An optical biosensor for monitoring recombinant proteins in process media

Chemical and biological characterisation of a sensor surface for bioprocess monitoring

This paper describes the step-wise fabrication and characterisation of a multi-layer dual polarization interferometry (DPI) based biosensor utilising Protein G (ProG) as the bio-recognition layer for the detection of a fragment antibody (Fab'). The biosensor is capable of monitoring the concentration of Fab' product within the extracellular medium of a fed-batch fermentation after leakage from Escherichia coli (E.coli). The activity, stability and functionality of each sensor layer were analysed in situ using DPI, whilst the chemical identity and homogeneity of the chemical layers were assessed ex situ using X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). Two different biotin linkers were found to produce hugely differing surfaces after the capture of NeutrAvidin™ (NA) and biotinylated Protein G (b-ProG). The hydrophilic (PEG)(4)-biotin linker resulted in a surface where the b-ProG layer was deposited and organised above the NA layer producing an active and stable surface, whilst the hydrophobic LC-biotin linker generated a surface where the b-ProG layer was buried within the NA layer leading to variable surfaces and poor binding of the Fab' target. The biosensor has a detection limit of 1.7 μg/ml with a dynamic range covering two orders of magnitude. The sensor can detect the onset of Fab' leakage as early as 2h following product induction, with high signal-to-noise ratios and little interference from extracellular components. Leakage of Fab' followed a biphasic profile, switching to a more rapid rate 20 h after induction, indicating accelerated product loss and the need for cultivation harvest.

Chemiluminescent bead-based hybridization assay for the detection of genomic DNA from E. coli in purified plasmid samples

A bead-based hybridization assay was developed for detection of traces of E. coli genomic DNA (gDNA) present in purified plasmid DNA (pDNA) samples. Standards of gDNA and pDNA samples were sheared by sonication and adsorbed onto aminopropyl controlled pore glass (CPG) particles (130 microm). A preliminary study was conducted to optimize the amount of DNA adsorbed on the particles. Results indicated that maximum attachment efficiency was obtained by adsorbing DNA for 2 h in 0.2 x SSC, pH 5.7. The DNA-bound particles were hybridized overnight with a 181-bp digoxigenin-labeled probe, specific for gDNA. Following a chemiluminescent detection protocol, signal intensities of the standards were plotted as a function of initial gDNA concentration. The calculated detection limit (LOD) was 1.4 pM of gDNA. The assay was able to detect gDNA in pure plasmid preparations at the 1% level even in the presence of 1,000-fold excess of noncomplementary target. Hybridization results were compared with a quantitative real-time PCR assay. Both methods afforded similar accurate results at the 95% confidence level.

An improved coating for the isolation and quantitation of interferon-γ in spiked plasma using surface plasmon resonance (SPR)

A study was initiated to investigate the use of surface plasmon resonance (SPR) for the detection in plasma of a high pI model protein, recombinant human interferon-gamma (IFN-gamma). Initially a number of self-assembled monolayers (SAMs) and hydrogel-derivatised SAM-coatings were characterised for the adsorptive and desorptive properties of plasma components. Next a monoclonal anti-IFN-gamma antibody, MD-2, was covalently attached to dextran-modified mercaptoundecanoic acid surfaces that performed best. On coatings consisting of carboxyl-modified dextran (CMD) a difference in interaction behaviour was observed when IFN-gamma was injected in either buffer or diluted plasma. During the injection of IFN-gamma in buffer, an acceleration of the interaction process was observed and the signal continued to increase after the injection plug had passed. Upon injection of diluted plasma spiked with IFN-gamma, the response increased without acceleration of the binding process. After the injection was finished, some of the bound material desorbed as expected, resulting in a signal decrease. On non-charged dextrans, the interaction between the antibody-modified surface and IFN-gamma in either plasma or buffer was similar. During sample injection the response increased with a binding rate depending on the concentration of IFN-gamma present in solution. When the injection was finished, some of the bound material was washed away from the surface and only a minor contribution of non-specific adsorbed plasma components was noticeable. From the coatings tested, the non-modified dextran-coated SPR sensor disks prove to be best suited for the detection of IFN-gamma in complex matrices like plasma. The interaction of IFN-gamma in both diluted plasma and buffer is comparable and concentrations of IFN-gamma of 250 ng ml-1 and higher can be detected in both buffer and 100x-diluted plasma. The non-specific adsorption of plasma components is low, whereas the specific IFN-gamma response is relatively high.

