Surface plasmon resonance biomolecular interaction analysis mass spectrometry. 2. Fiber optic-based analysis

Surface plasmon resonance for cell-based clinical diagnosis

Non-invasive real-time observations and the evaluation of living cell conditions and functions are increasingly demanded in life sciences. Surface plasmon resonance (SPR) sensors detect the refractive index (RI) changes on the surface of sensor chips in label-free and on a real-time basis. Using SPR sensors, we and other groups have developed techniques to evaluate living cells' reactions in response to stimuli without any labeling in a real-time manner. The SPR imaging (SPRI) system for living cells may visualize single cell reactions and has the potential to expand application of SPR cell sensing for clinical diagnosis, such as multi-array cell diagnostic systems and detection of malignant cells among normal cells in combination with rapid cell isolation techniques.

Lab-on-a-plate: extending the functionality of MALDI-MS and LDI-MS targets

We review the literature that describes how (matrix-assisted) laser desorption/ionization (MA)LDI target plates can be used not only as sample supports, but beyond that: as functional parts of analytical protocols that incorporate detection by MALDI-MS or matrix-free LDI-MS. Numerous steps of analytical procedures can be performed directly on the (MA)LDI target plates prior to the ionization of analytes in the ion source of a mass spectrometer. These include homogenization, preconcentration, amplification, purification, extraction, digestion, derivatization, synthesis, separation, detection with complementary techniques, data storage, or other steps. Therefore, we consider it helpful to define the "lab-on-a-plate" as a format for carrying out extensive sample treatment as well as bioassays directly on (MA)LDI target plates. This review introduces the lab-on-plate approach and illustrates it with the aid of relevant examples from the scientific and patent literature.

Use of multidimensional separation protocols for the purification of trace components in complex biological samples for proteomics analysis

The routine detection of low abundance components in complex samples for detailed proteomics analysis continues to be a challenge. Whilst the potential of multidimensional chromatographic fractionation for this purpose has been proposed for some years, and was used effectively for the purification to homogeneity of trace components in bulk biological samples for N-terminal sequence analysis, its practical application in the proteomics arena is still limited. This article reviews some of the recent data using these approaches, including the use of microaffinity purification as part of multidimensional protocols for downstream proteomics analysis.

Self-assembled monolayers for MALDI-TOF mass spectrometry for immunoassays of human protein antigens

This paper reports a method that combines self-assembled monolayers with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry to perform immunoassays on clinical samples. The immunosensors are prepared by immobilizing His-tagged protein G (or A) to a monolayer presenting the Ni2+ chelates, followed by immobilization of IgG antibodies with specificity for the intended analyte. The SAMDI mass spectrometry technique confirms the presence of the two proteins on the immunosensor and additionally provides a label-free analysis of antigens that bind to the sensor. This paper reports examples of detecting several proteins from human serum, including multianalyte assays that resolve each analyte according to their mass-to-charge ratio in the SAMDI spectra. An example is described wherein SAMDI is used to identify a proteolytic fragment of cystatin C in cerebral spinal fluids from patients diagnosed with multiple sclerosis. The SAMDI-TOF immunoassay, which combines well-defined surface chemistries for the selective and reproducible localization of analytes with mass spectrometry for label-free detection of analytes, may offer an alternative methodology to address many of the issues associated with standardized clinical diagnostics.

Mass spectrometry-based immunoassays for the next phase of clinical applications

Recent applications of affinity mass spectrometry into clinical laboratories brought a renewed interest in immunoaffinity mass spectrometry as a more specific affinity method capable of selectively targeting and studying protein biomarkers. In mass spectrometry-based immunoassays, proteins are affinity retrieved from biological samples via surface-immobilized antibodies, and are then detected via mass spectrometric analysis. The assays benefit from dual specificity, which is brought about by the affinity of the antibody and the protein mass readout. The mass spectrometry aspect of the assays enables single-step detection of protein isoforms and their individual quantification. This review offers a comprehensive review of mass spectrometry-based immunoassays, from historical perspectives in the development of the immunoaffinity mass spectrometry, to current applications of the assays in clinical and population proteomic endeavors. Described in more detail are two types of mass spectrometry-based immunoassays, one of which incorporates surface plasmon resonance detection for protein quantification. All mass spectrometry-based immunoassays offer high-throughput targeted protein investigation, with clear implications in clinical research, encompassing biomarker discovery and validation, and in diagnostic settings as the next-generation immunoassays.

