Reaction-Based Probes for Imaging Mobile Zinc in Live Cells and Tissues

Zinc-chelating BET bromodomain inhibitors equally target islet endocrine cell types

Inhibition of the bromodomain and extraterminal domain (BET) protein family is a potential strategy to prevent and treat diabetes; however, the clinical use of BET bromodomain inhibitors (BETis) is associated with adverse effects. Here, we explore a strategy for targeting BETis to β cells by exploiting the high-zinc (Zn2+) concentration in β cells relative to other cell types. We report the synthesis of a novel, Zn2+-chelating derivative of the pan-BETi (+)-JQ1, (+)-JQ1-DPA, in which (+)-JQ1 was conjugated to dipicolyl amine (DPA). As controls, we synthesized (+)-JQ1-DBA, a non-Zn2+-chelating derivative, and (-)-JQ1-DPA, an inactive enantiomer that chelates Zn2+. Molecular modeling and biophysical assays showed that (+)-JQ1-DPA and (+)-JQ1-DBA retain potent binding to BET bromodomains in vitro. Cellular assays demonstrated (+)-JQ1-DPA attenuated NF-ĸB target gene expression in β cells stimulated with the proinflammatory cytokine interleukin 1β. To assess β-cell selectivity, we isolated islets from a mouse model that expresses green fluorescent protein in insulin-positive β cells and mTomato in insulin-negative cells (non-β cells). Surprisingly, Zn2+ chelation did not confer β-cell selectivity as (+)-JQ1-DPA was equally effective in both β and α cells; however, (+)-JQ1-DPA was less effective in macrophages, a nonendocrine islet cell type. Intriguingly, the non-Zn2+-chelating derivative (+)-JQ1-DBA displayed the opposite selectivity, with greater effect in macrophages compared with (+)-JQ1-DPA, suggesting potential as a macrophage-targeting molecule. These findings suggest that Zn2+-chelating small molecules confer endocrine cell selectivity rather than β-cell selectivity in pancreatic islets and provide valuable insights and techniques to assess Zn2+ chelation as an approach to selectively target small molecules to pancreatic β cells.NEW & NOTEWORTHY Inhibition of BET bromodomains is a novel potential strategy to prevent and treat diabetes mellitus. However, BET inhibitors have negative side effects. We synthesized a BET inhibitor expected to exploit the high zinc concentration in β cells to accumulate in β cells. We show our inhibitor targeted pancreatic endocrine cells; however, it was less effective in immune cells. A control inhibitor showed the opposite effect. These findings help us understand how to target specific cells in diabetes treatment.

Fluorescent Sensor Based on 1H-Pyrazolo[3,4-b]quinoline Derivative for Detecting Zn2+ Cations

The photophysical and sensory properties of the donor-acceptor pyrazoloquinoline derivative (PQPc) were investigated using absorption, steady-state, and time-resolved fluorescence measurements. The compound synthesized from commercial, readily available substrates exhibited absorptions in the UV-Vis range, with a maximum of the longwave band around 390 nm. The maximum fluorescence was around 460-480 nm, depending on the solvent. The quantum yield was between 12.87% (for n-hexane) and 0.75% (for acetonitrile) and decreased with increasing solvent polarity. The PET mechanism was implicated as the cause of fluorescence quenching. Divalent ions such as Zn2+, Pb2+, Cd2+, Ca2+, Mg2+, Co2+, Ni2+, and Cu2+ were introduced to study the fluorescent response of PQPc. A 13-times increase in fluorescence quantum yield was observed after the addition of Zn2+ ions. Detailed research was carried out for the PQPc-Zn2+ system in order to check the possibility of analytical applications of PQPc as a fluorescent sensor. A detection limit of Zn2+ was set at the value level 1.93 × 10-7 M. PQPc-Zn2+ complexes had a stoichiometry of 1:1 with a binding constant of 859 M-1. Biological studies showed that the sensor was localized in cells near the membrane and cytoplasm and may be used to detect zinc ions in eukaryotic cells.

