Cobalt(III) as a stable and inert mediator ion between NTA and His6-tagged proteins

Achieving cinnamic acid amides in water by a variant of acyltransferase from Mycobacterium smegmatis and its immobilized form using Ni-NTA modified aspen powder as a carrier

An aqueous N-acylation reaction for preparing cinnamic acid amides was realized by using a variant of acyltransferase from Mycobacterium smegmatis (MsAcT-L12A), whereas the wild-type MsAcT showed no activity. MsAcT-L12A exhibited broad substrate adaptability, and preferred the substrates with electron-donating group. When the vinyl cinnamate (1a, 40 mM) and p-methoxyaniline (2a, 4 mM) were involved in the reaction, the excellent yield reached to 86.7 % ± 2.1 % within 3 h by MsAcT-L12A (1 mgpro./mL) in a PBS buffer (100 mM, pH 8.0) at 25 °C. The aqueous N-acylation reaction could be further improved by using an immobilized MsAcT-L12A. The biomass aspen powder (AP) as a carrier provided a low-cost, green, and environmental-friendly immobilization strategy. After it was modified by Ni-NTA, the obtained Ni-NAP could realize one-step purification and immobilization of MsAcT-L12A. The accomplished MsAcT-L12A-Ni-NAP exhibited excellent stability and recyclability, and retained its relative yield as 83.3 % ± 2.2 % even after the 7th cycle of reuse. Using only PBS buffer as a reaction medium, the operation for MsAcT-L12A-catalyzed acyl transfer was greatly simplified, and the improved stabilities of MsAcT-L12A-Ni-NAP could enhance its application potential.

DNA nanostructures as biomolecular scaffolds for antigen display

Nanoparticle-based vaccines offer a multivalent approach for antigen display, efficiently activating T and B cells in the lymph nodes. Among various nanoparticle design strategies, DNA nanotechnology offers an innovative alternative platform, featuring high modularity, spatial addressing, nanoscale regulation, high functional group density, and lower self-antigenicity. This review delves into the potential of DNA nanostructures as biomolecular scaffolds for antigen display, addressing: (1) immunological mechanisms behind nanovaccines and commonly used nanoparticles in their design, (2) techniques for characterizing protein NP-antigen complexes, (3) advancements in DNA nanotechnology and DNA-protein assembly approach, (4) strategies for precise antigen presentation on DNA scaffolds, and (5) current applications and future possibilities of DNA scaffolds in antigen display. This analysis aims to highlight the transformative potential of DNA nanoscaffolds in immunology and vaccinology. This article is categorized under: Biology-Inspired Nanomaterials > Nucleic Acid-Based Structures Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

A Simple and Versatile Strategy for Oriented Immobilization of His-Tagged Proteins on Magnetic Nanoparticles

Oriented and covalent immobilization of proteins on magnetic nanoparticles (MNPs) is particularly challenging as it requires both the functionality of the protein and the colloidal stability of the MNPs to be preserved. Here, we describe a simple, straightforward, and efficient strategy for MNP functionalization with proteins using metal affinity binding. Our method involves a single-step process where MNPs are functionalized using a preformed, ready-to-use nitrilotriacetic acid-divalent metal cation (NTA-M2+) complex and polyethylene glycol (PEG) molecules. As a proof-of-concept, we demonstrate the oriented immobilization of a recombinant cadherin fragment engineered with a hexahistidine tag (6His-tag) onto the MNPs. Our developed methodology is simple and direct, enabling the oriented bioconjugation of His-tagged cadherins to MNPs while preserving protein functionality and the colloidal stability of the MNPs, and could be extended to other proteins expressing a polyhistidine tag. When compared to the traditional method where NTA is first conjugated to the MNPs and afterward free metal ions are added to form the complex, this novel strategy results in a higher functionalization efficiency while avoiding MNP aggregation. Additionally, our method allows for covalent bonding of the cadherin fragments to the MNP surface while preserving functionality, making it highly versatile. Finally, our strategy not only ensures the correct orientation of the protein fragments on the MNPs but also allows for the precise control of their density. This feature enables the selective targeting of E-cadherin-expressing cells only when MNPs are decorated with a high density of cadherin fragments.

NTA-Cholesterol Analogue for the Nongenetic Liquid-Ordered Phase-Specific Functionalization of Lipid Membranes with Proteins

The nongenetic modification of cell membranes with proteins is a straightforward way of cellular engineering. In these processes, it is important to specifically address the proteins to liquid-ordered (Lo) or liquid-disordered (Ld) domains as this can largely affect their biological functions. Herein, we report a cholesterol analogue (CHIM) with a nitrilotriacetic acid (NTA) headgroup, named CHIM-NTA. CHIM-NTA integrates into lipid membranes similar to the widely used phospholipid-derived DGS-NTA and, when loaded with Ni2+, allows for specific membrane immobilization of any polyhistidine-tagged proteins of choice. Yet, unlike DGS-NTA, it localizes to the Lo phase in phase-separated giant unilamellar vesicles (GUVs) and allows addressing His-tagged proteins to Lo domains. Furthermore, CHIM-NTA readily integrates into the membranes of live cells and thus enables the nongenetic modification of the cell surface with proteins. Overall, CHIM-NTA provides a facile and flexible way to modify biological membranes, in particular Lo domains, with His-tagged proteins and can serve as a broadly applicable molecular tool for cell surface engineering.

