Oxidation-Controlled, Strain-Promoted Tellurophene-Alkyne Cycloaddition (OSTAC): A Bioorthogonal Tellurophene-Dependent Conjugation Reaction

Tellurophene-bearing small molecules have emerged as valuable tools for localizing cellular activities in vivo using mass cytometry. To broaden the utility of tellurophenes in chemical biology, we have developed a bioorthogonal reaction to facilitate tagging of tellurophene-bearing conjugates for downstream applications. Using TePhe, a tellurophene-based phenylalanine analogue, labeled recombinant proteins were generated for reaction development. Using these proteins, we demonstrate an oxidation-controlled, strain-promoted tellurophene-alkyne cycloaddition (OSTAC) reaction. Mild oxidation of the tellurophene ring with N-chlorosuccinimide produces a reactive Te(IV) species which undergoes rapid (k > 100 M-1 s-1) cycloaddition with bicyclo[6.1.0]nonyne (BCN) yielding a benzo-fused cyclooctane. Selective labeling of TePhe-containing proteins can be achieved in complex protein mixtures and on fixed cells. OSTAC reactions can be combined with strain-promoted azide alkyne cycloaddition (SPAAC) and copper-catalyzed azide alkyne click (CuAAC) reactions. Demonstrating the versatility of this approach, we observe the expected staining patterns for 5-ethynyl-2'-deoxyuridine (DNA synthesis-CuAAC) and immunohistochemistry targets in combination with TePhe (protein synthesis-OSTAC) in fixed cells. The favorable properties of the OSTAC reaction suggest its broad applicability in chemical biology.

A series of potent BODIPY photosensitisers featuring tellurophene motifs at boron

This article describes the synthesis and photophysical properties of a series of BODIPY photosensitisers that feature tellurophene motifs appended at the boron centre. These compounds were obtained via nucleophilic substitution of various F-BODIPYs with lithiated tellurophene. The synthetic scope, photophysical characteristics and photosensitisation properties are discussed. Structural modifications around the BODIPY core resulted in an eight-fold improvement in light IC50 values compared to previous designs.

Mass tag-encoded nanointerfaces for multiplexed mass spectrometric analysis and imaging of biomolecules

Revealing multiple biomolecules in the physiopathological environment simultaneously is crucial in biological and biomedical research. Mass spectrometry (MS) features unique technical advantages in multiplexed and label-free analyses. However, owing to comparably low abundance and poor ionization efficiency of target biomolecules, direct MS profiling of these biological species in vitro or in situ remains a challenge. An emerging route to solve this issue is to devise mass tag (MT)-encoded nanointerfaces which specifically convert the abundance or activity of biomolecules into amplified ion signals of mass tags, offering an ideal strategy for synchronous MS assaying and mapping of multiple targets in biofluids, cells and tissues. This review provides a thorough and organized overview of recent advances in MT-encoded nanointerfaces elaborately tailored for several practical applications in multiplexed MS bioanalysis and biomedical research. First, we start with elucidation of the structural characteristics and working principle of MT-encoded nanointerfaces in specific labeling and sensing of multiple biological targets. In addition, we further discuss the application scenarios of MT-encoded nanointerfaces particularly in multiplexed biomarker assays, cell analysis, and tissue imaging. Finally, the current challenges are pointed out and future prospects of these nanointerfaces in MS analysis are forecast.

Reagents for Mass Cytometry

Mass cytometry (cytometry by time-of-flight detection [CyTOF]) is a bioanalytical technique that enables the identification and quantification of diverse features of cellular systems with single-cell resolution. In suspension mass cytometry, cells are stained with stable heavy-atom isotope-tagged reagents, and then the cells are nebulized into an inductively coupled plasma time-of-flight mass spectrometry (ICP-TOF-MS) instrument. In imaging mass cytometry, a pulsed laser is used to ablate ca. 1 μm² spots of a tissue section. The plume is then transferred to the CyTOF, generating an image of biomarker expression. Similar measurements are possible with multiplexed ion bean imaging (MIBI). The unit mass resolution of the ICP-TOF-MS detector allows for multiparametric analysis of (in principle) up to 130 different parameters. Currently available reagents, however, allow simultaneous measurement of up to 50 biomarkers. As new reagents are developed, the scope of information that can be obtained by mass cytometry continues to increase, particularly due to the development of new small molecule reagents which enable monitoring of active biochemistry at the cellular level. This review summarizes the history and current state of mass cytometry reagent development and elaborates on areas where there is a need for new reagents. Additionally, this review provides guidelines on how new reagents should be tested and how the data should be presented to make them most meaningful to the mass cytometry user community.

