Engineering Bioactive Surfaces with Fischer Carbene Complex: Protein A on Self-Assembled Monolayer for Antibody Sensing

Organometallic catalysis in aqueous and biological environments: harnessing the power of metal carbenes

Translating the power of transition metal catalysis to the native habitats of enzymes can significantly expand the possibilities of interrogating or manipulating natural biological systems, including living cells and organisms. This is especially relevant for organometallic reactions that have shown great potential in the field of organic synthesis, like the metal-catalyzed transfer of carbenes. While, at first sight, performing metal carbene chemistry in aqueous solvents, and especially in biologically relevant mixtures, does not seem obvious, in recent years there has been a growing number of reports demonstrating the feasibility of the task. Either using small molecule metal catalysts or artificial metalloenzymes, a number of carbene transfer reactions that tolerate aqueous and biorelevant media are being developed. This review intends to summarize the most relevant contributions, and establish the state of the art in this emerging research field.

Restricted Proteolysis and LC-MS/MS To Evaluate the Orientation of Surface-Immobilized Antibodies

The molecular orientation of antibodies immobilized on solid surfaces plays a significant role in the sensitivity of immunoassays and efficiency of protein isolation using antibody-decorated nanoparticles. Optimally, nearly all antibody binding sites should be available to bind. Here we report for the first time an LC-MS/MS approach to probe antibody orientation directly, utilizing sterically restricted proteolysis. Trypsin-decorated magnetic beads (MBs, 1.5 μm) were much larger than average antibody-free areas (55 × 55 nm) of oriented antibodies on MBs, restricting proteolysis to mainly Fab regions. Randomly attached antibodies on MB surfaces served as controls. The tryptic-hydrolyzed peptides were quantified using LC-MS/MS peptide analysis as markers for average positions of Fc and Fab of antibodies on the beads. Different patterns of digestion rates were found due to proteolysis of the oriented and nonoriented antibodies on MBs. For oriented antibodies, the peptides from outer Fab regions gave a much higher digestion rate than those from Fc regions, while for randomly immobilized antibodies digestion rates for Fab and Fc peptides were similar. This novel approach is a useful and convenient tool to characterize antibody orientation for immunoassays and other applications. The relative degree of orientation can be assessed using a metric R_o denoting amount of Fab marker peptides found divided by Fc + Fab marker peptides × 100%. Oriented antibodies on the MBs also provided more efficient antigen capture compared to randomly immobilized antibodies.

Sensors Based on Bio and Biomimetic Receptors in Medical Diagnostic, Environment, and Food Analysis

Analytical chemistry is now developing mainly in two areas: automation and the creation of complexes that allow, on the one hand, for simultaneously analyzing a large number of samples without the participation of an operator, and on the other, the development of portable miniature devices for personalized medicine and the monitoring of a human habitat. The sensor devices, the great majority of which are biosensors and chemical sensors, perform the role of the latter. That last line is considered in the proposed review. Attention is paid to transducers, receptors, techniques of immobilization of the receptor layer on the transducer surface, processes of signal generation and detection, and methods for increasing sensitivity and accuracy. The features of sensors based on synthetic receptors and additional components (aptamers, molecular imprinted polymers, biomimetics) are discussed. Examples of bio- and chemical sensors' application are given. Miniaturization paths, new power supply means, and wearable and printed sensors are described. Progress in this area opens a revolutionary era in the development of methods of on-site and in-situ monitoring, that is, paving the way from the "test-tube to the smartphone".

Label-Free and Recalibrated Multilayer MoS2 Biosensor for Point-of-Care Diagnostics

Molybdenum disulfide (MoS₂) field-effect transistor (FET)-based biosensors have attracted significant attention as promising candidates for highly sensitive, label-free biomolecule detection devices. In this paper, toward practical applications of biosensors, we demonstrate reliable and quantitative detection of a prostate cancer biomarker using the MoS₂ FET biosensor in a nonaqueous environment by reducing nonspecific molecular binding events and realizing uniform chemisorption of anti-PSA onto the MoS₂ surface. A systematic and statistical study on the capability of the proposed device is presented, and the biological binding events are directly confirmed and characterized through intensive structural and electrical analysis. Our proposed biosensor can reliably detect various PSA concentrations with a limit of 100 fg/mL. Moreover, rigorous theoretical simulations provide a comprehensive understanding of the operating mechanism of the MoS₂ FET biosensors, and further suggests the enhancement of the sensitivity through engineering device design parameters.

