Self-assembled monolayers containing terminal mono-, bis-, and tris-nitrilotriacetic acid groups: characterization and application

Molecule Clustering Dynamics in the Molecular Doping Process of Si(111) with Diethyl-propyl-phosphonate

The molecular doping (MD) process is based on the deposition of dopant-containing molecules over the surface of a semiconductor substrate, followed by the thermal diffusion step. Previous studies suggest that, during the deposition, the molecules nucleate clusters, and at prolonged deposition times, they grow into self-assembled layers on the sample to be doped. Little is known about the influence of nucleation kinetics on the final properties of these layers and how they change when we modify the solution properties. In this work, we examine the nucleation rate and the molecular surface coverage kinetics of diethyl-propyl phosphonate on silicon at different solution concentrations and how these conditions influence the final electrical properties of the doped samples. We present a high-resolution morphological characterization of the as-deposited molecules together with the electrical results of the final doped samples. The experimental results show a non-obvious behavior, explained through understanding of the competition between the molecules' physisorption and chemisorption mechanisms. As a consequence, due to the deeper knowledge of the deposition phase, a finer tuning of the conductive properties of MD-doped samples is achieved.

Rapid and regenerable surface plasmon resonance determinations of biomarker concentration and biomolecular interaction based on tris-nitrilotriacetic acid chips

The tris-nitrilotriacetic acid (tris-NTA) chip has been used for surface plasmon resonance (SPR) kinetic studies involving histidine (His)-tagged proteins. However, its full potential, especially for analyte quantification in complex biological media, has not been realized due to a lack of systematic studies on the factors governing ligand immobilization, surface regeneration, and data analysis. We demonstrate that the tris-NTA chip not only retains His-tagged proteins more strongly than its mono-NTA counterpart, but also orients them more uniformly than protein molecules coupled to carboxymethylated dextran films. We accurately and rapidly quantified immunoglobulin (IgG) molecules in sera by using the initial association phase of their conjugation with His-tagged protein G densely immobilized onto the tris-NTA chip, and established criteria for selecting the optimal time for constructing the calibration curve. The method is highly reproducible (less than 2% RSD) and three orders of magnitude more sensitive than immunoturbidimetry. In addition, we found that the amount of His-protein immobilized is highly dependent on the protein isoelectric point (pI). Reliable kinetic data in a multi-channel SPR instrument can also be rapidly obtained by using a low density of immobilized His-tagged protein. The experimental parameters and procedures outlined in this study help expand the range of SPR applications involving His-tagged proteins.

Probing fibronectin adsorption on chemically defined surfaces by means of single molecule force microscopy

Atomic force microscope (AFM) based single molecule force spectroscopy (SMFS) and a quartz crystal microbalance (QCM) were respectively employed to probe interfacial characteristics of fibronectin fragment FNIII^8–14 and full-length fibronectin (FN) on CH₃–, OH–, COOH–, and NH₂-terminated alkane-thiol self-assembled monolayers (SAMs). Force-distance curves acquired between hexahistidine-tagged FNIII^8–14 immobilised on trisNTA-Ni²⁺ functionalized AFM cantilevers and the OH and COOH SAM surfaces were predominantly ‘loop-like’ (76% and 94% respectively), suggesting domain unfolding and preference for ‘end-on’ oriented binding, while those generated with NH₂ and CH₃ SAMs were largely ‘mixed type’ (81% and 86%, respectively) commensurate with unravelling and desorption, and ‘side-on’ binding. Time-dependent binding of FN to SAM-coated QCM crystals occurred in at least two phases: initial rapid coverage over the first 5 min; and variably diminishing adsorption thereafter (5–70 min). Loading profiles and the final hydrated surface concentrations reached (~ 950, ~ 1200, ~ 1400, ~ 1500 ng cm⁻² for CH₃, OH, COOH and NH₂ SAMs) were consistent with: space-filling ‘side-on’ orientation and unfolding on CH₃ SAM; greater numbers of FN molecules arranged ‘end-on’ on OH and especially COOH SAMs; and initial ‘side-on’ contact, followed by either (1) gradual tilting to a space-saving ‘end-on’ configuration, or (2) bi-/multi-layer adsorption on NH₂ SAM.