Tellurophene-tagging of teniposide facilitates monitoring by mass cytometry

Target engagement and the biodistribution of exogenously administered small molecules is rarely homogenous. Methods to determine the biodistribution at the cellular level are limited by the ability to detect the small molecule and simultaneously identify the cell types or tissue structures with which it is associated. The highly multiplexed nature of mass cytometry could facilitate these studies provided a heavy isotope label was available in the molecule of interest. Here we show it is possible to append a tellurophene to a known chemotherapeutic, teniposide, to follow this molecule in vivo . A semi-synthetic approach offers an efficient route to the teniposide analogue which is found to have indistinguishable characteristics when compared with the parent teniposide in vitro . Using mass cytometry and imaging mass cytometry we find the teniposide analogue has significant non-specific binding to cells. In vivo the tellurium bearing teniposide produces the expected DNA damage in a PANC-1 xenograft model. The distribution of Te in the tissue is near the limits of detection and further work will be required to characterize the localization of this analogue with respect to cell type distributions.

Hybrid Fluorescent Mass-Tag Nanotrackers as Universal Reagents for Long-Term Live-Cell Barcoding

Barcoding and pooling cells for processing as a composite sample are critical to minimize technical variability in multiplex technologies. Fluorescent cell barcoding has been established as a standard method for multiplexing in flow cytometry analysis. In parallel, mass-tag barcoding is routinely used to label cells for mass cytometry. Barcode reagents currently used label intracellular proteins in fixed and permeabilized cells and, therefore, are not suitable for studies with live cells in long-term culture prior to analysis. In this study, we report the development of fluorescent palladium-based hybrid-tag nanotrackers to barcode live cells for flow and mass cytometry dual-modal readout. We describe the preparation, physicochemical characterization, efficiency of cell internalization, and durability of these nanotrackers in live cells cultured over time. In addition, we demonstrate their compatibility with standardized cytometry reagents and protocols. Finally, we validated these nanotrackers for drug response assays during a long-term coculture experiment with two barcoded cell lines. This method represents a new and widely applicable advance for fluorescent and mass-tag barcoding that is independent of protein expression levels and can be used to label cells before long-term drug studies.

Introducing the Tellurophene-Appended BODIPY: PDT Agent with Mass Cytometry Tracking Capabilities

The synthesis and characterization of the first BODIPY appended to the five-membered heterocylic tellurophene [Te] moiety is reported. By incorporating tellurophene at the meso position, the tellurophene-appended boron-dipyrromethene dye (BODIPY) acts as a multimodal agent, becoming a potent photosensitizer with a mass cytometry tag. To synthesize the compound, we developed a method to enable late-stage Suzuki-Miyaura coupling by preparing and isolating tellurophene-2-BPin in a one-step procedure from the parent tellurophene. Coupling to a meso-substituted BODIPY functionalized with a pendant aryl bromide provides the desired tellurophene-appended BODIPY. This compound demonstrated a singlet oxygen quantum yield of 0.26 ± 0.01 and produced a light dose-dependent cytotoxicity with nanomolar IC50 values against 2D cultured HeLa cells and high efficacy against 3D cultured HeLa tumor spheroids, proving to be a strong photosensitizer. The presence of the tellurophene moiety could be detected using mass cytometry, thus showcasing the ability of a tellurophene-appended BODIPY as a novel photodynamic-therapy-mass-cytometry theranostic agent.