Advances in reaction-based synthetic fluorescent probes for studying the role of zinc and copper ions in living systems

Recently, the behavior of essential trace metal elements in living organisms has attracted more and more attention as their dynamics have been found to be tightly regulated by metallothionines, transporters, etc. As the physiological and/or pathological roles of such metal elements are critical, there have been many non-invasive methods developed to determine their cellular functions, mainly by small molecule fluorescent probes. In this review, we focus on probes that detect intracellular zinc and monovalent copper. Both zinc and copper act not only as tightly bound cofactors of enzymes and proteins but also as signaling factors as labile or loosely bound species. Many fluorescent probes that detect mobile zinc or monovalent copper are recognition-based probes, whose detection is hindered by the abundance of intracellular chelators such as glutathione which interfere with the interaction between probe and metal. In contrast, reaction-based probes release fluorophores triggered by zinc or copper and avoid interference from such intracellular chelators, allowing the detection of even low concentrations of such metals. Here, we summarize the current status of the cumulative effort to develop such reaction-based probes and discuss the strategies adopted to overcome their shortcomings.

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.

A sensitive zinc probe operating via enhancement of excited-state intramolecular charge transfer

Novel highly sensitive fluorescent probes for zinc cations based on the diketopyrrolopyrrole scaffold were designed and synthesized. Large bathochromic shifts (≈80 nm) of fluorescence are observed when the Zn²⁺-recognition unit (di-(2-picolyl)amine) is bridged with the fluorophore possessing an additional pyridine unit able to participate in the coordination process. This effect originates from the dipolar architecture and the increasing electron-withdrawing properties of the diketopyrrolopyrrole core upon addition of the cation. The new, greenish-yellow emitting probes, which operate via modulation of intramolecular charge transfer, are very sensitive to the presence of Zn²⁺. Introduction of a morpholine unit in the diketopyrrolopyrrole structure induces a selective six-fold increase of the emission intensity upon zinc coordination. Importantly, the presence of other divalent biologically relevant metal cations has negligible effects and typically even at a 100-fold higher concentration of Mg²⁺/Zn²⁺, the effect is comparable. Computational studies rationalize the strong bathochromic shift upon Zn²⁺-complexation. Decorating the probes with the triphenylphosphonium cation and morpholine unit enables selective localization in the mitochondria and the lysosome of cardiac H9C2 cells, respectively.

Luminescent Metal Complexes as Emerging Tools for Lipid Imaging

Fluorescence microscopy is a key tool in the biological sciences, which finds use as a routine laboratory technique (e.g., epifluorescence microscope) or more advanced confocal, two-photon, and super-resolution applications. Through continued developments in microscopy, and other analytical methods, the importance of lipids as constituents of subcellular organelles, signalling or regulating molecules continues to emerge. The increasing recognition of the importance of lipids to fundamental cell biology (in health and disease) has prompted the development of protocols and techniques to image the distribution of lipids in cells and tissues. A diverse suite of spectroscopic and microscopy tools are continuously being developed and explored to add to the "toolbox" to study lipid biology. A relatively recent breakthrough in this field has been the development and subsequent application of metal-based luminescent complexes for imaging lipids in biological systems. These metal-based compounds appear to offer advantages with respect to their tunability of the photophysical properties, in addition to capabilities centred around selectively targeting specific lipid structures or classes of lipids. The presence of the metal centre also opens the path to alternative imaging modalities that might not be applicable to traditional organic fluorophores. This review examines the current progress and developments in metal-based luminescent complexes to study lipids, in addition to exploring potential new avenues and challenges for the field to take.