Simplified detection of polyhistidine-tagged proteins in gels and membranes using a UV-excitable dye and a multiple chelator head pair

The polyhistidine tag (His-tag) is one of the most popular protein tags used in the life sciences. Traditionally, the detection of His-tagged proteins relies on immunoblotting with anti-His antibodies. This approach is laborious for certain applications, such as protein purification, where time and simplicity are critical. The His-tag can also be directly detected by metal ion-loaded nickel-nitrilotriacetic acid-based chelator heads conjugated to fluorophores, which is a convenient alternative method to immunoblotting. Typically, such chelator heads are conjugated to either green or red fluorophores, the detection of which requires specialized excitation sources and detection systems. Here, we demonstrate that post-run staining is ideal for His-tag detection by metal ion-loaded and fluorescently labeled chelator heads in PAGE and blot membranes. Additionally, by comparing the performances of different chelator heads, we show how differences in microscopic affinity constants translate to macroscopic differences in the detection limits in environments with limited diffusion, such as PAGE. On the basis of these results, we devise a simple approach, called UVHis-PAGE, that uses metal ion-loaded and fluorescently labeled chelator heads to detect His-tagged proteins in PAGE and blot membranes. Our method uses a UV transilluminator as an excitation source, and the results can be visually inspected by the naked eye.

The GDP-Bound State of Mitochondrial Mfn1 Induces Membrane Adhesion of Apposing Lipid Vesicles through a Cooperative Binding Mechanism

Mitochondria are double-membrane organelles that continuously undergo fission and fusion. Outer mitochondrial membrane fusion is mediated by the membrane proteins mitofusin 1 (Mfn1) and mitofusin 2 (Mfn2), carrying a GTP hydrolyzing domain (GTPase) and two coiled-coil repeats. The detailed mechanism on how the GTP hydrolysis allows Mfns to approach adjacent membranes into proximity and promote their fusion is currently under debate. Using model membranes built up as giant unilamellar vesicles (GUVs), we show here that Mfn1 promotes membrane adhesion of apposing lipid vesicles. The adhesion forces were sustained by the GDP-bound state of Mfn1 after GTP hydrolysis. In contrast, the incubation with the GDP:AlF4−, which mimics the GTP transition state, did not induce membrane adhesion. Due to the flexible nature of lipid membranes, the adhesion strength depended on the surface concentration of Mfn1 through a cooperative binding mechanism. We discuss a possible scenario for the outer mitochondrial membrane fusion based on the modulated action of Mfn1.

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.

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.

The Scaffold Design of Trivalent Chelator Heads Dictates Affinity and Stability for Labeling His-tagged Proteins in vitro and in Cells

Small chemical/biological interaction pairs are at the forefront in tracing protein function and interaction at high signal-to-background ratios in cellular pathways. However, the optimal design of scaffold, linker, and chelator head still deserve systematic investigation to achieve the highest affinity and kinetic stability for in vitro and especially cellular applications. We report on a library of N-nitrilotriacetic acid (NTA)-based multivalent chelator heads (MCHs) built on linear, cyclic, and dendritic scaffolds and compare these with regard to their binding affinity and stability for the labeling of cellular His-tagged proteins. Furthermore, we describe a new approach for tracing cellular target proteins at picomolar probe concentrations in cells. Finally, we outline fundamental differences between the MCH scaffolds and define a cyclic trisNTA chelator that displays the highest affinity and kinetic stability of all reported reversible, low-molecular-weight interaction pairs.