New Structure Mass Tag based on Zr-NMOF for Multiparameter and Sensitive Single-Cell Interrogating in Mass Cytometry

Mass cytometry, also called cytometry by time-of-flight (CyTOF), is an emerging powerful proteomic analysis technique that utilizes metal chelated polymer (MCP) as mass tags for interrogating high-dimensional biomarkers simultaneously on millions of individual cells. However, under the typical polymer-based mass tag system, the sensitivity and multiplexing detection ability has been highly restricted. Herein, a new structure mass tag based on a nanometal organic framework (NMOF) is reported for multiparameter and sensitive single-cell biomarker interrogating in CyTOF. A uniform-sized Zr-NMOF (33 nm) carrying 105 metal ions is synthesized under modulator/reaction time coregulation, which is monodispersed and colloidally stable in water for over one-year storage. On functionalization with an antibody, the Zr mass tag exhibits specific molecular recognition properties and minimal cross-reaction toward nontargeted cells. In addition, the Zr-mass tag is compatible with MCP mass tags in a multiparameter assay for mouse spleen cells staining, which exploits four additional channels, m/z = 90, 91, 92, 94, for single-cell immunoassays in CyTOF. Compared to the MCP mass tag, the Zr-mass tag provides an additional fivefold signal amplification. This work provides the fundamental technical capability for exploiting NMOF-based mass tags for CyTOF application, which opens up possibility of high-dimensional single-cell immune profiling, low abundant antigen detection, and development of new barcoding systems.

Incorporation of TePhe into Expressed Proteins is Minimally Perturbing

Tellurium is a versatile heavy chalcogen with numerous applications in chemical biology, providing valuable probes in mass cytometry, fluorescence imaging and structural biology. L-Tellurienylalanine (TePhe) is an analogue of the proteinogenic amino acid L-phenylalanine (Phe) in which the phenyl side chain has been replaced by a 5-membered tellurophene moiety. High incorporation level of TePhe in expressed proteins at defined sites is expected to facilitate studies in proteomics, protein NMR spectroscopy, and structure elucidation. As a model we chose immunoglobulin-binding Protein G, B1 domain (GB1) to validate TePhe as a suitable structural analogue for Phe. We demonstrate that approximately 1 in 2 of all Phe sites within GB1 can be substituted with TePhe through expression in standard non-Phe-auxotrophic E. coli in Phe-deficient media containing glyphosate, an inhibitor of aromatic amino acid biosynthesis. The TePhe content of the GB1 sample can be further increased to 85 % through HPLC. Using NMR and CD spectroscopy, we confirm that the Phe-to-TePhe substitution has negligible impact on the global structure and stability of GB1.

Mass Cytometry Tags: Where Chemistry Meets Single-Cell Analysis

Mass cytometry is a highly multiparametric proteomic technology that allows the measurement and quantification of nearly 50 markers with single-cell resolution. Mass cytometry reagents are probes tagged with metal isotopes of defined mass and act as reporters. Metals are detected using inductively coupled plasma time-of-flight mass spectrometry (ICP-TOF-MS). Many different types of mass-tag reagents have been developed to afford myriad applications. We have classified these compounds into polymer-based mass-tag reagents, nonpolymer-based mass-tag reagents, and inorganic nanoparticles. Metal-chelating polymers (MCPs) are widely used to profile and quantify cellular biomarkers; however, both the range of metals that can be detected and the metal signals have to be improved. Several strategies such as the inclusion of chelating agents or highly branched polymers may overcome these issues. Biocompatible materials such as polystyrene and inorganic nanoparticles are also of profound interest in mass cytometry. While polystyrene allows the inclusion of a wide variety of metals, the high metal content of inorganic nanoparticles offers an excellent opportunity to increase the signal from the metals to detect low-abundance biomarkers. Nonpolymer-based mass-tag reagents offer multiple applications: cell detection, cell cycle property determination, biomarker detection, and mass-tag cellular barcoding (MCB). Recent developments have been achieved in live cell barcoding by targeting proteins (CD45, b2m, and CD298), by using small and nonpolar probes or by ratiometric barcoding. From this perspective, the principal applications, strengths, and shortcomings of mass-tag reagents are highlighted, summarized, and discussed, with special emphasis on mass-tag reagents for MCB. Thereafter, the future perspectives of mass-tag reagents are discussed considering the current state-of-the-art technologies.