High-Dose Exposure to Polymer-Coated Iron Oxide Nanoparticles Elicits Autophagy-Dependent Ferroptosis in Susceptible Cancer Cells

Ferroptosis, a form of iron-dependent, lipid peroxidation-driven cell death, has been extensively investigated in recent years, and several studies have suggested that the ferroptosis-inducing properties of iron-containing nanomaterials could be harnessed for cancer treatment. Here we evaluated the potential cytotoxicity of iron oxide nanoparticles, with and without cobalt functionalization (Fe₂O₃ and Fe₂O₃@Co-PEG), using an established, ferroptosis-sensitive fibrosarcoma cell line (HT1080) and a normal fibroblast cell line (BJ). In addition, we evaluated poly (ethylene glycol) (PEG)-poly(lactic-co-glycolic acid) (PLGA)-coated iron oxide nanoparticles (Fe₃O₄-PEG-PLGA). Our results showed that all the nanoparticles tested were essentially non-cytotoxic at concentrations up to 100 μg/mL. However, when the cells were exposed to higher concentrations (200-400 μg/mL), cell death with features of ferroptosis was observed, and this was more pronounced for the Co-functionalized nanoparticles. Furthermore, evidence was provided that the cell death triggered by the nanoparticles was autophagy-dependent. Taken together, the exposure to high concentrations of polymer-coated iron oxide nanoparticles triggers ferroptosis in susceptible human cancer cells.

Biosensors Based on the Binding Events of Nitrilotriacetic Acid-Metal Complexes

Molecular immobilization and recognition are two key events for the development of biosensors. The general ways for the immobilization and recognition of biomolecules include covalent coupling reactions and non-covalent interactions of antigen-antibody, aptamer-target, glycan-lectin, avidin-biotin and boronic acid-diol. Tetradentate nitrilotriacetic acid (NTA) is one of the most common commercial ligands for chelating metal ions. The NTA-metal complexes show high and specific affinity toward hexahistidine tags. Such metal complexes have been widely utilized in protein separation and immobilization for diagnostic applications since most of commercialized proteins have been integrated with hexahistidine tags by synthetic or recombinant techniques. This review focused on the development of biosensors with NTA-metal complexes as the binding units, mainly including surface plasmon resonance, electrochemistry, fluorescence, colorimetry, surface-enhanced Raman scattering spectroscopy, chemiluminescence and so on.

Histidine-Mediated Synthesis of Chiral Cobalt Oxide Nanoparticles for Enantiomeric Discrimination and Quantification

Chiral transition metal oxide nanoparticles (CTMOs) are attracting a lot of attention due to their fascinating properties. Nevertheless, elucidating the chirality induction mechanism often remains a major challenge. Herein, the synthesis of chiral cobalt oxide nanoparticles mediated by histidine (Co₃ O₄ @L-His and Co₃ O₄ @D-His for nanoparticles synthesized in the presence of L- and D-histidine, respectively) is investigated. Interestingly, these CTMOs exhibit remarkable and tunable chiroptical properties. Their analysis by x-ray photoelectron, Fourier transform infrared, and ultraviolet-visible absorption spectroscopy indicates that the ratio of Co²⁺ /Co³⁺ and their interactions with the imidazole groups of histidine are behind their chiral properties. In addition, the use of chiral Co₃ O₄ nanoparticles for the development of sensitive, rapid, and enantioselective circular dichroism-based sensors is demonstrated, allowing direct molecular detection and discrimination between cysteine or penicillamine enantiomers. The circular dichroism response of the chiral Co₃ O₄ exhibits a limit of detection and discrimination of cysteine and penicillamine enantiomers as low as 10 µm. Theoretical calculations suggest that the ligand exchange and the coexistence of both species adsorbed on the oxide surface are responsible for the enantiomeric discrimination. This research will enrich the synthetic approaches to obtain CTMOs and enable the extension of the applications and the discovery of new chiroptical properties.

Trivalent Cobalt Complexes with NNS Tridentate Thiosemicarbazones: Preparation, Structural Study and Investigation of Antibacterial Activity and Cytotoxicity against Human Breast Cancer Cells

New complexes of trivalent cobalt with substituted thiosemicarbazone ligands having an NNS donor system {HL1 = 4-(4-nitrophenyl)-1-((pyridin-2-yl)methylene)thiosemicarbazide and HL2 = 4-(2,5-dimethoxyphenyl)-1-((pyridin-2-yl)methylene)thiosemicarbazide} were synthesized via the in situ oxidation of divalent cobalt chloride accompanying its addition to the ligands. The complexes C1 and C2 were characterized via elemental (CHNS) analysis and 1H NMR, FT-IR and UV-Vis. spectroscopic data. Further, conductometric studies on the DMF solutions of the complexes indicated their 1:1 nature, and their diamagnetism revealed the low-spin trivalent oxidation state of the cobalt in the complexes. The X-ray diffraction analysis of complex C1 indicated that it crystallizes in the triclinic space group P-1. The metal exhibits an octahedral environment built by two anionic ligands bound via pyridine nitrogen, imine nitrogen and thiol sulfur atoms. The complex is counterbalanced by a chloride ion. In addition, two lattice water molecules were detected in the asymmetric unit of the unit cell. The ligand HL2 (20 mg/mL in DMSO) displayed inhibition zones of 10 mm against both S. aureus and E. coli, and the same concentration of the respective complex raised this activity to 15 and 12 mm against these bacterial strains, respectively. As a comparison, ampicillin inhibited these bacterial strains by 21 and 25 mm, respectively. Screening assay by HL1 on four human cancer cells revealed the most enhanced activity against the breast MCF-7 cells. The induced growth inhibitions in the MCF-7 cells by all compounds (0–100 μg/mL) have been detected. The ligands {HL1 and HL2} and complex C2 gave inhibitions with IC50 values of 52.4, 145.4 and 49.9 μM, respectively. These results are more meaningful in comparison with similar cobalt complexes, but less efficient compared with the inhibition with IC50 of 9.66 μM afforded by doxorubicin. In addition, doxorubicin, HL1 and HL2 induced cytotoxicity towards healthy BHK cells with IC50 values of 36.42, 54.8 and 110.6 μM, but surviving fractions of 66.1% and 62.7% of these cells were detected corresponding to a concentration of 100 μg/mL of the complexes (136.8 μM of C1 and 131.4 μM of C2).