Controlling Protein Surface Orientation by Strategic Placement of Oligo-Histidine Tags

We report oriented immobilization of proteins using the standard hexahistidine (His6)-Ni2+:NTA (nitrilotriacetic acid) methodology, which we systematically tuned to give control of surface coverage. Fluorescence microscopy and surface plasmon resonance measurements of self-assembled monolayers (SAMs) of red fluorescent proteins (TagRFP) showed that binding strength increased by 1 order of magnitude for each additional His6-tag on the TagRFP proteins. All TagRFP variants with His6-tags located on only one side of the barrel-shaped protein yielded a 1.5 times higher surface coverage compared to variants with His6-tags on opposite sides of the so-called β-barrel. Time-resolved fluorescence anisotropy measurements supported by polarized infrared spectroscopy verified that the orientation (and thus coverage and functionality) of proteins on surfaces can be controlled by strategic placement of a His6-tag on the protein. Molecular dynamics simulations show how the differently tagged proteins reside at the surface in "end-on" and "side-on" orientations with each His6-tag contributing to binding. Also, not every dihistidine subunit in a given His6-tag forms a full coordination bond with the Ni2+:NTA SAMs, which varied with the position of the His6-tag on the protein. At equal valency but different tag positions on the protein, differences in binding were caused by probing for Ni2+:NTA moieties and by additional electrostatic interactions between different fractions of the β-barrel structure and charged NTA moieties. Potential of mean force calculations indicate there is no specific single-protein interaction mode that provides a clear preferential surface orientation, suggesting that the experimentally measured preference for the end-on orientation is a supra-protein, not a single-protein, effect.

Layer-by-Layer Deposition with Polymers Containing Nitrilotriacetate, A Convenient Route to Fabricate Metal- and Protein-Binding Films

This paper describes a convenient synthesis of nitrilotriacetate (NTA)-containing polymers and subsequent layer-by-layer adsorption of these polymers on flat surfaces and in membrane pores. The resulting films form NTA-metal-ion complexes and capture 2-3 mmol of metal ions per mL of film. Moreover, these coatings bind multilayers of polyhistidine-tagged proteins through association with NTA-metal-ion complexes. Inclusion of acrylic acid repeat units in NTA-containing copolymers promotes swelling to increase protein binding in films on Au-coated wafers. Adsorption of NTA-containing films in porous nylon membranes gives materials that capture ∼46 mg of His-tagged ubiquitin per mL. However, the binding capacity decreases with the protein molecular weight. Due to the high affinity of NTA for metal ions, the modified membranes show modest leaching of Ni(2+) in binding and rinsing buffers. Adsorption of NTA-containing polymers is a simple method to create metal- and protein-binding films and may, with future enhancement of stability, facilitate development of disposable membranes that rapidly purify tagged proteins.

Investigating organic multilayers by spectroscopic ellipsometry: specific and non-specific interactions of polyhistidine with NTA self-assembled monolayers

BACKGROUND

A versatile strategy for protein-surface coupling in biochips exploits the affinity for polyhistidine of the nitrilotriacetic acid (NTA) group loaded with Ni(II). Methods based on optical reflectivity measurements such as spectroscopic ellipsometry (SE) allow for label-free, non-invasive monitoring of molecule adsorption/desorption at surfaces.

RESULTS

This paper describes a SE study about the interaction of hexahistidine (His6) on gold substrates functionalized with a thiolate self-assembled monolayer bearing the NTA end group. By systematically applying the difference spectra method, which emphasizes the small changes of the ellipsometry spectral response upon the nanoscale thickening/thinning of the molecular film, we characterized different steps of the process such as the NTA-functionalization of Au, the adsorption of the His6 layer and its eventual displacement after reaction with competitive ligands. The films were investigated in liquid, and ex situ in ambient air. The SE investigation has been complemented by AFM measurements based on nanolithography methods (nanografting mode).

CONCLUSION

Our approach to the SE data, exploiting the full spectroscopic potential of the method and basic optical models, was able to provide a picture of the variation of the film thickness along the process. The combination of δΔ i +1 ,i (λ), δΨ i +1 ,i (λ) (layer-addition mode) and δΔ(†) i ', i +1(λ), δΨ(†) i ', i +1(λ) (layer-removal mode) difference spectra allowed us to clearly disentangle the adsorption of His6 on the Ni-free NTA layer, due to non specific interactions, from the formation of a neatly thicker His6 film induced by the Ni(II)-loading of the NTA SAM.