Identification and characterization of DNA-binding proteins by mass spectrometry

Mass spectrometry is the most sensitive and specific analytical technique available for protein identification and quantification. Over the past 10 years, by the use of mass spectrometric techniques hundreds of previously unknown proteins have been identified as DNA-binding proteins that are involved in the regulation of gene expression, replication, or DNA repair. Beyond this task, the applications of mass spectrometry cover all aspects from sequence and modification analysis to protein structure, dynamics, and interactions. In particular, two new, complementary ionization techniques have made this possible: matrix-assisted laser desorption/ionization and electrospray ionization. Their combination with different mass-over-charge analyzers and ion fragmentation techniques, as well as specific enzymatic or chemical reactions and other analytical techniques, has led to the development of a broad repertoire of mass spectrometric methods that are now available for the identification and detailed characterization of DNA-binding proteins. These techniques, how they work, what their requirements and limitations are, and selected examples that document their performance are described and discussed in this chapter.

Sensitivity enhancement of surface plasmon resonance biosensing of small molecules

Surface plasmon resonance (SPR) biosensor formats using gold nanoparticle or protein signal amplification for the sensitive assay of small molecules were developed using progesterone as a model compound. Progesterone was immobilized to a dextran surface in the Biacore biosensor through in situ covalent immobilization using an oligoethylene glycol linker attached to the 4 position of the steroid. This surface produced stable antibody binding for in excess of 1100 assay cycles. Using this surface, assays were developed for progesterone using 10- and 20-nm gold-streptavidin labels attached to biotinylated monoclonal antibody in both label prebinding and sequential binding formats. Prelabeling formats gave no signal enhancement but produced assays with limits of detection of 143 pg/ml, compared with approximately 1 ng/ml in previous studies. Sequential binding formats gave signal enhancements of 2.2-fold over the monoclonal antibody and a limit of detection of 23.1 pg/ml. It was found that secondary antibody labeling gave 8.1-fold signal enhancements and a limit of detection of 20.1 pg/ml, whereas use of secondary antibody-25 nm gold complexes provided more signal enhancement (13-fold) and a further improvement in limit of detection of 8.6 pg/ml.

Design and use of multi-affinity surfaces in biomolecular interaction analysis-mass spectrometry (BIA/MS): a step toward the design of SPR/MS arrays

The feasibility of multi-affinity ligand surfaces in biomolecular interaction analysis-mass spectrometry (BIA/MS) was explored in this work. Multi-protein affinity surfaces were constructed by utilizing antibodies to beta-2-microglobulin, cystatin C, retinol binding protein, transthyretin, serum amyloid P and C-reactive protein. In the initial experiments, all six antibodies were immobilized on a single site (flow cell) on the sensor chip surface, followed by verification of the surface activity via separate injections of purified proteins. After an injection of diluted human plasma aliquot over the antibodies-derivatized surfaces, and subsequent MALDI-TOF MS analysis, signals representing five out of the six targeted proteins were observed in the mass spectra. Further, to avoid the complexity of the spectra, the six proteins were divided into two groups (according to their molecular weight) and immobilized on two separate surfaces on a single sensor chip, followed by an injection of human plasma aliquot. The resulting mass spectra showed signals from all proteins. Also, the convolution resulting from the multiply charged ion species was eliminated. The ability to create such multi-affinity surfaces indicates that smaller-size ligand areas/spots can be employed in the BIA/MS protein interaction screening experiments, and opens up the possibilities for construction of novel multi-arrayed SPR-MS platforms and methods for high-throughput parallel protein interaction investigations.

Delineating protein-protein interactions via biomolecular interaction analysis-mass spectrometry

The utility of biomolecular interaction analysis-mass spectrometry (BIA/MS) in screening for protein-protein interactions was explored in this work. Experiments were performed in which proteins served as ligands for screening of possible interactions with other proteins from human plasma and urine. The proteins utilized were beta-2-microglobulin, cystatin C (cysC), retinol binding protein (RBP), transthyretin (TTR), alpha-1-microglobulin, C-reactive protein, transferrin and papain. The immobilization of functionally active proteins was confirmed via interactions with antibodies to the corresponding proteins. Various dilutions of human urine and plasma were injected over the protein-derivatized surfaces. It was observed that the urine injections generally yielded smaller SPR responses than those observed after the plasma injections. The BIA/MS experiments did not reveal novel protein-protein interactions, although several established interactions (such as those between RBP and TTR, and cysC and papain) were validated. Few protein ligand deficiencies (such as truncations) leading to false negative and false positive BIA/MS results were also discovered.

Contributions of mass spectrometry in the study of nucleic acid-binding proteins and of nucleic acid-protein interactions

Nucleic-acid-protein (NA-P) interactions play essential roles in a variety of biological processes-gene expression regulation, DNA repair, chromatin structure regulation, transcription regulation, RNA processing, and translation-to cite only a few. Such biological processes involve a broad spectrum of NA-P interactions as well as protein-protein (P-P) interactions. These interactions are dynamic, in terms of the chemical composition of the complexes involved and in terms of their mere existence, which may be restricted to a given cell-cycle phase. In this review, the contributions of mass spectrometry (MS) to the deciphering of these intricate networked interactions are described along with the numerous applications in which it has proven useful. Such applications include, for example, the identification of the partners involved in NA-P or P-P complexes, the identification of post-translational modifications that (may) regulate such complexes' activities, or even the precise molecular mapping of the interaction sites in the NA-P complex. From a biological standpoint, we felt that it was worth the reader's time to be as informative as possible about the functional significance of the analytical methods reviewed herein. From a technical standpoint, because mass spectrometry without proper sample preparation would serve no purpose, each application described in this review is detailed by duly emphasizing the sample preparation-whenever this step is considered innovative-that led to significant analytical achievements.