Binding of His-tagged fluorophores to lipid bilayers of giant vesicles

His-tagged molecules can be attached to lipid bilayers via certain anchor lipids, a method that has been widely used for the biofunctionalization of membranes and vesicles. To observe the membrane-bound molecules, it is useful to consider His-tagged molecules that are fluorescent as well. Here, we study two such molecules, green fluorescence protein (GFP) and green-fluorescent fluorescein isothiocyanate (FITC), both of which are tagged with a chain of six histidines (6H) that bind to the anchor lipids within the bilayers. The His-tag 6H is much smaller than the GFP molecule but somewhat larger than the FITC dye. The lipid bilayers form giant unilamellar vesicles (GUVs), the behavior of which can be directly observed in the optical microscope. We apply and compare three well-established preparation methods for GUVs: electroformation on platinum wire, polyvinyl alcohol (PVA) hydrogel swelling, and electroformation on indium tin oxide (ITO) glass. Microfluidics is used to expose the GUVs to a constant fluorophore concentration in the exterior solution. The brightness of membrane-bound 6H-GFP exceeds the brightness of membrane-bound 6H-FITC, in contrast to the quantum yields of the two fluorophores in solution. In fact, 6H-FITC is observed to be strongly quenched by the anchor lipids which bind the fluorophores via Ni²⁺ ions. For both 6H-GFP and 6H-FITC, the membrane fluorescence is measured as a function of the fluorophores' molar concentration. The theoretical analysis of these data leads to the equilibrium dissociation constants K_d = 37.5 nM for 6H-GFP and K_d = 18.5 nM for 6H-FITC. We also observe a strong pH-dependence of the membrane fluorescence.

Highly shape- and size-tunable membrane nanopores made with DNA

Membrane nanopores are key for molecular transport in biology, portable DNA sequencing^1-4, label-free single-molecule analysis^5-14 and nanomedicine⁵. Transport traditionally relies on barrel-like channels of a few nanometres width, but there is considerable scientific and technological interest for much wider structures of tunable shape. Yet, these nanopores do not exist in nature and are challenging to build using existing de novo routes for proteins^10,15-17. Here, we show that rational design with DNA can drastically expand the structural and functional range of membrane nanopores. Our design strategy bundles DNA duplexes into pore subunits that modularly arrange to form tunable pore shapes and lumen widths of up to tens of nanometres. Functional units for recognition or signalling can be optionally attached. By dialling in essential parameters, we demonstrate the utility and potential of the custom-engineered nanopores by electrical direct single-molecule sensing of 10-nm-sized proteins using widely used research and hand-held analysis devices. The designer nanopores illustrate how DNA nanotechnology can deliver functional biomolecular structures to be used in synthetic biology, single-molecule enzymology and biophysical analysis, as well as portable diagnostics and environmental screening.

Injectable Liposome-based Supramolecular Hydrogels for the Programmable Release of Multiple Protein Drugs

Directing biological functions is at the heart of next-generation biomedical initiatives in tissue and immuno-engineering. However, the ambitious goal of engineering complex biological networks requires the ability to precisely perturb specific signaling pathways at distinct times and places. Using lipid nanotechnology and the principles of supramolecular self-assembly, we developed an injectable liposomal nanocomposite hydrogel platform to precisely control the release of multiple protein drugs. By integrating modular lipid nanotechnology into a hydrogel, we introduced multiple mechanisms of release based on liposome surface chemistry. To validate the utility of this system for multi-protein delivery, we demonstrated synchronized, sustained, and localized release of IgG antibody and IL-12 cytokine in vivo, despite the significant size differences between these two proteins. Overall, liposomal hydrogels are a highly modular platform technology with the ability the mediate orthogonal modes of protein release and the potential to precisely coordinate biological cues both in vitro and in vivo.

Direct Measurements of the Cobalt-thiolate Bonds Strength in Rubredoxin by Single-Molecule Force Spectroscopy

Cobalt is a trace transition metal. Although it is not abundant on earth, tens of cobalt-containing proteins exist in life. Moreover, the characteristic spectrum of Co(II) ion makes it a powerful probe for the characterization of metal-binding proteins through the formation of cobalt-ligand bonds. Since most of these natural and artificial cobalt-containing proteins are stable, we believe that these cobalt-ligand bonds in the protein system are also mechanically stable. To prove this, we used atomic force microscopy-based single-molecule force spectroscopy (AFM-SMFS) to directly measure the rupture force of Co(II)-thiolate bond in Co-substituted rubredoxin (CoRD). By combining the chemical denature/renature method for building metalloprotein and cysteine coupling-based polyprotein construction strategy, we successfully prepared the polyprotein sample (CoRD) n suitable for single-molecule study. Thus, we quantified the strength of Co(II)-thiolate bonds in  rubredoxin with a rupture force of ~140 pN, revealing that the bond is a stable chemical bond. In addition, the Co-S bond is more labile than the Zn-S bond in proteins, similar to the result from the metal-competing titration experiment.

An Earth-abundant cobalt based photocatalyst: visible light induced direct (het)arene C-H arylation and CO2 capture

In this work, we have reported a noble metal free heterogeneous photocatalyst to carry out direct (het)arene C-H arylation and solvent-free CO₂ capture via single-electron transfer processes at room temperature and under pressure. The catalytic system comprises a cobalt(III) complex grafted over the silica coated magnetic support for the efficient recovery of the photocatalytic moiety without hampering its light-harvesting capability. The novel Earth-abundant cobalt(III) based photocatalyst possesses various fascinating properties such as high surface area to volume ratios, large pore volume, crystalline behaviour, high metal loading, excellent stability and reusability. The general efficacy of the highly abundant and low-cost cobalt based heterogeneous nanocatalyst was checked for the selective conversion of aryldiazonium salts into synthetically and pharmaceutically significant biaryl motifs under ambient conditions upon irradiation with visible light. The highly efficient photocatalytic conversion of carbon dioxide (CO₂) to a value-added chemical was accomplished under mild reaction conditions with high selectivity, showing the added benefit of operational simplicity.