Identifying and quantifying two ligand-binding sites while imaging native human membrane receptors by AFM

A current challenge in life sciences is to image cell membrane receptors while characterizing their specific interactions with various ligands. Addressing this issue has been hampered by the lack of suitable nanoscopic methods. Here we address this challenge and introduce multifunctional high-resolution atomic force microscopy (AFM) to image human protease-activated receptors (PAR1) in the functionally important lipid membrane and to simultaneously localize and quantify their binding to two different ligands. Therefore, we introduce the surface chemistry to bifunctionalize AFM tips with the native receptor-activating peptide and a tris-N-nitrilotriacetic acid (tris-NTA) group binding to a His10-tag engineered to PAR1. We further introduce ways to discern between the binding of both ligands to different receptor sites while imaging native PAR1s. Surface chemistry and nanoscopic method are applicable to a range of biological systems in vitro and in vivo and to concurrently detect and localize multiple ligand-binding sites at single receptor resolution.

ω-Thio nitrilotriacetic chemically modified gold electrode for iron determination in natural waters with different salinity

The preparation, characterization and analytical application of a chemically modified gold electrode (CME), based on ω-thio nitrilotriacetic acid derivative (N-[5-[[[[20-(acetylthio)-3,6,9-trioxaeicos-1-yl]oxo]carbonyl]amino]-1carboxypentyl]iminodiacetic acid) self-assembled monolayer (SAM), have been described. The electrode has been characterized by electrochemical techniques and tested for its response towards metallic ions, demonstrating to be effective for the determination of ionized iron at sub-μg L(-1) level by differential pulse cathodic stripping voltammetry (DPCSV). The analytical response towards iron in natural water (tap water, marine water) and the interference of ions usually present and chelating agents (humic acids and EDTA as model ligand of high complexing capacity) have been evaluated.

Quantitative real-time imaging of protein-protein interactions by LSPR detection with micropatterned gold nanoparticles

Localized surface plasmon resonance (LSPR) offers powerful means for sensitive label-free detection of protein-protein interactions in a highly multiplexed format. We have here established self-assembly and surface modification of plasmonic nanostructures on solid support suitable for quantitative protein-protein interaction analysis by spectroscopic and microscopic LSPR detection. These architectures were obtained by layer-by-layer assembly via electrostatic attraction. Gold nanoparticles (AuNP) were adsorbed on a biocompatible amine-terminated poly(ethylene glycol) (PEG) polymer brush and further functionalized by poly-l-lysine graft PEG (PLL-PEG) copolymers. Stable yet reversible protein immobilization was achieved via tris(nitrilotriacetic acid) groups incorporated into the PLL-PEG coating. Thus, site-specific immobilization of His-tagged proteins via complexed Ni(II) ions was achieved. Functional protein immobilization on the surface was confirmed by real-time detection of LSPR scattering by reflectance spectroscopy. Association and dissociation rate constants obtained for a reversible protein-protein interaction were in good agreement with the data obtained by other surface-sensitive detection techniques. For spatially resolved detection, AuNP were assembled into micropatterns by means of photolithographic uncaging of surface amines. LSPR imaging of reversible protein-protein interactions was possible in a conventional wide field microscope, yielding detection limits of ∼30 protein molecules within a diffraction-limited surface area.

Site-specific and covalent attachment of his-tagged proteins by chelation assisted photoimmobilization: a strategy for microarraying of protein ligands