Unusual Reactivity and Metal Affinity of Water-Soluble Dipyrrins

Dipyrrins are a versatile class of organic ligands capable of fluorogenic complexation of metal ions. The primary goal of our study was to evaluate dipyrrins functionalized with ester and amide groups in 2,2'-positions in sensing applications. While developing the synthesis, we found that 3,3',4,4'-tetraalkyldipyrrins 2,2'-diesters as well as 2,2'-diamides can undergo facile addition of water at the meso-bridge, transforming into colorless meso-hydroxydipyrromethanes. Spectroscopic and computational investigation revealed that this transformation proceeds via dipyrrin cations, which exist in equilibrium with the hydroxydipyrromethanes. While trace amounts of acid favor conversion of dipyrrins to hydroxydipyrromethanes, excess acid shifts the equilibrium toward the cations. Similarly, the presence of Zn²⁺ facilitates elimination of water from hydroxydipyrromethanes with chromogenic regeneration of the dipyrrin system. In organic solutions in the presence of Zn²⁺, dipyrrin-2,2'-diesters exist as mixtures of mono-(LZnX) and bis-(L₂Zn) complexes. In L₂Zn, the dipyrrin ligands are oriented in a nonorthogonal fashion, causing strong exciton coupling. In aqueous solutions, dipyrrins bind Zn²⁺ in a 1:1 stoichiometry, forming mono-dipyrrinates (LZnX). Unexpectedly, dipyrrins with more electron-rich 2,2'-carboxamide groups revealed ∼20-fold lower affinity for Zn²⁺ than the corresponding 2,2'-diesters. Density Functional Theory (DFT) calculations with explicit inclusion of water reproduced the observed trends and allowed us to trace the low affinity of the dipyrrin-diamides to the stabilization of the corresponding free bases via hydrogen bonding with water molecules. Overall, our results reveal unusual trends in the reactivity of dipyrrins and provide clues for the design of dipyrrin-based sensors for biological applications.

Rational Design of Silicon-Based Zinc Ionophores

Ionophores transport ions across biological membranes and have wide-ranging applications, but a platform for their rapid development does not exist. We report a platform for developing ionophores from metal-ion chelators, which are readily available with wide-ranging affinities and specificities, and structural data that can aid rational design. Specifically, we fine-tuned the binding affinity and lipophilicity of a Zn^II -chelating ligand by introducing silyl groups proximal to the Zn^II -binding pocket, which generated ionophores that performed better than most of the currently known Zn^II ionophores. Furthermore, these silicon-based ionophores were specific for Zn^II over other metals and exhibited better antibacterial activity and less toxicity to mammalian cells than several known Zn^II ionophores, including pyrithione. These studies establish rational design principles for the rapid development of potent and specific ionophores and a new class of antibacterial agents.

Emerging diabetes therapies: Bringing back the β-cells

BACKGROUND

Stem cell therapies are finally coming of age as a viable alternative to pancreatic islet transplantation for the treatment of insulin-dependent diabetes. Several clinical trials using human embryonic stem cell (hESC)-derived β-like cells are currently underway, with encouraging preliminary results. Remaining challenges notwithstanding, these strategies are widely expected to reduce our reliance on human isolated islets for transplantation procedures, making cell therapies available to millions of diabetic patients. At the same time, advances in our understanding of pancreatic cell plasticity and the molecular mechanisms behind β-cell replication and regeneration have spawned a multitude of translational efforts aimed at inducing β-cell replenishment in situ through pharmacological means, thus circumventing the need for transplantation.

SCOPE OF REVIEW

We discuss here the current state of the art in hESC transplantation, as well as the parallel quest to discover agents capable of either preserving the residual mass of β-cells or inducing their proliferation, transdifferentiation or differentiation from progenitor cells.

MAJOR CONCLUSIONS

Stem cell-based replacement therapies in the mold of islet transplantation are already around the corner, but a permanent cure for type 1 diabetes will likely require the endogenous regeneration of β-cells aided by interventions to restore the immune balance. The promise of current research avenues and a strong pipeline of clinical trials designed to tackle these challenges bode well for the realization of this goal.