Oriented immobilization of antibodies onto sensing platforms - A critical review

The immunosensor has been proven a versatile tool to detect various analytes, such as food contaminants, pathogenic bacteria, antibiotics and biomarkers related to cancer. To fabricate robust and reproducible immunosensors with high sensitivity, the covalent immobilization of immunoglobulins (IgGs) in a site-specific manner contributes to better performance. Instead of the random IgG orientations result from the direct yet non-selective immobilization techniques, this review for the first time introduces the advances of stepwise yet site-selective conjugation strategies to give better biosensing efficiency. Noncovalently adsorbing IgGs is the first but decisive step to interact specifically with the Fc fragment, then following covalent conjugate can fix this uniform and antigens-favorable orientation irreversibly. In this review, we first categorized this stepwise strategy into two parts based on the different noncovalent interactions, namely adhesive layer-mediated interaction onto homofunctional support and layer-free interaction onto heterofunctional support (which displays several different functionalities on its surface that are capable to interact with IgGs). Further, the influence of ligands characteristics (synthesis strategies, spacer requirements and matrices selection) on the heterofunctional support has also been discussed. Finally, conclusions and future perspectives for the real-world application of stepwise covalent conjugation are discussed. This review provides more insights into the fabrication of high-efficiency immunosensor, and special attention has been devoted to the well-orientation of full-length IgGs onto the sensing platform.

Stability of HA2 Prefusion Structure and pH-Induced Conformational Changes in the HA2 Domain of H3N2 Hemagglutinin

Influenza hemagglutinin is the fusion protein that mediates fusion of the viral and host membranes through a large conformational change upon acidification in the developing endosome. The "spring-loaded" model has long been used to describe the mechanism of hemagglutinin and other type 1 viral glycoproteins. This model postulates a metastable conformation of the HA2 subunit, caged from adopting a lower-free energy conformation by the HA1 subunit. Here, using a combination of biochemical and spectroscopic methods, we study a truncated construct of HA2 (HA2*, lacking the transmembrane domain) recombinantly expressed in Escherichia coli as a model for HA2 without the influence of HA1. Our data show that HA2* folds into a conformation like that of HA2 in full length HA and forms trimers. Upon acidification, HA2* undergoes a conformational change that is consistent with the change from pre- to postfusion HA2 in HA. This conformational change is fast and occurs on a time scale that is not consistent with aggregation. These results suggest that the prefusion conformation of HA2 is stable and the change to the postfusion conformation is due to protonation of HA2 itself and not merely uncaging by HA1.

Gold nanoparticle enhanced multiplexed biosensing on a fiber optic surface plasmon resonance probe

We present an innovative multiplexing concept on a fiber optic surface plasmon resonance (FO-SPR) platform and demonstrate for the first time the simultaneous detection of two targets using the same FO sensor probe. Co(III)-NTA chemistry was used for oriented and stable co-immobilization of two different His₆-tagged bioreceptors. T2C2 and MDTCS (i.e. fragments of the ADAMTS13 metalloprotease linked to the thrombotic thrombocytopenic purpura disorder) served as model system bioreceptors together with their respective targets (4B9 and II-1 antibodies). Gold nanoparticles were used here in an original way for discriminating the two targets in the same sample, in addition to their traditional signal amplification-role. After verifying the specificity of the selected model system, we studied the bioreceptor surface density and immobilization order. Innovative approach to lower the bioreceptor concentration below surface saturation resulted in an optimal detection of both targets, whereas the order of immobilization of the two bioreceptors did not give any significant difference. By sequentially immobilizing the T2C2 and MDTC bioreceptors, we established calibration curves in buffer and 100-fold diluted human blood plasma. This resulted in calculated limits of detection of 3.38 and 2.31 ng/mL in diluted plasma for 4B9 and II-1, respectively, indicating almost the same sensitivity as in buffer. Importantly, we also proved the applicability of the established calibration curves for quantifying the targets at random and more realistic ratios, directed by the design of experiments. This multiplexing study further expands the repertoire of applications on the FO-SPR biosensing platform, which together with its intrinsic features opens up great opportunities for diagnostics and life sciences.

DNA origami-based protein networks: from basic construction to emerging applications

Natural living systems are driven by delicate protein networks whose functions are precisely controlled by many parameters, such as number, distance, orientation, and position. Focusing on regulation rather than just imitation, the construction of artificial protein networks is important in many research areas, including biomedicine, synthetic biology and chemical biology. DNA origami, sophisticated nanostructures with rational design, can offer predictable, programmable, and addressable scaffolds for protein assembly with nanometer precision. Recently, many interdisciplinary efforts have achieved the precise construction of DNA origami-based protein networks, and their emerging application in many areas. To inspire more fantastic research and applications, herein we highlight the applicability and potentiality of DNA origami-based protein networks. After a brief introduction to the development and features of DNA origami, some important factors for the precise construction of DNA origami-based protein networks are discussed, including protein-DNA conjugation methods, networks with different patterns and the controllable parameters in the networks. The discussion then focuses on the emerging application of DNA origami-based protein networks in several areas, including enzymatic reaction regulation, sensing, bionics, biophysics, and biomedicine. Finally, current challenges and opportunities in this research field are discussed.

