significantly enhances the binding rate, although it is not directly involved in force generation. Taking into account the findings from another study on F1 with mutated phosphate-binding residues, it was proposed that progressive bond formation between the phosphate region and catalytic residues is responsible for the rotation-coupled change in affinity.

Remote control of myosin and kinesin motors using light-activated gearshifting

Cytoskeletal motors perform critical force generation and transport functions in eukaryotic cells. Engineered modifications of motor function provide direct tests of protein structure-function relationships and potential tools for controlling cellular processes or for harnessing molecular transport in artificial systems. Here, we report the design and characterization of a panel of cytoskeletal motors that reversibly change gears--speed up, slow down or switch directions--when exposed to blue light. Our genetically encoded structural designs incorporate a photoactive protein domain to enable light-dependent conformational changes in an engineered lever arm. Using in vitro motility assays, we demonstrate robust spatiotemporal control over motor function and characterize the kinetics of the optical gearshifting mechanism. We have used a modular approach to create optical gearshifting motors for both actin-based and microtubule-based transport.

Complete ON/OFF photoswitching of the motility of a nanobiomolecular machine

To apply motor proteins as natural nanomolecular machines to transporting systems in nanotechnology, complete temporal control over ON/OFF switching of the motility is necessary. We have studied the photoresponsive inhibition properties of azobenzene-tethered peptides for regulation of kinesin-microtubule motility. Although a compound containing a peptide having an amino acid sequence derived from the kinesin's C-terminus (a known inhibitor of kinesin's motor domain) and also featuring a terminal azobenzene unit exhibited an inhibition effect, the phototunability of this behavior upon irradiation with UV or visible light was only moderate. Unexpectedly, newly synthesized peptides featuring the reverse sequence of amino acids of the C-terminus of kinesin exhibited excellent photoresponsive inhibition. In particular, azobenzene-CONH-IPKAIQASHGR completely stopped and started the motility of kinesin-microtubules in its trans- and cis-rich states, respectively, obtained after irradiation with visible and UV light, respectively. A gliding motility system containing this photoresponsive inhibitor allowed in situ control of the motion of microtubules on a kinesin-coated glass substrate. It is expected that the present results on the photoresponsive nanomotor system open up new opportunities to design nanotransportation systems.

A non-nucleoside triphosphate for powering kinesin-microtubule motility with photo-tunable velocity

Three non-nucleoside triphosphates, one containing an azobenzene moiety, power a kinesin-microtubule system with high motile activity, with an observed maximum velocity of almost half of that obtained with ATP. The cis-trans photoisomerization of the azobenzene unit allows reversible and repeated control over the motile properties of kinesin.

Reversible control of F(1)-ATPase rotational motion using a photochromic ATP analog at the single molecule level

Motor enzymes such as F1-ATPase and kinesin utilize energy from ATP for their motion. Molecular motions of these enzymes are critical to their catalytic mechanisms and were analyzed thoroughly using a single molecule observation technique. As a tool to analyze and control the ATP-driven motor enzyme motion, we recently synthesized a photoresponsive ATP analog with a p-tert-butylazobenzene tethered to the 2' position of the ribose ring. Using cis/trans isomerization of the azobenzene moiety, we achieved a successful reversible photochromic control over a kinesin-microtubule system in an in vitro motility assay. Here we succeeded to control the hydrolytic activity and rotation of the rotary motor enzyme, F1-ATPase, using this photosensitive ATP analog. Subsequent single molecule observations indicated a unique pause occurring at the ATP binding angle position in the presence of cis form of the analog.

Synthesis and fluorescence characteristics of ATP-based FRET probes

Adenosine triphosphate (ATP) analogues labelled with two dyes suitable for undergoing Förster Resonance Energy Transfer (FRET) have the potential to be valuable tools to continuously study the enzymatic activity of ATP consuming enzymes. Here, we present a synthesis strategy that allows obtaining these ATP analogues in a straight-forward manner. Earlier studies indicate that modifying ATP at the O2'- and the γ-position is a very promising starting point for the design of these probes. We synthesized probes modified with five different combinations of dyes attached to these positions and investigated their fluorescence characteristics in the non-cleaved state as well as after enzymatic hydrolysis. All presented probes largely change their fluorescence characteristics upon cleavage. They include ratiometric FRET probes as well as dark quenched analogues. For typical in vitro applications a combination of the sulfonated polymethine dyes Sulfo-Cy3 and Sulfo-Cy5 seems to be most promising due to their excellent solubility in aqueous buffer and a large change of fluorescence characteristics upon cleavage. For this combination of dyes we also synthesized analogues modified at the γ- and the C2- or the O3'-position, respectively, as these attachment sites are also well accepted by certain ATP consuming enzymes. These analogues show comparably large changes in fluorescence characteristics. Overall, we present new ATP-based FRET probes that have the potential to enable monitoring the enzymatic activity of ATP consuming enzymes.