Harnessing reaction-based probes to preferentially target pancreatic β-cells and β-like cells

Highly sensitive approaches to target insulin-expressing cells would allow more effective imaging, sorting, and analysis of pancreatic β-cells. Here, we introduce the use of a reaction-based probe, diacetylated Zinpyr1 (DA-ZP1), to image pancreatic β-cells and β-like cells derived from human pluripotent stem cells. We harness the high intracellular zinc concentration of β-cells to induce a fluorescence signal in cells after administration of DA-ZP1. Given its specificity and rapid uptake by cells, we used DA-ZP1 to purify live stem cell-derived β-like cells as confirmed by immunostaining analysis. We tested the ability of DA-ZP1 to image transplanted human islet grafts and endogenous mouse pancreatic islets in vivo after its systemic administration into mice. Thus, DA-ZP1 enables purification of insulin-secreting β-like cells for downstream applications, such as functional studies, gene-expression, and cell-cell interaction analyses and can be used to label engrafted human islets and endogenous mouse islets in vivo.

A luminescent Cd(ii) coordination polymer as a multi-responsive fluorescent sensor for Zn2+, Fe3+ and Cr2O72− in water with fluorescence enhancement or quenching

A luminescent Cd(ii) coordination polymer can act as a multi-responsive sensor for efficiently detecting Zn²⁺, Fe³⁺ and Cr₂O₇²⁻ ions.

DNAzyme-Metal-Organic Framework Two-Photon Nanoprobe for In situ Monitoring of Apoptosis-Associated Zn2+ in Living Cells and Tissues

Monitoring Zn²⁺ in living cells is critical for fully elucidating the biological process of apoptosis. However, the quantitative intracellular sensing of Zn²⁺ using DNAzyme remains challenging because of issues related to penetration of the signal through tissue, targeted cellular uptake and activation, and susceptibility toward enzymatic degradation. In this study, we developed a novel phosphate ion-activated DNAzyme-metal-organic frameworks (MOFs) nanoprobe for two-photon imaging of Zn²⁺ in living cells and tissues. The design of this nanoprobe involved the loading of a Zn²⁺-specific, RNA-cleaving DNAzyme onto the MOFs through strong coordination between the phosphonate O atoms of the DNAzyme backbone and Zr atoms in the MOFs. This coordination restrained the extracellular activity of DNAzyme; however, after cell entry, the DNAzyme was released from the MOFs through a competitive binding by the phosphate ions present at a high intracellular concentration. Following their release, the two-photon (TP) fluorophore-labeled substrate strands of DNAzyme were cleaved with the aid of Zn²⁺, which resulted in a strong fluorescence signal. The incorporation of a TP fluorophore into the nanoprobe facilitated near-infrared excitation, which allowed the highly sensitive and specific imaging of Zn²⁺ in living cells and tissues at greater depths than possible previously. The TP-DNAzyme-MOFs nanoprobe achieved a low detection limit of 3.53 nM, extraordinary selectivity toward Zn²⁺, and a tissue signal penetration of 120 μm. More importantly, this nanoprobe was successfully used to monitor cell apoptosis, and this application of the DNAzyme-MOFs probe holds great potential for future use in biological studies and medical diagnostics.

Tools and techniques for illuminating the cell biology of zinc

Zinc (Zn²⁺) is an essential micronutrient that is required for a wide variety of cellular processes. Tools and methods have been instrumental in revealing the myriad roles of Zn²⁺ in cells. This review highlights recent developments fluorescent sensors to measure the labile Zn²⁺ pool, chelators to manipulate Zn²⁺ availability, and fluorescent tools and proteomics approaches for monitoring Zn²⁺-binding proteins in cells. Finally, we close with some highlights on the role of Zn²⁺ in regulating cell function and in cell signaling.

Activity-Based Sensing: A Synthetic Methods Approach for Selective Molecular Imaging and Beyond

Emerging from the origins of supramolecular chemistry and the development of selective chemical receptors that rely on lock-and-key binding, activity-based sensing (ABS)-which utilizes molecular reactivity rather than molecular recognition for analyte detection-has rapidly grown into a distinct field to investigate the production and regulation of chemical species that mediate biological signaling and stress pathways, particularly metal ions and small molecules. Chemical reactions exploit the diverse chemical reactivity of biological species to enable the development of selective and sensitive synthetic methods to decipher their contributions within complex living environments. The broad utility of this reaction-driven approach facilitates application to imaging platforms ranging from fluorescence, luminescence, photoacoustic, magnetic resonance, and positron emission tomography modalities. ABS methods are also being expanded to other fields, such as drug and materials discovery.