A novel strategy for site-specific and covalent attachment of proteins has been developed, intended for robust and controllable immobilization of histidine (His)-tagged ligands in protein microarrays. The method is termed chelation assisted photoimmobilization (CAP) and was demonstrated using human IgG-Fc modified with C-terminal hexahistidines (His-IgGFc) as the ligand and protein A as the analyte. Alkanethiols terminated with either nitrilotriacetic acid (NTA), benzophenone (BP), or oligo(ethylene glycol) were synthesized and mixed self-assembled monolayers (SAMs) were prepared on gold and thoroughly characterized by infrared reflection absorption spectroscopy (IRAS), ellipsometry, and contact angle goniometry. In the process of CAP, NTA chelates Ni(2+) and the complex coordinates the His-tagged ligand in an oriented assembly. The ligand is then photoimmobilized via BP, which forms covalent bonds upon UV light activation. In the development of affinity biosensors and protein microarrays, site-specific attachment of ligands in a fashion where analyte binding sites are available is often preferred to random coupling. Analyte binding performance of ligands immobilized either by CAP or by standard amine coupling was characterized by surface plasmon resonance in combination with IRAS. The relative analyte response with randomly coupled ligand was 2.5 times higher than when site-specific attachment was used. This is a reminder that also when immobilizing ligands via residues far from the binding site, there are many other factors influencing availability and activity. Still, CAP provides a valuable expansion of protein immobilization techniques since it offers attractive microarraying possibilities amenable to applications within proteomics.

Quantitation of affinity, avidity, and binding kinetics of protein analytes with a dynamically switchable biosurface

A label-free method for the analysis of interactions of proteins with surface-tethered ligands is introduced. Short DNA levers are electrically actuated on microelectrodes by ac potentials, and their switching dynamics are measured in real-time by fluorescence energy transfer. Binding of proteins to ligands attached to the top of the DNA levers is detected by time-resolved measurements of the levers' dynamic motion. We demonstrate the quantitation of binding kinetics (k(on), k(off) rate constants), dissociation constants (K(D) in the pM regime), and the influence of competitive binders (EC(50) values). Moreover, the "switchSENSE" method reveals avidity effects and allows discriminating between analytes with one or more binding sites. In a comparative study, interactions of six hexa-histidine-tagged proteins with tris-nitrilotriacetic acid (NTA(3)) ligands are quantitated. Their binding kinetics and affinities are found to vary over up to 2 orders of magnitude, evidencing that the proteins' individual chemical environments significantly influence the His(6)-NTA(3) interaction.

Stochastic sensing of proteins with receptor-modified solid-state nanopores

Solid-state nanopores are capable of the label-free analysis of single molecules. It is possible to add biochemical selectivity by anchoring a molecular receptor inside the nanopore, but it is difficult to maintain single-molecule sensitivity in these modified nanopores. Here, we show that metallized silicon nitride nanopores chemically modified with nitrilotriacetic acid receptors can be used for the stochastic sensing of proteins. The reversible binding and unbinding of the proteins to the receptors is observed in real time, and the interaction parameters are statistically analysed from single-molecule binding events. To demonstrate the versatile nature of this approach, we detect His-tagged proteins and discriminate between the subclasses of rodent IgG antibodies.

Immobilization of histidine-tagged proteins on electrodes

The development of new enzyme immobilization techniques that do not affect catalytic activity or conformation of a protein is an important research task in biotechnology including biosensor applications and heterogeneous reaction systems. One of the most promising approaches for controlled protein immobilization is based on the immobilized metal ion affinity chromatography (IMAC) principle originally developed for protein purification. Here we describe the current status and future perspectives of immobilization of His-tagged proteins on electrode surfaces. Recombinant proteins comprising histidine-tags or histidine rich native proteins have a strong affinity to transition metal ions. For metal ion immobilization at the electrode surface different matrices can be used such as self-assembled monolayers or conductive polymers. This specific technique allows a reversible immobilization of histidine-tagged proteins at electrodes in a defined orientation which is an important prerequisite for efficient electron transfer between the electrode and the biomolecule. Any application requiring immobilized biocatalysts on electrodes can make use of this immobilization approach, making future biosensors and biocatalytic technologies more sensitive, simpler, reusable and less expensive while only requiring mild enzyme modifications.