Intracellular monitoring of superoxide dismutase expression in an Escherichia coli fed-batch cultivation using on-line disruption with at-line surface plasmon resonance detection

An on-line cell disruption system for at-line monitoring of the intracellular concentration of recombinant human superoxide dismutase (rhSOD) in a genetically modified Escherichia coli strain, HMS174(DE3) (pET11a/rhSOD), in bioreactor cultivations is described. The sampled bacteria were disrupted on-line by rapid mixing with a nonionic detergent. The recombinant protein content of the lysed bacterial sample was quantitated by a subsequent surface plasmon resonance biosensor with a specific monoclonal antibody. Extraction efficiency of the monitoring system was optimized with respect to the flow rate ratio of the cell suspension and the detergent at relevant cell densities with the aim to attain rapid monitoring. Monitoring was demonstrated for a shake flask culture and a glucose-limited fed-batch cultivation. The results are compared with a traditional enzyme-linked immunosorbent assay method showing a correlation coefficient of R2 = 0.97. Extraction efficiency of rhSOD reached 95-99% at a total processing time of 1.8-2.6 min and a contact time of 0.8-1.4 min. The possibility of extending the monitoring system to other intracellular proteins is discussed.

Design, synthesis, and screening of biomimetic ligands for affinity chromatography

Affinity chromatography is ideally suited to the purification of pharmaceutical proteins due to its unique bio-specificity characteristics. Tailor-made affinity ligands that represent a promising class of synthetic affinity ligands have been developed to target specific proteins and designed to mimic peptidal templates, natural biological recognition motifs, or complementary surface-exposed residues. These biomimetic ligands have been generated by a combination of rational design, combinatorial library synthesis, and subsequent screening of potential leads against target proteins. Small ligands based on a triazine scaffold also present exceptional selectivity and stability, which allows their use in harsh manufacturing environments.

Imaging surface plasmon resonance system for screening affinity ligands

A surface plasmon resonance (SPR) system for screening ligands for application in affinity chromatography is described. A combinatorial library of 13 ligands was synthesised, characterised and immobilised to agarose beads and gold SPR devices. Binding and elution behaviour and a range of K(AX) values (10(3) to 10(5) M(-1)) were measured against two target proteins, an insulin analogue (MI3) and a recombinant clotting factor (rFVIIa), in order to create a relational database between the traditional chromatographic format and the new SPR screening system. The SPR transducer surface was fabricated with affinity ligands in a two-dimensional, spatially addressable format, which was durable (>100 cycles) and stable over 6 months. The imaging SPR system comprised a direct optical, CCD-based, instrument capable of imaging the change in refractive index created by biochemical interactions and allowed affinity ligands to be evaluated 15-fold faster with 130-fold less target protein than conventional chromatographic methods. The binding and elution data from both the SPR and chromatographic systems for both target proteins were comparable, with the K(AX) value generating a nearly linear correlation (R(2)=0.875) and a slope bias of approximately 2.5+/-0.25-fold higher for the SPR system. The imaging SPR system has proven capable of screening and evaluating affinity ligands for potential use in the recovery of biopharmaceutical proteins.

Optical biosensors. Monitoring studies of glycopeptide antibiotic fermentation using white light interference

This paper describes the design, characterization, and use of an optical biosensor suited for the process control of biotechnological processes. The detector principle is based on reflectometric interference spectroscopy (RIfS). RIfS enables a label-free, product-specific monitoring, with a future outline for on-line process control. The potential of the RIfS biosensor is exemplified by the qualitative and quantitative monitoring of the microbial production of vancomycin-type glycopeptide antibiotics.

Binding of Biotin to Gold Surfaces Functionalized by Self-Assembled Monolayers of Cystamine and Cysteamine: Combined FT-IRRAS and XPS Characterization

As part of our project of developing a new IR-based immunosensor, we investigated the functionalization of gold substrates with thin organic films containing biotin ligands. A two-step procedure was developed consisting of the chemisorption of short amine-terminated organosulfur compounds, followed by their reaction at the solid liquid interface with an activated ester derivative of biotin. Covalent binding of biotin to these attachment layers was assessed by Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS) and X-ray photoelectron spectroscopy (XPS). The interaction of activated biotin with alcohol- and carboxylic acid-terminated monolayers was also investigated, and, as expected, no binding occurred. Moreover, mixed layers of short alcohol- and amine-terminated thiolates were successfully constructed at the gold surfaces and were shown to be the most efficient for the covalent binding of biotin thanks to the blocking effect of the thioalcohol, which prevented direct adsorption of biotin to the gold surface. Copyright 2001 Academic Press.