Native Zinc Catalyzes Selective and Traceless Release of Small Molecules in β-Cells

The loss of insulin-producing β-cells is the central pathological event in type 1 and 2 diabetes, which has led to efforts to identify molecules to promote β-cell proliferation, protection, and imaging. However, the lack of β-cell specificity of these molecules jeopardizes their therapeutic potential. A general platform for selective release of small-molecule cargoes in β-cells over other islet cells ex vivo or other cell-types in an organismal context will be immensely valuable in advancing diabetes research and therapeutic development. Here, we leverage the unusually high Zn(II) concentration in β-cells to develop a Zn(II)-based prodrug system to selectively and tracelessly deliver bioactive small molecules and fluorophores to β-cells. The Zn(II)-targeting mechanism enriches the inactive cargo in β-cells as compared to other pancreatic cells; importantly, Zn(II)-mediated hydrolysis triggers cargo activation. This prodrug system, with modular components that allow for fine-tuning selectivity, should enable the safer and more effective targeting of β-cells.

Benzoxazine-based fluorescent probes with different auxochrome groups for cysteine detection

Three 5H-benzo[a]phenoxazin-5-one-based (benzoresorufin and nile-red) Cysteine (Cys) detection probes have been comparatively designed and synthesized in this paper. The optical experiments exhibit probe 1b with a crotonoyl group has no response toward Cys; while probes 1a and 1c have the same reaction site (acryloyl group), their optical responses to Cys are quite different. The benzoresorufin-based-probe 1a shows a turn-on fluorescence response (118-fold) to Cys at 631 nm and affords a very low detection limit (DL = 19.8 nM). Compared with probe 1a, the nile-red-based probe 1c displays gradually diminishing fluorescence intensity with increased Cys concentration at 665 nm. And the notable different fluorescence response mechanisms of probes 1a and 1c toward Cys can be interpreted by HRMS and time-dependent density functional theorety (TDDFT) calculations. Furthermore, both of the two probes indicate high sensitivity and selectivity toward Cys over other similar structured amino acids including homocysteine (Hcy) and glutathione (GSH). Further cellular applications of the two probes have been successfully performed in HeLa cells.

A fluorogenic probe using a catalytic reaction for the detection of trace intracellular zinc

A reaction-based fluorescent probe with cephem scaffold has been applied for signal amplification system to detect trace intracellular zinc.

Execution of julolidine based derivative as bifunctional chemosensor for Zn2+ and Cu2+ ions: Applications in bio-imaging and molecular logic gate

A simple julolidine based chemosensor (JT) was designed and synthesized by single condensation step. JT displayed excellent selectivity and sensitivity with on-off responses towards Zn²⁺ and Cu²⁺ over other biologically relevant metal ions in aqueous media. Upon addition of Zn²⁺ ions, JT exhibited a significant blue shift in emission followed by turn-on enhancement while with Cu²⁺, the fluorescence intensity of JT was completely vanished. The 1:1 binding affinity between JT and Zn²⁺/Cu²⁺ was proposed by Job's plot analysis. The detection limit for Zn²⁺ and Cu²⁺ ions reached at 3.5 × 10^-8 M and 1.46 × 10^-6 M, respectively. The sensing mechanism of JT with Zn²⁺/Cu²⁺ was supported by DFT calculations. Based on photophysical studies and its reversibility environment with EDTA, molecular logic gates were fabricated. Furthermore, JT was successfully established to detect intracellular Zn²⁺ ions in live cells by turn-on response.