Developing Eco-Friendly and Cost-Effective Porous Adsorbent for Carbon Dioxide Capture

To address the issue of global warming and climate change issues, recent research efforts have highlighted opportunities for capturing and electrochemically converting carbon dioxide (CO₂). Despite metal doped polymers receiving widespread attention in this respect, the structures hitherto reported lack in ease of synthesis with scale up feasibility. In this study, a series of mesoporous metal-doped polymers (MRFs) with tunable metal functionality and hierarchical porosity were successfully synthesized using a one-step copolymerization of resorcinol and formaldehyde with Polyethyleneimine (PEI) under solvothermal conditions. The effect of PEI and metal doping concentrations were observed on physical properties and adsorption results. The results confirmed the role of PEI on the mesoporosity of the polymer networks and high surface area in addition to enhanced CO₂ capture capacity. The resulting Cobalt doped material shows excellent thermal stability and promising CO₂ capture performance, with equilibrium adsorption of 2.3 mmol CO₂/g at 0 °C and 1 bar for at a surface area 675.62 m²/g. This mesoporous polymer, with its ease of synthesis is a promising candidate for promising for CO₂ capture and possible subsequent electrochemical conversion.

Assessing changes in the expression levels of cell surface proteins with a turn-on fluorescent molecular probe

Tri-nitrilotriacetic acid (NTA)-based fluorescent probes were developed and used to image His-tagged-labelled outer membrane protein C (His-OmpC) in live Escherichia coli. One of these probes was designed to light up upon binding, which provided the means to assess changes in the His-OmpC expression levels by taking a simple fluorescence spectrum.

Dynamic Stereoselection of Peptide Helicates and Their Selective Labeling of DNA Replication Foci in Cells*

Although largely overlooked in peptide engineering, coordination chemistry offers a new set of interactions that opens unexplored design opportunities for developing complex molecular structures. In this context, we report new artificial peptide ligands that fold into chiral helicates in the presence of labile metal ions such as Fe^II and Co^II . Heterochiral β-turn-promoting sequences encode the stereoselective folding of the peptide ligands and define the physicochemical properties of their corresponding metal complexes. Circular dichroism and NMR spectroscopy in combination with computational methods allowed us to identify and determine the structure of two isochiral ΛΛ-helicates, folded as topological isomers. Finally, in addition to the in-vitro characterization of their selective binding to DNA three-way junctions, cell-microscopy experiments demonstrated that a rhodamine-labeled Fe^II helicate was internalized and selectively stains DNA replication factories in functional cells.

Self-assembling nitrilotriacetic acid nanofibers for tracking and enriching His-tagged proteins in living cells

Specific and expeditious identification and enrichment of target proteins in living cells is often a challenging task. The hexahistidine (6His) tag is frequently used to label artificially engineered proteins produced in prokaryotic or eukaryotic cells. Utilizing the interaction between 6His-tag and nitrilotriacetic acid (NTA) mediated by divalent metal ions (Ni²⁺, Cu²⁺, Zn²⁺ or Co²⁺), we designed and synthesized a series of Nap-G/Biotin/ANA-FFpYGK-NTA probes that, assisted by alkaline phosphatase (ALP), self-assemble into nanofibers. The probe consists of an NTA group that specifically binds to 6His-tag, an FFpY group that promotes self-assembly facilitated by ALP, and a hydrophobic (Nap-G/ANA/Biotin) capping group for various applications. We demonstrate that the ANA-FFpYGK-NTA(Ni²⁺) nanofibers are fit for real-time tracking of His-tagged protein in living cells, and the Biotin-FFpYGK-NTA(Ni²⁺) nanofibers are for isolating His-tagged proteins and other proteins that they interact with.

Measuring Binding Constants of His-Tagged Proteins Using Affinity Chromatography and Ni-NTA Immobilized Enzymes

Affinity chromatography is one way to measure the binding constants of a protein-ligand interaction. Here, we describe a method of measuring a binding constant using Ni-NTA resin to immobilize a His-tagged enzyme and the method of frontal analysis. While other methods of immobilization are possible, using the strong affinity interaction between His-tagged proteins and Ni-NTA supports results in a fast, easy, and gentle method of immobilization. Once the affinity support is created, frontal analysis can be used to measure the binding constant between the protein and various analytes.

Expanding a Portfolio of (FO-) SPR Surface Chemistries with the Co(III)-NTA Oriented Immobilization of His6-Tagged Bioreceptors for Applications in Complex Matrices

Cobalt-nitrilotriacetic acid (Co(III)-NTA) chemistry is a recognized approach for oriented patterning of His₆-tagged bioreceptors. We have applied the matching strategy for the first time on a surface plasmon resonance (SPR) platform, namely, the commercialized fiber optic (FO)-SPR. To accomplish this, His₆-tagged bioreceptor (scFv-33H1F7) and its target PAI-1 were used as a model system, after scrutinizing the specificity of their interaction. When benchmarked to traditional carboxyl-based self-assembled monolayers (SAM), NTA allowed (1) more efficient FO-SPR surface coverage with bioreceptors compared with the former and (2) realization of thus far difficult-to-attain label-free bioassays on the FO-SPR platform in both buffer and 20-fold diluted human plasma. Moreover, Co(III)-NTA surface proved to be compatible with traditional gold nanoparticle-mediated signal amplification in the buffer as well as in 10-fold diluted human plasma, thus expanding the dynamic detection range to low ng/mL. Both types of bioassays revealed that scFv-33H1F7 immobilized on the FO-SPR surface using different concentrations (20, 10, or 5 μg/mL) had no impact on the bioassay sensitivity, accuracy, or reproducibility despite the lowest concentration effectively resulting in close to 20% fewer bioreceptors. Collectively, these results highlight the importance of Co(III)-NTA promoting the oriented patterning of bioreceptors on the FO-SPR sensor surface for securing robust and sensitive bioassays in complex matrices, both in label-free and labeled formats.