Stability of di-butyl-dichalcogenide-capped gold nanoparticles: experimental data and theoretical insights

Metals capped with organochalcogenides have attracted considerable interest due to their practical applications, which include catalysis, sensing, and biosensing, due to their optical, magnetic, electrochemical, adhesive, lubrication, and antibacterial properties. There are numerous reports of metals capped with organothiol molecules; however, there are few studies on metals capped with organoselenium or organotellurium. Thus, there is a gap to be filled regarding the properties of organochalcogenide systems which can be improved by replacing sulfur with selenium or tellurium. In the last decade, there has been significant development in the synthesis of selenium and tellurium compounds; however, it is difficult to find commercial applications of these compounds because there are few studies showing the feasibility of their synthesis and their advantages compared to organothiol compounds. Stability against oxidation by molecular oxygen under ambient conditions is one of the properties which can be improved by choosing the correct organochalcogenide; this can confer important advantages for many more suitable applications. This paper reports the successful synthesis and characterization of gold nanoparticles functionalized with organochalcogenide molecules (dibutyl-disulfide, dibutyl-diselenide and dibutyl-ditelluride) and evaluates the oxidation stability of the organochalcogenides. Spherical gold nanoparticles with diameters of 24 nm were capped with organochalcogenides and were investigated using X-ray photoelectron spectroscopy (XPS) to show the improved stability of organoselenium compared with organothiol and organotellurium. The results suggest that the organoselenium is a promising candidate to replace organothiol because of its enhanced stability towards oxidation by molecular oxygen under ambient conditions and its slow oxidation rate. The observed difference in the oxidation processes, as discussed, is also in agreement with theoretical calculations.

Validation of l-Tellurienylalanine as a Phenylalanine Isostere

Mass cytometry (MC) and imaging mass cytometry (IMC^TM ) have emerged as important tools for the study of biological heterogeneity. We recently demonstrated the use of l-2-tellurienylalanine (TePhe), a mimic of phenylalanine (Phe), as an MC- and IMC-compatible protein synthesis reporter. In this work, the biochemical similarity of TePhe and its cognate analogue, Phe, are examined in the context of the RNase S complex. Isothermal titration calorimetry studies show that incorporation of TePhe preserves the interaction of S-peptide with S-protein, and the dissociation constants for the interaction of the Phe and TePhe peptides are within a factor of two. The resulting RNase S complex is catalytically active without significant alterations in the enzyme's kinetic parameters. Furthermore, circular dichroism spectroscopy does not reveal any changes to the secondary structure of TePhe-substituted RNase S. These findings provide strong evidence that TePhe functions as a Phe isostere in the context of a folded protein. It is anticipated that incorporation of TePhe into peptides or peptidomimetic scaffolds will enable facile generation of MC and IMC^TM probes.

Methods for analyzing tellurium imaging mass cytometry data

Imaging mass cytometry (IMC) is a technique allowing visualization and quantification of over 40 biological parameters in a single experiment with subcellular spatial resolution, however most IMC experiments are limited to endpoint analysis with antibodies and DNA stains. Small molecules containing tellurium are promising probes for IMC due to their cell permeability, synthetic versatility, and most importantly their application to sequential labelling with isotopologous probes (SLIP) experiments. SLIP experiments with tellurium-containing probes allow quantification of intracellular biology at multiple timepoints with IMC. Despite the promise of tellurium in IMC, there are unique challenges in image processing associated with tellurium IMC data. Here, we address some of these issues by demonstrating the removal of xenon background signal, combining channels to improve signal-to-noise ratio, and calculating isotope transmission efficiency biases. These developments add accuracy to the unique temporal resolution afforded by tellurium IMC probes.

TePhe, a tellurium-containing phenylalanine mimic, allows monitoring of protein synthesis in vivo with mass cytometry

Protein synthesis is central to maintaining cellular homeostasis and its study is critical to understanding the function and dysfunction of eukaryotic systems. Here we report L-2-tellurienylalanine (TePhe) as a noncanonical amino acid for direct measurement of protein synthesis. TePhe is synthetically accessible, nontoxic, stable under biological conditions, and the tellurium atom allows its direct detection with mass cytometry, without postexperiment labeling. TePhe labeling is competitive with phenylalanine but not other large and aromatic amino acids, demonstrating its molecular specificity as a phenylalanine mimic; labeling is also abrogated in vitro and in vivo by the protein synthesis inhibitor cycloheximide, validating TePhe as a translation reporter. In vivo, imaging mass cytometry with TePhe visualizes translation dynamics in the mouse gut, brain, and tumor. The strong performance of TePhe as a probe for protein synthesis, coupled with the operational simplicity of its use, suggests TePhe could become a broadly applied molecule for measuring translation in vitro and in vivo.