Diketopyrrolopyrrole-based fluorescence probes for the imaging of lysosomal Zn2+ and identification of prostate cancer in human tissue

A series of diketopyrrolopyrrole-based fluorescent probes (DPP-C2, LysoDPP-C2, LysoDPP-C3, and LysoDPP-C4) have been developed for the detection of low pH and Zn2+ in an AND logic fashion. The chelation of Zn2+ or the protonation of a morpholine moiety within these probes results in a partial increase in the fluorescence intensity, an effect ascribed to suppression of one possible photo-induced electron transfer (PET) pathway. In contrast, a large increase in the observed fluorescence intensity is observed at low pH and in the presence of Zn2+; this is rationalized in terms of both possible PET pathways within the probes being blocked. Job plots, fluorescence titration curves, and isothermal titration calorimetry proved consistent with a 1 : 1 Zn2+ complexation stoichiometry. Each probe demonstrated an excellent selectivity towards Zn2+ and the resulting Zn2+ complexes demonstrated pH sensitivity over the 3.5-9 pH range. Fluorescence imaging experiments confirmed that LysoDPP-C4 was capable of imaging lysosomal Zn2+ in live cells. Little evidence of cytotoxicity was seen. LysoDPP-C4 was successfully applied to the bioimaging of nude mice, wherein it was shown capable of imaging the prostate. Histological studies using a human sample revealed that LysoDPP-C4 can discriminate cancerous prostate tissue from healthy prostate tissue.

Detecting Zn(II) Ions in Live Cells with Near-Infrared Fluorescent Probes

Two near-infrared fluorescent probes (A and B) containing hemicyanine structures appended to dipicolylamine (DPA), and a dipicolylamine derivative where one pyridine was substituted with pyrazine, respectively, were synthesized and tested for the identification of Zn(II) ions in live cells. In both probes, an acetyl group is attached to the phenolic oxygen atom of the hemicyanine platform to decrease the probe fluorescence background. Probe A displays sensitive fluorescence responses and binds preferentially to Zn(II) ions over other metal ions such as Cd2+ ions with a low detection limit of 0.45 nM. In contrast, the emission spectra of probe B is not significantly affected if Zn(II) ions are added. Probe A possesses excellent membrane permeability and low cytotoxicity, allowing for sensitive imaging of both exogenously supplemented Zn(II) ions in live cells, and endogenously releases Zn(II) ions in cells after treatment of 2,2-dithiodipyridine.

Sensors for measuring subcellular zinc pools

Zinc homeostasis is essential for normal cellular function, and defects in this process are associated with a number of diseases including type 2 diabetes (T2D), neurological disorders and cardiovascular disease. Thus, variants in the SLC30A8 gene, encoding the vesicular/granular zinc transporter ZnT8, are associated with altered insulin release and increased T2D risk while the zinc importer ZIP12 is implicated in pulmonary hypertension. In light of these, and findings in other diseases, recent efforts have focused on the development of refined sensors for intracellular free zinc ions that can be targeted to subcellular regions including the cytosol, endoplasmic reticulum (ER), secretory granules, Golgi apparatus, nucleus and the mitochondria. Here, we discuss recent advances in Zn²⁺ probe engineering and their applications to the measurement of labile subcellular zinc pools in different cell types.

An imidazole derivative-based chemodosimeter for Zn2+ and Cu2+ ions through “ON–OFF–ON” switching with intracellular Zn2+ detection

Imidazole derivative-based chemodosimeter (HL) for selective detection of Zn²⁺ and Cu²⁺ metal ions and intracellular Zn²⁺ sensing.

