SpiroZin1: a reversible and pH-Insensitive, reaction-based, red-fluorescent probe for imaging biological mobile zinc

Small-molecule photoswitches for fluorescence bioimaging: engineering and applications

Fluorescence microscopy has revolutionised our understanding of biological systems, enabling the visualisation of biomolecular structures and dynamics in complex systems. The possibility to reversibly control the optical or biochemical properties of fluorophores can unlock advanced applications ranging from super-resolution microscopy to the design of multi-stimuli responsive and functional biosensors. In this Highlight, we review recent progress in small-molecule photoswitches applied to biological imaging with an emphasis on molecular engineering strategies and promising applications, while underlining the main challenges in their design and implementation.

Mobile zinc as a modulator of sensory perception

Mobile zinc is an abundant transition metal ion in the central nervous system, with pools of divalent zinc accumulating in regions of the brain engaged in sensory perception and memory formation. Here, we present essential tools that we developed to interrogate the role(s) of mobile zinc in these processes. Most important are (a) fluorescent sensors that report the presence of mobile zinc and (b) fast, Zn-selective chelating agents for measuring zinc flux in animal tissue and live animals. The results of our studies, conducted in collaboration with neuroscientist experts, are presented for sensory organs involved in hearing, smell, vision, and learning and memory. A general principle emerging from these studies is that the function of mobile zinc in all cases appears to be downregulation of the amplitude of the response following overstimulation of the respective sensory organs. Possible consequences affecting human behavior are presented for future investigations in collaboration with interested behavioral scientists.

Spiropyran-Based Nanocarrier: A New Zn2+ -Responsive Delivery System with Real-Time Intracellular Sensing Capabilities

A new spiropyran-based stimuli-responsive delivery system is fabricated. It encapsulates and then releases an extraneous compound in response to elevated levels of Zn²⁺ , a critical factor in cell apoptosis. A C₁₂ -alkyl substituent on the spiropyran promotes self-assembly into a micelle-like nanocarrier in aqueous media, with nanoprecipitation and encapsulation of added payload. Zn²⁺ binding occurs to an appended bis(2-pyridylmethyl)amine group at biologically relevant micromolar concentration. This leads to switching of the spiropyran (SP) isomer to the strongly fluorescent ring opened merocyanine-Zn²⁺ (MC-Zn²⁺ ) complex, with associated expansion of the nanocarriers to release the encapsulated payload. Payload release is demonstrated in solution and in HEK293 cells by encapsulation of a blue fluorophore, 7-hydroxycoumarin, and monitoring its release using fluorescence spectroscopy and microscopy. Furthermore, the use of the nanocarriers to deliver a caspase inhibitor, Azure B, into apoptotic cells in response to an elevated Zn²⁺ concentration is demonstrated. This then inhibits intracellular caspase activity, as evidenced by confocal microscopy and in real-time by time-lapsed microscopy. Finally, the nanocarriers are shown to release an encapsulated proteasome inhibitor (5) in Zn²⁺ -treated breast carcinoma cell line models. This then inhibits intracellular proteasome and induces cytotoxicity to the carcinoma cells.

Green, fast, and large-scale synthesis of highly fluorescent Au nanoclusters for Cu2+ detection and temperature sensing

Gold nanoclusters have attracted widespread attention because of their unique optical and physical properties. However, traditional synthesis methods are complicated and require additional reducing agents, while the yield is often very low. Such resource and time-consuming synthesis processes limit their further application. Herein, a rapid sonochemical route is used to synthesize fluorescent Au nanoclusters in large quantities using glutathione (GSH) both as a capping and reducing agent. These Au nanoclusters are synthesized quickly (∼40 min) due to the presence of ultrasonic waves, and show orange red photoluminescence (Em = 598 nm), small size (∼2 nm) and good dispersion in aqueous solution. Moreover, GSH, as a protecting agent on the surface of resultant Au nanoclusters, has many functional groups including carboxyl and amino groups because of which the nanoclusters show high photo-, storage-, metal- and pH-stability. A stable Au nanoclusters-based nano-sensor is designed for highly sensitive and selective label-free detection of Cu2+ with a low limit of detection of 7 ppb (based on S/N = 3). The fluorescent probe can be used in versatile nanothermometry devices, because their photoluminescence intensity correlates strongly with temperature and varies considerably over a wide temperature range (20-80 °C). Therefore, the novel fluorescent sensing probe has great application prospects in Cu2+ detection and temperature sensing.