Substrate-mediated nucleic acid delivery from self-assembled monolayers

Substrate-mediated nucleic acid (NA) delivery involves the immobilization of NAs or NA delivery vehicles to biomaterials for localized transfection of cells. Self-assembled monolayers (SAMs) offer an easy system to immobilize delivery vectors. SAMs form well-defined surfaces; therefore, the effect of surface composition on vector immobilization and transfection efficiency can also be studied. To date, the most effective SAM-mediated delivery systems have utilized nonspecific interactions for immobilization; however, systems that rely on specific interactions between vector and surface can impart higher control of spatial and/or temporal delivery. This review summarizes systems that use both specific and nonspecific interactions for gene delivery from SAMs; highlights progress and remaining challenges; and explores other specific recognition modalities that might be employed for future applications in surface-mediated NA delivery.

Oriented immobilization of prion protein demonstrated via precise interfacial nanostructure measurements

Nanopatterning of biomolecules on functionalized surfaces offers an excellent route for ultrasensitive protein immobilization, for interaction measurements, and for the fabrication of devices such as protein nanoarrays. An improved understanding of the physics and chemistry underlying the device properties and the recognition process is necessary for performance optimization. This is especially important for the recognition and immobilization of intrinsically disordered proteins (IDPs), like the prion protein (PrP), a partial IDP, whose folding and stability may be influenced by local environment and confinement. Atomic force microscopy allows for both highly controllable nanolithography and for sensitive and accurate direct detection, via precise topographic measurements on ultraflat surfaces, of protein interactions in a liquid environment, thus different environmental parameters affecting the biorecognition phenomenon can be investigated in situ. Using nanografting, a tip-induced lithographic technique, and an affinity immobilization strategy based on two different histidine tagged antibodies, with high nM affinity for two different regions of PrP, we successfully demonstrated the immobilization of recombinant mouse PrP onto nanostructured surfaces, in two different orientations. Clear discrimination of the two molecular orientations was shown by differential height (i.e., topographic) measurements, allowing for the estimation of binding parameters and the full characterization of the nanoscale biorecognition process. Our work opens the way to several high sensitivity diagnostic applications and, by controlling PrP orientation, allows for the investigation of unconventional interactions with partially folded proteins, and may serve as a platform for protein misfolding and refolding studies on PrP and other thermodynamically unstable, fibril forming, proteins.

Grading the commercial optical biosensor literature-Class of 2008: 'The Mighty Binders'

Optical biosensor technology continues to be the method of choice for label-free, real-time interaction analysis. But when it comes to improving the quality of the biosensor literature, education should be fundamental. Of the 1413 articles published in 2008, less than 30% would pass the requirements for high-school chemistry. To teach by example, we spotlight 10 papers that illustrate how to implement the technology properly. Then we grade every paper published in 2008 on a scale from A to F and outline what features make a biosensor article fabulous, middling or abysmal. To help improve the quality of published data, we focus on a few experimental, analysis and presentation mistakes that are alarmingly common. With the literature as a guide, we want to ensure that no user is left behind.

Protein-resistant NTA-functionalized polymer brushes for selective and stable immobilization of histidine-tagged proteins

Protein-resistant polymeric coatings that allow highly selective immobilization of specific biomolecules are essential for biomedical applications such as microarrays, biosensing, heterogeneous catalysis, and bioengineering. Polymer brushes are particularly interesting for this purpose because their chemical structure and physical properties can easily be tailored to meet specific needs. This article explores the functionalization of two protein-resistant polymer brushes, poly(oligoethylene glycol methacrylate) (POEGMA) and poly(hydroxyethyl methacrylate) (PHEMA), with nitrilotriacetic acid (NTA) moieties that can complex histidine-tagged (His-tagged) proteins selectively and reversibly. Using fluorescence microscopy, IR spectroscopy, X-ray photoelectron spectroscopy, surface plasmon resonanace, and ellipsometry, we demonstrate that His-tagged green fluorescent protein can be immobilized on NTA brushes with high stability and loading. The loading saturation reached for NTA-POEGMA is higher than that for NTA-PHEMA because of increased swelling of the former brush. Despite this higher loading capacity, NTA-POEGMA remained highly protein-resistant, which shows its potential for "clean" and specific protein immobilization. Finally, we showed that the preserved protein resistance of NTA-POEGMA brushes can be used to generate well-defined binary biofunctional patterns via a simple protocol of incubations and washes. These patterns may find applications in cell arraying and screening.