Applications of Nanomaterials Based on Magnetite and Mesoporous Silica on the Selective Detection of Zinc Ion in Live Cell Imaging

Functionalized magnetite nanoparticles (FMNPs) and functionalized mesoporous silica nanoparticles (FMSNs) were synthesized by the conjugation of magnetite and mesoporous silica with the small and fluorogenic benzothiazole ligand, that is, 2(2-hydroxyphenyl)benzothiazole (hpbtz). The synthesized fluorescent nanoparticles were characterized by FTIR, XRD, XRF, ¹³C CP MAS NMR, BET, and TEM. The photophysical behavior of FMNPs and FMSNs in ethanol was studied using fluorescence spectroscopy. The modification of magnetite and silica scaffolds with the highly fluorescent benzothiazole ligand enabled the nanoparticles to be used as selective and sensitive optical probes for zinc ion detection. Moreover, the presence of hpbtz in FMNPs and FMSNs induced efficient cell viability and zinc ion uptake, with desirable signaling in the normal human kidney epithelial (Hek293) cell line. The significant viability of FMNPs and FMSNs (80% and 92%, respectively) indicates a potential applicability of these nanoparticles as in vitro imaging agents. The calculated limit of detections (LODs) were found to be 2.53 × 10⁻⁶ and 2.55 × 10⁻⁶ M for Fe₃O₄-H@hpbtz and MSN-Et₃N-IPTMS-hpbtz-f1, respectively. FMSNs showed more pronounced zinc signaling relative to FMNPs, as a result of the more efficient penetration into the cells.

A diketopyrrolopyrrole-based fluorescent probe for investigating mitochondrial zinc ions

We report a diketopyrrolopyrrole-based fluorescent probe DPP-Mito, which is low toxicity and mitochondrial-specific localization and could monitor the changes in the intracellular Zn²⁺ concentrations.

Polymerization led selective detection and removal of Zn2+and Cd2+ions: isolation of Zn- and Cd-MOFs and reversibility studies

Two post-functionalized chemosensors display remarkable sensing of Zn²⁺and Cd²⁺ionsviagenerating corresponding metal–organic frameworks (MOFs), whereas nitrate and nitrite ions reverse the MOF-polymerization process.

Report of the International Society for Zinc Biology 5th Meeting, in Collaboration with Zinc-Net (COST Action TD1304)-UCLan Campus, Pyla, Cyprus

From 18 to 22 June 2017, the fifth biennial meeting of the International Society for Zinc Biology was held in conjunction with the final dissemination meeting of the Network for the Biology of Zinc (Zinc-Net) at the University of Central Lancashire, Cyprus campus. The meeting attracted over 160 participants, had 17 scientific symposia, 4 plenary speakers and 2 poster discussion sessions. In this report, we give an overview of the key themes of the meeting and some of the highlights from the scientific programme.

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.

A functional applied material on recognition of metal ion zinc based on the double azine compound

A colorimetric and fluorescent probe L has been designed and synthesized, which bearing the double azine moiety and showing a detection limit of 2.725 × 10⁻⁷ M towards Zn²⁺. Based on the basic recognition mechanism of ESIPT and CHEF effect, the L has high selectivity and sensitivity to only Zn²⁺ (not Fe³⁺, Hg²⁺, Ag⁺, Ca²⁺, Co²⁺, Ni²⁺, Cd²⁺, Pb²⁺, Cr³⁺, and Mg²⁺) within the physiological pH range (pH = 7.0–8.4) and showed a fluorescence switch. Moreover, this detection progress occured in the DMSO/H₂O ∼ HEPES buffer (80/20, v/v; pH 7.23) solution which can conveniently used on test strip.

The zinc paradigm for metalloneurochemistry

Neurotransmission and sensory perception are shaped through metal ion–protein interactions in various brain regions. The term "metalloneurochemistry" defines the unique field of bioinorganic chemistry focusing on these processes, and zinc has been the leading target of metalloneurochemists in the almost 15 years since the definition was introduced. Zinc in the hippocampus interacts with receptors that dictate ion flow and neurotransmitter release. Understanding the intricacies of these interactions is crucial to uncovering the role that zinc plays in learning and memory. Based on receptor similarities and zinc-enriched neurons (ZENs) in areas of the brain responsible for sensory perception, such as the olfactory bulb (OB), and dorsal cochlear nucleus (DCN), zinc participates in odor and sound perception. Development and improvement of methods which allow for precise detection and immediate manipulation of zinc ions in neuronal cells and in brain slices will be critical in uncovering the synaptic action of zinc and, more broadly, the bioinorganic chemistry of cognition.