Ultrafast in-gel detection by fluorescent super-chelator probes with HisQuick-PAGE

Polyacrylamide gel electrophoresis (PAGE) and immunoblotting (Western blotting) are the most common methods in life science. In conjunction with these methods, the polyhistidine-tag has proven to be a superb fusion tag for protein purification as well as specific protein detection by immunoblotting, which led to a vast amount of commercially available antibodies. Nevertheless, antibody batch-to-batch variations and nonspecific binding complicate the laborious procedure. The interaction principle applied for His-tagged protein purification by metal-affinity chromatography using N-nitrilotriacetic acid (NTA) was employed to develop small high-affinity lock-and-key molecules coupled to a fluorophore. These multivalent NTA probes allow specific detection of His-tagged proteins by fluorescence. Here, we report on HisQuick-PAGE as a fast and versatile immunoblot alternative, using such high-affinity fluorescent super-chelator probes. The procedure allows direct, fast, and ultra-sensitive in-gel detection and analysis of soluble proteins as well as intact membrane protein complexes and macromolecular ribonucleoprotein particles.

Brüchert et al. describe the application of high-affinity super-chelator probes that are linked to a fluorophore for the detection of His-tagged proteins using PAGE. Their system has the advantage over conventional antibody staining in the ease of application and ultra-sensitive detection.

Bioconjugates of Co(III) complexes with Schiff base ligands and cell penetrating peptides: Solid phase synthesis, characterization and antiproliferative activity

In this work we synthesized a chelating Schiff base by a single condensation of salicylaldehyde with 3,4-diamino benzoic acid (1). This ligand was used further for complexation to CoCl₂·6H₂O under nitrogen. In the next step, three six-coordinate Co(III) complexes were synthesized by coordinating this complex with imidazole (2), 2-methyimidazole (3) and N-Boc-l-histidine methyl ester (4) (Boc: tert.-butoxycarbonyl) in axial positions with simultaneous oxidation of Co(II) to Co(III) under ambient environment. All Co(III) complexes were characterized by multinuclear NMR spectroscopy (¹H, ¹³C and ⁵⁹Co NMR), FT-IR, mass spectrometry and HPLC. The Co(III) complexes were conjugated to three different cell penetrating peptides: FFFF (P1), RRRRRRRRRGAL (P2) and FFFFRRRRRRRRRGAL (P3). Standard solid-phase peptide chemistry was used for the synthesis of cell penetrating peptides. Coupling of N-terminal peptides with the cobalt complexes, possessing a carboxylic group on the tetradentate Schiff base ligand, afforded Co(III)-peptide bioconjugates, which were purified by semi-preparative HPLC and characterized by analytical HPLC and mass spectrometry. The antiproliferative activity of the synthesized compounds was studied against different human tumour cell lines: lung cancer A549, liver cancer HepG2 and normal human fibroblasts GM5657T, in comparison with the activity of cisplatin as a reference drug. The bioconjugate 21 containing the Co complex 4 and the combined phenylalanine and polyarginine cell penetrating sequence P3 shows better activity against the liver cancer line HepG2 than the parent Co(III) complex 4.

Tris-(Nitrilotriacetic Acid)-Decorated Polymer Conjugates as Tools for Immobilization and Visualization of His-Tagged Proteins

Recombinant proteins are commonly expressed with artificial affinity tags for purification, immobilization and characterization. The most frequently used tag, His-tag, is a sequence of consecutive histidine residues fused to the protein of interest. Specialized small molecules that bind His-tag are primarily used for purification, while antibodies are used for protein analysis. However, various issues may be encountered with the use of antibodies. Low inherent stability, the difficulty of introducing chemical modifications, and often-unreliable batch-to-batch consistency are among the limiting factors that call for better alternatives. Recently described polymer conjugates of N-(2-hydroxypropyl) methacrylamide and low-molecular-weight functional ligands, so-called iBodies, are antibody mimetics capable of replacing antibodies in biochemical applications. We tailored this system for methods utilizing His-tag by accessorizing the polymer carrier with tris-nitrilotriacetic acid targeting ligands. These anti-polyHis iBodies are additionally accessorized with fluorophores, enabling detection, and biotin ligands, enabling immobilization. Here, we characterized anti-polyHis iBodies and explored their use as antibody mimetics. We tested their stability, as well as the influence of different metal mediators and His-tag lengths on binding. With high affinity and stability, iBodies represent a new alternative for immobilization and visualization of His-tagged proteins.

Single molecule force spectroscopy reveals that the oxidation state of cobalt ions plays an important role in enhancing the mechanical stability of proteins

Engineered bi-histidine (biHis)-based metal chelation is a general and robust method to enhance the mechanical stability of proteins. Here we used single molecule force spectroscopy techniques to investigate the effect of binding of Co²⁺/Co³⁺ on the mechanical stability of an engineered biHis mutant of protein GB1, G6-53. We found that the binding of Co²⁺/Co³⁺ can lead to an enhancement of the mechanical stability of G6-53, but the degree of enhancement is drastically different. The binding of Co²⁺ can only lead to marginal enhancement of G6-53's mechanical stability, while Co³⁺ has a much stronger effect. This large difference is likely due to the large difference in thermodynamic stability and kinetic lability of Co²⁺ and Co³⁺ complexes. These results opened up new avenues towards fine tuning the mechanical properties of proteins.

Protein adaptors assemble functional proteins on DNA scaffolds

DNA is an attractive molecular building block to construct nanoscale structures for a variety of applications. In addition to their structure and function, modification the DNA nanostructures by other molecules opens almost unlimited possibilities for producing functional DNA-based architectures. Among the molecules to functionalize DNA nanostructures, proteins are one of the most attractive candidates due to their vast functional variations. DNA nanostructures loaded with various types of proteins hold promise for applications in the life and material sciences. When loading proteins of interest on DNA nanostructures, the nanostructures by themselves act as scaffolds to specifically control the location and number of protein molecules. The methods to arrange proteins of interest on DNA scaffolds at high yields while retaining their activity are still the most demanding task in constructing usable protein-modified DNA nanostructures. Here, we provide an overview of the existing methods applied for assembling proteins of interest on DNA scaffolds. The assembling methods were categorized into two main classes, noncovalent and covalent conjugation, with both showing pros and cons. The recent advance of DNA-binding adaptor mediated assembly of proteins on the DNA scaffolds is highlighted and discussed in connection with the future perspectives of protein assembled DNA nanoarchitectures.