Metal-isotope-tagged monoclonal antibodies for high-dimensional mass cytometry

Advances in single-cell mass cytometry have increasingly improved highly multidimensional characterization of immune cell heterogeneity. The immunoassay multiplexing capacity relies on monoclonal antibodies labeled with stable heavy-metal isotopes. To date, a variety of rare-earth elements and noble and post-transition metal isotopes have been used in mass cytometry; nevertheless, the methods used for antibody conjugation differ because of the individual metal coordination chemistries and distinct stabilities of various metal cations. Herein, we provide three optimized protocols for conjugating monoclonal IgG antibodies with 48 high-purity heavy-metal isotopes: (i) 38 isotopes of lanthanides, 2 isotopes of indium, and 1 isotope of yttrium; (ii) 6 isotopes of palladium; and (iii) 1 isotope of bismuth. Bifunctional chelating agents containing coordinative ligands of monomeric DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) or polymeric pentetic acid (DTPA) were used to stably sequester isotopic cations in aqueous solutions and were subsequently coupled to IgG antibodies using site-specific biorthogonal reactions. Furthermore, quantification methods based on antibody inherent absorption at 280 nm and on extrinsic absorption at 562 nm after staining with bicinchoninic acid (BCA) are reported to determine metal-isotope-tagged antibodies. In addition, a freeze-drying procedure to prepare palladium isotopic mass tags is described. To demonstrate the utility, experiments using six palladium-tagged CD45 antibodies for barcoding assays of live immune cells in cytometry by time-of-flight (CyTOF) are described. Conjugation of pure isotopes of lanthanides, indium, or yttrium takes ~3.5 h. Conjugation of bismuth takes ~4 h. Preparation of palladium mass tags takes ~8 h. Conjugation of pure isotopes of palladium takes ~2.5 h. Antibody titration takes ~4 h.

Thermodynamics of Anion Binding by Chalcogen Bonding Receptors

The application of chalcogen bonding (ChB) to anion recognition is an underdeveloped area of host-guest supramolecular chemistry. The chemical instability of heavier chalcogen derivatives may in part be responsible for the lack of progress. Herein, the synthesis of a new structurally simple, tellurium-based ChB binding motif is reported, the robust stability of which has enabled the thermodynamic properties for ChB halide anion binding in polar aprotic and wet protic organic solvent media to be elucidated. The thermodynamic data reveals how the subtle interplay between ChB host, anion guest and solvent dictates halide binding selectivity and affinity trends. These findings help to provide a deeper insight into the nature of the ChB-anion interaction.

Chalcogen Bonding Macrocycles and [2]Rotaxanes for Anion Recognition

Electron-deficient heavy chalcogen atoms contain Lewis acidic σ-holes which are able to form attractive supramolecular interactions, known as chalcogen bonding (ChB), with Lewis bases. However, their potential in solution-phase anion binding applications is only just beginning to be realized in simple acyclic systems. Herein, we explore the 5-(methylchalcogeno)-1,2,3-triazole (chalcogen = Se, Te) motif as a novel ChB donor for anion binding. Other than being chemically robust enough to be incorporated into macrocyclic structures, thereby significantly expanding the scope and complexity of ChB host systems, we also demonstrate, by ¹H NMR and DFT calculations, that the chalcogen atoms oriented within the macrocycle cavity are able to chelate copper(I) endotopically. Exploiting this property, the first examples of mechanically interlocked [2]rotaxanes containing ChB-donor groups are prepared via an active metal template strategy. Solution-phase ¹H NMR and molecular modeling studies provide compelling evidence for the dominant influence of ChB in anion binding by these interlocked host systems. In addition, unprecedented charge-assisted ChB-mediated anion binding was also studied in aqueous solvent mixtures, which revealed considerable differences in anion recognition behavior in comparison with chalcogen-free host analogues. Moreover, DFT calculations and molecular dynamics simulations in aqueous solvent mixtures indicate that the selectivity is determined by the different hydrophilic characters of the anions allied to the hydration of the binding units in the presence of the anions. Exploiting the NMR-active nuclei of the ChB-donor chalcogen atoms, heteronuclear ⁷⁷Se and ¹²⁵Te NMR were used to directly study how anion recognition influences the local electronic environment of the chalcogen atoms in the mechanically bonded rotaxane binding sites in organic and aqueous solvent mixtures.

A β-galactosidase probe for the detection of cellular senescence by mass cytometry

Enzyme substrates for mass cytometry applications enable new dimensions in multiparametric cellular assays.