Site-selective orientated immobilization of antibodies and conjugates for immunodiagnostics development

Immobilized antibody systems are the key to develop efficient diagnostics and separations tools. In the last decade, developments in the field of biomolecular engineering and crosslinker chemistry have greatly influenced the development of this field. With all these new approaches at our disposal, several new immobilization methods have been created to address the main challenges associated with immobilized antibodies. Few of these challenges that we have discussed in this review are mainly associated to the site-specific immobilization, appropriate orientation, and activity retention. We have discussed the effect of antibody immobilization approaches on the parameters on the performance of an immunoassay.

Single Molecule Characterization of UV-Activated Antibodies on Gold by Atomic Force Microscopy

The interaction between proteins and solid surfaces can influence their conformation and therefore also their activity and affinity. These interactions are highly specific for the respective combination of proteins and solids. Consequently, it is desirable to investigate the conformation of proteins on technical surfaces, ideally at single molecule level, and to correlate the results with their activity. This is in particular true for biosensors where the conformation-dependent target affinity of an immobilized receptor determines the sensitivity of the sensor. Here, we investigate for the first time the immobilization and orientation of antibodies (Abs) photoactivated by a photonic immobilization technique (PIT), which has previously demonstrated to enhance binding capabilities of antibody receptors. The photoactivated immunoglobulins are immobilized on ultrasmooth template stripped gold films and investigated by atomic force microscopy (AFM) at the level of individual molecules. The observed protein orientations are compared with results of nonactivated antibodies adsorbed on similar gold films and mica reference samples. We find that the behavior of Abs is similar for mica and gold when the protein are not treated (physisorption), whereas smaller contact area and larger heights are measured when Abs are treated (PIT). This is explained by assuming that the activated antibodies tend to be more upright compared with nonirradiated ones, thereby providing a better exposure of the binding sites. This finding matches the observed enhancement of Abs binding efficiency when PIT is used to functionalize gold surface of QCM-based biosensors.

Sensitive detection and glycoprofiling of a prostate specific antigen using impedimetric assays

This study presents a proof-of-concept for the development of an impedimetric biosensor for ultra-sensitive glycoprofiling of prostate specific antigen (PSA). The biosensor exhibits three unique characteristics: (1) analysis of PSA with limit of detection (LOD) down to 4 aM; (2) analysis of the glycan part of PSA with LOD down to 4 aM level and; (3) both assays (i.e., PSA quantification and PSA glycoprofiling) can be performed on the same interface due to label-free analysis.

Nucleophilic substitution in ionizable Fischer thiocarbene complexes: steric effect of the alkyl substituent on the heteroatom

A detailed kinetic study has been carried out for the aminolysis of ionizable Fischer thiocarbene complexes (CO)5M[double bond, length as m-dash]C(SR)CH3 (M = Cr, W; R = iPr, nBu, cHex, tBu) with five primary amines and one secondary amine in aqueous acetonitrile solutions (50% MeCN-50% water (v/v)). The observed rate constants for the reaction with primary amines showed a first-order dependence on the amine concentration, while with morpholine, the rate constant has second-order dependence. The general base catalysis process was confirmed by the variation of the rate constants with the concentration of an external catalyst and the pH. The results agree with a stepwise mechanism where the nucleophilic addition to the carbene carbon to produce a tetrahedral intermediate (T±) is the first step, followed by a rapid deprotonation of to form the anion T- which leads to the products by general-acid catalysed leaving group (-SR) expulsion. In general, it was found that the chromium complexes are less reactive than the tungsten analogues. The obtained Brønsted parameters for the nucleophilic addition (βnuc) indicate that C-N bond formation has made little progress at the transition state. By using Charton's correlation, the role that the steric factor plays throughout the mechanism has been unraveled. The nucleophilic addition to the thiocarbenes is less sensitive to steric effects than the alkoxycarbenes regardless of the nature of the metal centre. Conversely, the steric effects on the general-base catalysis can be strong depending on the volume of the catalyst and the metal centre. On the basis of the structure-reactivity coefficients β and ψ and comparison with alkoxycarbene complexes, esters and thiolesters, insights into the main factors ruling the reactivity in terms of transition state imbalances are discussed.