Kinetics of Histidine-Tagged Protein Association to Nickel-Decorated Liposome Surfaces

Nickel-chelating lipids offer a convenient platform for reversible immobilization of histidine-tagged proteins to liposome surfaces. This interaction recently found utility as a model system for studying membrane remodeling triggered by protein crowding. Despite its wide array of utility, the molecular details of transient protein association to the lipid surfaces decorated with such chelator lipids remains poorly understood. In this study, we explore the kinetics of protein-liposome association across a wide concentration range using stopped-flow fluorescence. The fluorescence of histidine-tagged protein containing an intrinsic fluorophore (superfolder green fluorescent protein, SfGFP) was quenched upon binding to Ni-NTA-modified liposomes containing the quencher Dabsyl-PE lipids. Stopped-flow fluorescence reveals a complex, multiexponential binding behavior with a fast (k_obs ∼ 10-20 s^-1) phase and slower (k_obs < 4 s^-1) phase. Interestingly, the observed rates for the slower phase increase initially under low concentrations but start decreasing once a critical concentration is reached. Despite differences in the binding time scales, we observe that the trend of decreasing rates is reproducible irrespective of the chelator lipid doping level, protein surface charge, or lipid composition. Consideration of the protein footprint and membrane surface area occupancy leads us to conclude that the multiphasic binding behavior is reflective of protein binding via two distinct binding conformations. We propose that preliminary steps in protein association involve binding of a sterically occlusive side-on conformation followed by reorganization that leads to an end-on conformation with increased packing density. These results are important for the improvement of histidine-tag-based immobilization strategies and offer mechanistic insight into intermediates preceding membrane bending driven by protein crowding.

Cobalt-Cross-Linked, Redox-Responsive Spy Network Protein Hydrogels

Although assembly of recombinant proteins by SpyTag/SpyCatcher chemistry has proven to be a versatile approach for creating bioactive hydrogels, the resulting Spy networks often exhibit weak mechanics due to the poor efficiency of interchain cross-linking. Here we leverage metal/ligand (i.e., cobalt/His6-tag) coordination interactions to modulate the bulk mechanics of the protein networks. The drastic difference between the Co²⁺ and Co³⁺ complexes in thermodynamic and kinetic properties enabled us to regulate the materials' properties and to immobilize and release recombinant proteins in a redox-dependent manner. The resulting hydrogels are capable of not only supporting cell growth and proliferation, but also influencing specific cell signaling via immobilized growth factors such as leukemia inhibitory factor (LIF). The integrated use of stimuli-responsive metal coordination and SpyTag/SpyCatcher chemistry opens up a new dimension for designing bioactive protein materials.

Exploiting His-Tags for Absolute Quantitation of Exogenous Recombinant Proteins in Biological Matrices: Ruthenium as a Protein Tracer

Metal labeling and ICP MS detection offer an alternative to commonly accepted techniques that are currently used to quantitate exogenous proteins in vivo, but modifying the protein surface with metal-containing groups inevitably changes its biophysical properties and is likely to affect trafficking and biodistribution. The approach explored in this work takes advantage of the presence of hexa-histidine tags in many recombinant proteins, which have high affinity toward a range of metals. While many divalent metals bind to poly histidine sequences reversibly, oxidation of imidazole-bound Co^II or Ru^II is known to result in a dramatic increase of the binding strength. In order to evaluate the feasibility of using imidazole-bound metal oxidation as a means of attaching permanent tags to polyhistidine segments, a synthetic peptide YPDFEDYWMKHHHHHH was used as a model. Ru^II can be oxidized under ambient (aerobic) conditions, allowing any oxidation damage to the peptide beyond the metal-binding site to be avoided. The resulting peptide-Ru^III complex is very stable, with the single hexa-histidine segment capable of accommodating up to three metal ions. Localization of Ru^III within the hexa-histidine segment of the peptide was confirmed by tandem mass spectrometry. The Ru^III/peptide binding appears to be irreversible, with both low- and high-molecular weight biologically relevant scavengers failing to strip the metal from the peptide. Application of this protocol to labeling a recombinant form of an 80 kDa protein transferrin allowed Ru^III to be selectively placed within the His-tag segment. The metal label remained stable in the presence of ubiquitous scavengers and did not interfere with the receptor binding, while allowing the protein to be readily detected in serum at sub-nM concentrations. The results of this work suggest that ruthenium lends itself as an ideal metal tag for selective labeling of His-tag containing recombinant proteins to enable their sensitive detection and quantitation with ICP MS.

Oriented Immobilization and Quantitative Analysis Simultaneously Realized in Sandwich Immunoassay via His-Tagged Nanobody

Despite the advantages of the nanobody, the unique structure limits its use in sandwich immunoassay. In this study, a facile protocol of sandwich immunoassay using the nanobody was established. In brief, β amyloid and SH2, an anti-β amyloid nanobody, were used as capture antibody and antigen, respectively. The SH2 fused with His-tag was first purified and absorbed on Co²⁺-NTA functional matrix and then immobilized through H₂O₂ oxidation of Co²⁺ to Co³⁺ under the optimized conditions. Then, 150 mM imidazole and 20 mM EDTA were introduced to remove the unbound SH2. The immobilized SH2 showed highly-sensitive detection of β amyloid. It is interesting that the quantification of the sandwich immunoassay was carried out by determining the His-tag of the detection nanobody, without interference from the His-tag of the capture nanobody. The immobilized SH2 detached exhibited outstanding stability during 30 days of storage. Taken together, His6-tag facilitated both the oriented immobilization of capture antibody and quantitative assay of detection antibody in sandwich immunoassay. We propose a facile and efficient sandwich immunoassay method that opens new avenue to the study of His-tagged protein interactions.