A highly selective and sensitive fluorescent chemosensor for Zn2+based on a diarylethene derivative

A promising photochromic fluorescent chemosensor1olinked with Schiff base unit was synthesized and the sensitivity test of1otoward Zn²⁺has been performed with detection limit up to 8.10 × 10⁻⁸M without any interference from Cd²⁺.

Triazole-Linked Quinoline Conjugate of Glucopyranose: Selectivity Comparison among Zn2+, Cd2+, and Hg2+ Based on Spectroscopy, Thermodynamics, and Microscopy, and Reversible Sensing of Zn2+ and the Structure of the Complex Using DFT

A water-soluble triazole-linked quinoline conjugate of glucopyranose (L) has been synthesized and characterized, and its single-crystal X-ray diffraction (XRD) structure has been established. Binding of L toward different biologically relevant metal ions has been studied using fluorescence and absorption spectroscopy in HEPES buffer at pH 7.4. The conjugate L detects Zn²⁺ and Cd²⁺ with 30 ± 2 and 14 ± 1-fold fluorescence enhancement, respectively, but in the case of Hg²⁺, only a fluorescence quench was observed. The stoichiometry of the complex is 1:2 metal ion to the ligand in the case of Zn²⁺ and Cd²⁺ resulting in [Zn(L)₂] and [Cd(L)₂], and it is 1:1 in the case of Hg²⁺, as confirmed from their electrospray ionization mass spectrometry (ESIMS) spectra. Zn²⁺ shows greater exothermicity over Cd²⁺, whereas Hg²⁺ shows endothermicity , which supports the differences in their binding strength and the nature of the corresponding complex. L exhibits rod-shaped particles and upon complexation with Zn²⁺, it exhibits sphere-like morphological features in scanning electron microscopy (SEM) images. However, clustered aggregates are observed in Cd²⁺, whereas the [HgL] complex exhibits small fused spherical structures, and therefore the signature of these ions is seen in microscopy images. The computational studies revealed that the syn-[Zn(L)₂] complex is stabilized by 9.7 kcal mol^-1 more than that in the case of anti-[Zn(L)₂] owing to the formation of hydrogen bonds between the two glucosyl moieties within the syn-complex. Among the anions studied, [Zn(L)₂] is sensitive and selective toward the phosphate ion (H₂PO₄ ^-) with a minimum detection limit of 16 ± 2 ppb. Similarly, the [HgL] can act as a secondary sensor for CN^- while also exhibiting reversibility. Based on the input-output characteristics, INHIBIT logic gate was built in the case of Zn²⁺ vs H₂PO₄ ^- and IMPLICATION logic gate was built in the case of Hg²⁺ vs CN^-.

Both visual and fluorescent sensors for Zn(2+) based on bis(pyrrol-2-yl-methyleneamine) platform

Two bis(pyrrol-2-yl-methyleneamine) chemo-sensors, 1, 3- and 1, 4-bis[3,4-dimethyl-5-ethyloxy -carbonyl-pyrrol-2-yl-methyleneamine]benzene (H2L(1) and H2L(2), respectively) have been synthesized and characterized, which exhibit high selectivity as off-on fluorescence sensors toward Zn(2+) in CH3CN/H2O (9:1, v/v) solution. The detection limits of both sensors are at the parts per million level. Moreover, the probes H2L(1) and H2L(2) could sense Zn(2+) by "naked eye" with a color change from colorless to yellow, and from yellow to dark yellow, respectively. To test the practical use of the probes, the determination of Zn(2+) in real water samples was also evaluated.

Deciphering the CHEF-PET-ESIPT liaison mechanism in a Zn2+ chemosensor and its applications in cell imaging study

Proper fusion of fluorescence mechanisms in a single fluorophore unit is highly desirable to obtain better sensitivity, as well as selectivity towards a particular metal ion.