Superiority of SpiroZin2 Versus FluoZin-3 for monitoring vesicular Zn2+ allows tracking of lysosomal Zn2+ pools

Small-molecule fluorescent probes are powerful and ubiquitous tools for measuring the concentration and distribution of analytes in living cells. However, accurate characterization of these analytes requires rigorous evaluation of cell-to-cell heterogeneity in fluorescence intensities and intracellular distribution of probes. In this study, we perform a parallel and systematic comparison of two small-molecule fluorescent vesicular Zn²⁺ probes, FluoZin-3 AM and SpiroZin2, to evaluate each probe for measurement of vesicular Zn²⁺ pools. Our results reveal that SpiroZin2 is a specific lysosomal vesicular Zn²⁺ probe and affords uniform measurement of resting Zn²⁺ levels at the single cell level with proper calibration. In contrast, FluoZin-3 AM produces highly variable fluorescence intensities and non-specifically localizes in the cytosol and multiple vesicular compartments. We further applied SpiroZin2 to lactating mouse mammary epithelial cells and detected a transient increase of lysosomal free Zn²⁺ at 24-hour after lactation hormone treatment, which implies that lysosomes play a role in the regulation of Zn²⁺ homeostasis during lactation. This study demonstrates the need for critical characterization of small-molecule fluorescent probes to define the concentration and localization of analytes in different cell populations, and reveals SpiroZin2 to be capable of reporting diverse perturbations to lysosomal Zn²⁺.

Mechanism of Evolution of Koneramine Complexes from One-Pot Reactions: Snapshots of Intermediates Offer Facile Routes to New Dipicolylamines

Koneramines (L^R OR', R=Ph or Ts; R'=Me, iPr) and their complexes were found to emerge from the system of pyridine-2-carboxaldehyde and N-phenyl/tosylethylenediamine when a primary or secondary alcohol was used as solvent. Imidazolidinylpyridines (L^R , R=Ph or Ts) became major emergents whereas hemi-aminals (L^R OH, R=Ph or Ts) are minor emergents of the system when tertiary butanol was used as the solvent; the bulky tertiary butyl group prevented the addition of alcohol to the iminium ion that diverted the equilibrium towards imidazolidinylpyridines. By playing with the components of the reaction mixture, crystals of the metastable intermediates bound to copper(II) and/or zinc(II) were obtained and the structures were determined by X-ray diffraction analysis. The reported results shed light on how to control the emergents of the multicomponent reaction mixture that forms koneramines. Reactivity studies of the intermediates pave the way for a new type of koneramine complexes that are new dipicolylamines where the two pyridine moieties of the resulting koneramine are not the same.

Rationally Designed Probe for Reversible Sensing of Zinc and Application in Cells

Biologically compatible fluorescent ion sensors, particularly those that are reversible, represent a key tool for answering a range of fundamental biological questions. We report a rationally designed probe with a 6'-fluoro spiropyran scaffold (5) for the reversible sensing of zinc (Zn²⁺) in cells. The 6'-fluoro substituent overcomes several limitations normally associated with spiropyran-based sensors to provide an improved signal-to-background ratio and faster photoswitching times in aqueous solution. In vitro studies were performed with 5 and the 6'-nitro analogues (6) in HEK 293 and endothelial cells. The new spiropyran (5) can detect exogenous Zn²⁺ inside both cell types and without affecting the proliferation of endothelial cells. Studies were also performed on dying HEK 293 cells, with results demonstrating the ability of the key compound to detect endogenous Zn²⁺ efflux from cells undergoing apoptosis. Biocompatibility and photoswitching of 5 were demonstrated within endothelial cells but not with 6, suggesting the future applicability of sensor 5 to study intracellular Zn²⁺ efflux in these systems.

Near-Infrared Fluorescent Probes with Large Stokes Shifts for Sensing Zn(II) Ions in Living Cells

We report two new near-infrared fluorescent probes based on Rhodol counterpart fluorophore platforms functionalized with dipicolylamine Zn(II)-binding groups. The combinations of the pendant amines and fluorophores provide the probes with an effective three-nitrogen-atom and one-oxygen-atom binding motif. The fluorescent probes with large Stokes shifts offer sensitive and selective florescent responses to Zn(II) ions over other metal ions, allowing a reversible monitoring of Zn(II) concentration changes in living cells, and detecting intracellular Zn(II) ions released from intracellular metalloproteins.