Immunochemical binding assays for detection and quantification of trace impurities in biotechnological production

New, highly sensitive, biosensor concepts make it possible to assay biomacromolecules at concentrations that previously were far below the limit of detection. The previous generation of assays used in quality control situations during biotechnological production was designed primarily for monitoring target molecules, which typically appeared in high concentrations. Hence, novel analytical techniques with high sensitivity should become increasingly important in meeting the demands from regulatory agencies with regard to declaring levels of impurities in biopharmaceuticals. Such techniques also open up opportunities for a range of other challenging measurements, for example, in the area of biohazards. This review describes the development of immuno-based biosensors and exemplifies these by presenting analyses of common impurities in biopharmaceutical production.

High-affinity labeling and tracking of individual histidine-tagged proteins in live cells using Ni2+ tris-nitrilotriacetic acid quantum dot conjugates

Investigation of many cellular processes using fluorescent quantum dots (QDs) is hindered by the nontrivial requirements for QD surface functionalization and targeting. To address these challenges, we designed, characterized and applied QD-trisNTA, which integrates tris-nitrilotriacetic acid, a small and high-affinity recognition unit for the ubiquitous polyhistidine protein tag. Using QD-trisNTA, we demonstrate two-color QD tracking of the type-1 interferon receptor subunits in live cells, potentially enabling direct visualization of protein-protein interactions at the single molecule level.

Amphiphilic diblock copolymers for molecular recognition: metal-nitrilotriacetic acid functionalized vesicles

Here we describe the design, synthesis, and characterization of new, metal-functionalized, amphiphilic diblock copolymers for molecular recognition. Polybutadiene-block-polyethylenoxide copolymers were synthesized by living anionic polymerization and end group functionalized with nitrilotriacetic acid and tris(nitrilotriacetic acid). After complexation with nickel and copper, these groups are known to selectively bind to oligohistidine residues of proteins. The polymers were characterized by 1H NMR spectroscopy, size exclusion chromatography, electron paramagnetic resonance, and UV-vis spectroscopy. Mixtures of these polymers with the respective nonfunctionalized block copolymers self-assemble in aqueous solution into vesicular structures with a controlled density of the metal complex end-groups on their surface. The accessibility of these binding sites was tested using maltose binding protein carrying a terminal decahistidine moiety and His-tagged enhanced green fluorescent protein as model systems. Fluorescence correlation spectroscopy clearly showed a significant and selective binding of these proteins to the vesicle surface.

Protein-protein interactions in reversibly assembled nanopatterns

We describe herein a platform to study protein-protein interactions and to form functional protein complexes in nanoscopic surface domains. For this purpose, we employed multivalent chelator (MCh) templates, which were fabricated in a stepwise procedure combining dip-pen nanolithography (DPN) and molecular recognition-directed assembly. First, we demonstrated that an atomic force microscope (AFM) tip inked with an oligo(ethylene glycol) (OEG) disulfide compound bearing terminal biotin groups can be used to generate biotin patterns on gold achieving line widths below 100 nm, a generic platform for fabrication of functional nanostructures via the highly specific biotin-streptavidin recognition. Subsequently, we converted such biotin/streptavidin patterns into functional MCh patterns for reversible assembly of histidine-tagged (His-tagged) proteins via the attachment of a tris-nitriloacetic acid (trisNTA) biotin derivative. Fluorescence microscopy confirmed reversible immobilization of the receptor subunit ifnar2-His10 and its interaction with interferon-alpha2 labeled with fluorescent quantum dots in a 7 x 7 dot array consisting of trisNTA spots with a diameter of approximately 230 nm. Moreover, we carried out characterization of the specificity, stability, and reversibility as well as quantitative real-time analysis of protein-protein interactions at the fabricated nanopatterns by imaging surface plasmon resonance. Our work offers a route for construction and analysis of functional protein-based nanoarchitectures.