Physical properties, ligand substitution reactions, and biological activity of Co(iii)-Schiff base complexes

Four cobalt(iii) complexes of the general formula [Co(Schiff base)(L)2]+, where L is ammonia (NH3) or 3-fluorobenzylamine (3F-BnNH2), were synthesized. The complexes were characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography. Their electrochemical properties, ligand substitution mechanisms, and ligand exchange rates in aqueous buffer were investigated. These physical properties were correlated to the cellular uptake and anticancer activities of the complexes. The complexes undergo sequential, dissociative ligand substitution, with the exchange rates depending heavily on the axial ligands. Eyring analyses revealed that the relative ligand exchange rates were largely impacted by differences in the entropy, rather than enthalpy, of activation for the complexes. Performing the substitution reactions in the presence of ascorbate led to a change in the reaction profile and kinetics, but no change in the final product. The cytotoxic activity of the complexes correlates with both the ligand exchange rate and reduction potential, with the more easily reduced and rapidly substituted complexes showing higher toxicity. These relationships may be valuable for the rational design of Co(iii) complexes as anticancer or antiviral prodrugs.

Multivalent Chelators for In Vivo Protein Labeling

With the advent of single-molecule methods, chemoselective and site-specific labeling of proteins evolved to become a central aspect in chemical biology as well as cell biology. Protein labeling demands high specificity, rapid as well as efficient conjugation, while maintaining low concentration and biocompatibility under physiological conditions. Generic methods that do not interfere with the function, dynamics, subcellular localization of proteins, and crosstalk with other factors are crucial to probe and image proteins in vitro and in living cells. Alternatives to enzyme-based tags or autofluorescent proteins are short peptide-based recognition tags. These tags provide high specificity, enhanced binding rates, bioorthogonality, and versatility. Here, we report on recent applications of multivalent chelator heads, recognizing oligohistidine-tagged proteins. The striking features of this system has facilitated the analysis of protein complexes by single-molecule approaches.

Bifunctional nickel–iminodiacetic acid-core–shell silica nanoparticles for the exclusion of high molecular weight proteins and purification of His-tagged recombinant proteins

Silica nanoparticles were synthesized and chemically modified with iminodiacetic acid (IDA) and Ni²⁺ ions surrounded by a bis-acrylamide polymeric shell to obtain a new core–shell immobilized metal affinity chromatography (IMAC) based material.

Metal complexes as "protein surface mimetics"

A key challenge in chemical biology is to identify small molecule regulators for every single protein. However, protein surfaces are notoriously difficult to recognise with synthetic molecules, often having large flat surfaces that are poorly matched to traditional small molecules. In the surface mimetic approach, a supramolecular scaffold is used to project recognition groups in such a manner as to make multivalent non-covalent contacts over a large area of protein surface. Metal based supramolecular scaffolds offer unique advantages over conventional organic molecules for protein binding, including greater stereochemical and geometrical diversity conferred through the metal centre and the potential for direct assessment of binding properties and even visualisation in cells without recourse to further functionalisation. This feature article will highlight the current state of the art in protein surface recognition using metal complexes as surface mimetics.

Highly stable protein immobilizationviamaleimido-thiol chemistry to monitor enzymatic activity

Combining a novel protein immobilisation method with multivariate curve resolution enables the direct observation of biocatalysis by ATR-FTIR spectroscopy.

Selective binding, magnetic separation and purification of histidine-tagged protein using biopolymer magnetic core-shell nanoparticles

In previous studies, we synthesized the magnetic core-shell structured Fe₃O₄/PMG/IDA-Ni²⁺ nanoparticles. The Ni²⁺ on the surface of nanoparticles provides abundant docking sites for histidine, and the composite nanoparticles showed potential applications in the separation and purification of histidine-tagged (His-tagged) proteins. Meanwhile, the presence of the superparamagnetic core (Fe₃O₄) in the nanoparticles allows them to be quickly separated and purified by an external magnetic field. Herein, the ability of magnetic nanoparticles to purify His-tagged human superoxide dismutase 1 (hSOD1) was verified. SDS-PAGE and activity data showed His-tagged hSOD1 specifically bound to Fe₃O₄/PMG/IDA-Ni²⁺, and there was no significant competition for binding between final and three intermediate products. The binding capacity of nanoparticles can reach to 62.0 mg/g (dry weight of hSOD1/nanoparticles). The nanoparticle-bound hSOD1 exhibited better thermal and storage stability compared to free hSOD1. Furthermore, the purification efficiency of the magnetic nanoparticles in the separation and purification of His-tagged proteins was comparable to the other two commercial materials (High Affinity Ni-NTA Resin, HisPur Ni-NTA Magnetic Beads). Finally, the magnetic nanoparticles can be reused in the binding of His-tagged protein for multiple times. In conclusion, the nanoparticles are ready to be applied in the separation and purification of His-tagged protein.

Unexpected lability of the [RuIII(phtpy)Cl3] complex

Ruthenium(iii) complexes are known for their high stability and inertness. To the best of our knowledge, the only well-characterized example of a labile Ru(iii) complex is [Ru^III(edta)(H₂O)] as a consequence of an intramolecular hydrogen bonding leading to the formation of a large opening in the molecule front, thus changing the mechanism from dissociative to associative. Compelling experimental evidence is presented demonstrating that the [Ru^III(phtpy)Cl₃] complex is labile, also indicating that the Ru(iii)-phtpy bond is much weaker than expected, in contrast to the strongly π-back-bonding stabilized Ru(ii)-phtpy bond.