Microstructured Optical Fiber-based Biosensors: Reversible and Nanoliter-Scale Measurement of Zinc Ions

Sensing platforms that allow rapid and efficient detection of metal ions would have applications in disease diagnosis and study, as well as environmental sensing. Here, we report the first microstructured optical fiber-based biosensor for the reversible and nanoliter-scale measurement of metal ions. Specifically, a photoswitchable spiropyran Zn(2+) sensor is incorporated within the microenvironment of a liposome attached to microstructured optical fibers (exposed-core and suspended-core microstructured optical fibers). Both fiber-based platforms retains high selectivity of ion binding associated with a small molecule sensor, while also allowing nanoliter volume sampling and on/off switching. We have demonstrated that multiple measurements can be made on a single sample without the need to change the sensor. The ability of the new sensing platform to sense Zn(2+) in pleural lavage and nasopharynx of mice was compared to that of established ion sensing methodologies such as inductively coupled plasma mass spectrometry (ICP-MS) and a commercially available fluorophore (Fluozin-3), where the optical-fiber-based sensor provides a significant advantage in that it allows the use of nanoliter (nL) sampling when compared to ICP-MS (mL) and FluoZin-3 (μL). This work paves the way to a generic approach for developing surface-based ion sensors using a range of sensor molecules, which can be attached to a surface without the need for its chemical modification and presents an opportunity for the development of new and highly specific ion sensors for real time sensing applications.

Multicomponent One-pot Reactions Towards the Synthesis of Stereoisomers of Dipicolylamine Complexes

Reported are multi-component one-pot syntheses of chiral complexes [M(L(R) OR')Cl2 ] or [M(L(R) SR')Cl2 ] from the mixture of an N-substituted ethylenediamine, pyridine-2-carboxaldehyde, a primary alcohol or thiol and MCl2 utilizing in-situ formed cyclized Schiff bases where a C-O bond, two stereocenters, and three C-N bonds are formed (M=Zn, Cu, Ni, Cd; R=Et, Ph; R'=Me, Et, nPr, nBu). Tridentate ligands L(R) OR' and L(R) SR' comprise two chiral centers and a hemiaminal ether or hemiaminal thioether moiety on the dipicolylamine skeleton. Syn-[Zn(L(Ph) OMe)Cl2 ] precipitates out readily from the reaction mixture as a major product whereas anti-[Zn(L(Ph) OMe)Cl2 ] stays in solution as minor product. Both syn-[Zn(L(Ph) OMe)Cl2 ] and anti-[Zn(L(Ph) OMe)Cl2 ] were characterized using NMR spectroscopy and mass spectrometry. Solid-state structures revealed that syn-[Zn(L(Ph) OMe)Cl2 ] adopted a square pyramidal geometry while anti-[Zn(L(Ph) OMe)Cl2 ] possesses a trigonal bipyramidal geometry around the Zn centers. The scope of this method was shown to be wide by varying the components of the dynamic coordination assembly, and the structures of the complexes isolated were confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. Syn complexes were isolated as major products with Zn(II) and Cu(II) , and anti complexes were found to be major products with Ni(II) and Cd(II) . Hemiaminals and hemiaminal ethers are known to be unstable and are seldom observed as part of cyclic organic compounds or as coordinated ligands assembled around metals. It is now shown, with the support of experimental results, that linear hemiaminal ethers or thioethers can be assembled without the assistance of Lewis acidic metals in the multi-component assembly, and a possible pathway of the formation of hemiaminal ethers has been proposed.

A Far-Red Emitting Probe for Unambiguous Detection of Mobile Zinc in Acidic Vesicles and Deep Tissue

Imaging mobile zinc in acidic environments remains challenging because most small-molecule optical probes display pH-dependent fluorescence. Here we report a reaction-based sensor that detects mobile zinc unambiguously at low pH. The sensor responds reversibly and with a large dynamic range to exogenously applied Zn2+ in lysosomes of HeLa cells, endogenous Zn2+ in insulin granules of MIN6 cells, and zinc-rich mossy fiber boutons in hippocampal tissue from mice. This long-wavelength probe is compatible with the green-fluorescent protein, enabling multicolor imaging, and facilitates visualization of mossy fiber boutons at depths of >100 µm, as demonstrated by studies in live tissue employing two-photon microscopy.