Design of heat shock-resistant surfaces to prevent protein aggregation: Enhanced chaperone activity of immobilized α-Crystallin

α-Crystallin is a multimeric protein belonging to the family of small heat shock proteins, which function as molecular chaperones by resisting heat and oxidative stress induced aggregation of other proteins. We immobilized α-Crystallin on a self-assembled monolayer on glass surface and studied its activity in terms of the prevention of aggregation of aldolase. We discovered that playing with grafted protein density led to interesting variations in the chaperone activity of immobilized α-Crystallin. This result is in accordance with the hypothesis that dynamicity of subunits plays a vital role in the functioning of α-Crystallin and might be able to throw light on the structure-activity relationship. We showed that the chaperone activity of a certain number of immobilized α-Crystallins was superior compared to a solution containing an equivalent number of the protein and 10 times the number of the protein at temperatures >60 °C. The α-Crystallin grafted surfaces retained activity on reuse. This could also lead to the design of potent heat-shock resistant surfaces that can find wide applications in storage and shipping of protein based biopharmaceuticals.

Fischer carbene complexes remain favourite targets, and vehicles for new discoveries

Fifty years after their introduction, Fischer-type carbene complexes still enthral synthetic and theoretical chemists interested in their preparation and characterization.

Nano-visualization of oriented-immobilized IgGs on immunosensors by high-speed atomic force microscopy

Oriented immobilization of sensing molecules on solid phases is an important issue in biosensing. In case of immunosensors, it is essential to scrutinize not only the direction and shape of immunoglobulin G (IgG) in solution but also the real-time movement of IgGs, which cannot be achieved by conventional techniques. Recently, we developed bio-nanocapsules (BNCs) displaying a tandem form of the IgG Fc-binding Z domain derived from Staphylococcus aureus protein A (ZZ-BNC) to enhance the sensitivity and antigen-binding capacity of IgG via oriented-immobilization. Here, we used high-speed atomic force microscopy (HS-AFM) to reveal the fine surface structure of ZZ-BNC and observe the movement of mouse IgG3 molecules tethered onto ZZ-BNC in solution. ZZ-BNC was shown to act as a scaffold for oriented immobilization of IgG, enabling its Fv regions to undergo rotational Brownian motion. Thus, HS-AFM could decipher real-time movement of sensing molecules on biosensors at the single molecule level.

Noble polymeric surface conjugated with zwitterionic moieties and antibodies for the isolation of exosomes from human serum

New zwitterionic polymer-coated immunoaffinity beads were developed to resist nonspecific protein adsorption from undiluted human serum for diagnostic applications of exosomes. A zwitterionic sulfobetaine monomer with an amine functional group was employed for simple surface chemistry and antifouling properties. An exosomal biomarker protein, epithelial cell adhesion molecule (EpCAM), was selected as a target molecule in this work. The beads were coated with polyacrylic acids (PAA) for increasing biorecognition sites, and protein G was then conjugated with carboxylic acid groups on the surfaces for controlling EpCAM antibody orientation. The remaining free carboxylic acid groups were modified with sulfobetaine moieties, and anti-EpCAM antibody was finally introduced. The amount of anti-EpCAM on the beads was increased by 40% when compared with PAA-uncoated beads. The surfaces of the beads exhibited near-net-zero charge, and nonspecific protein adsorption was effectively suppressed by sulfobetaine moieties. EpCAM was captured from undiluted human serum with almost the same degree of efficiency as from PBS buffer solution using the newly developed immunoaffinity beads.

Facile modification of silica substrates provides a platform for direct-writing surface click chemistry

Please click here: a facile two-step functionalization strategy for silicon oxide-based substrates generates a stable platform for surface click chemistry via direct writing. The suitability of the obtained substrates is proven by patterning with two different direct-writing techniques and three different molecules.