Analysis of native proteins from biological fluids by biomolecular interaction analysis mass spectrometry (BIA/MS): exploring the limit of detection, identification of non-specific binding and detection of multi-protein complexes

Biomolecular interaction analysis mass spectrometry (BIA/MS) is a two-dimensional analytical technique that quantitatively and qualitatively detects analytes of interests. In the first dimension, surface plasmon resonance (SPR) is utilized for detection of biomolecules in their native environment. Because SPR detection is non-destructive, analyte(s) retained on the SPR-active sensor surface can be analyzed in a second dimension using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. The qualitative nature of the MALDI-TOF MS analysis complements the quantitative character of SPR sensing and overcomes the shortcomings of the SPR detection stemming from the inability to differentiate and characterize multi-protein complexes and non-specific binding. In this work, the benefit of performing MS analysis following SPR sensing is established. Retrieval and detection of four markers present in biological fluids (cystatin C, beta-2-microglobulin, urinary protein 1 and retinol binding protein) was explored to demonstrate the effectiveness of BIA/MS in simultaneous detection of clinically related biomarkers and delineation of non-specific binding. Furthermore, the BIA/MS limit of detection at very low SPR responses was investigated. Finally, detection of in-vivo assembled protein complexes was achieved for the first time using BIA/MS.

Immunosensors--principles and applications to clinical chemistry

INTRODUCTION

Immunosensors are affinity ligand-based biosensor solid-state devices in which the immunochemical reaction is coupled to a transducer. The fundamental basis of all immunosensors is the specificity of the molecular recognition of antigens by antibodies to form a stable complex. This is similar to the immunoassay methodology. Immunosensors can be categorized based on the detection principle applied. The main developments are electrochemical, optical, and microgravimetric immunosensors. In contrast to immunoassay, modern transducer technology enables the label-free detection and quantification of the immune complex.

METHODS

The analysis of trace substances in environmental science, pharmaceutical and food industries is a challenge since many of these applications demand a continuous monitoring mode. The use of immunosensors in these applications is most appropriate. Similarly, a series of clinical problems may be solved by continuous monitoring of certain analytes.

CONCLUSIONS

Clinical chemists should take advantage of immunosensors in clinical diagnostics. There are many recent developments in the immunosensor field which have potential impacts. The future role of this technique in intralaboratory, as well as bedside testing, will become even more important as the clinical laboratory is faced with increasing pressure to contain costs.

Analysis of human urine protein biomarkers via biomolecular interaction analysis mass spectrometry

Biomolecular interaction analysis mass spectrometry (BIA/MS) is a two-dimensional chip-based analytical technique geared toward quantitative and qualitative analysis of small volumes of biological samples. Interactions between surface-immobilized ligands and solute-borne analytes are quantitatively viewed in real time through surface plasmon resonance sensing, followed by qualitative matrix-assisted laser desorption/ionization time-of-flight MS analysis of the analyte(s) affinity-retained on the sensor surface. In this work, BIA/MS was used in the detection of a number of protein biomarkers from human urine. Small volumes of human urine were analyzed for cystatin C, beta(2)-microglobulin, urinary protein 1, and retinol-binding protein (RBP). Multiaffinity sensor surfaces were created to simultaneously and rapidly detect all four proteins in a single BIA/MS analysis on a two-flow cell sensor chip configuration. Furthermore, RBP was analyzed separately from both urine and plasma samples. Results indicate that BIA/MS can be used successfully in rapid screening of a number of urinary proteins indicated as putative biological markers for renal dysfunction.

Functional genomics with protein-protein interactions

Knowing the sequence of a gene does not mean knowing its function. Although, information stored at the DNA level can be used to predict biological processes, proteins are the final executors of the various response programs of a cell. Transient information, like posttranslational modifications or interactions among proteins, cannot be deduced from DNA sequences. The rapid accumulation of large amounts of DNA sequence data in genomics projects has led to an increasing demand for powerful tools to analyze proteins and their behaviour at a large scale. This review aims to compare different technologies used for identification of interacting proteins and discusses recent developments in the field of high-throughput protein-protein interaction mapping.

Multitoxin biosensor-mass spectrometry analysis: a new approach for rapid, real-time, sensitive analysis of staphylococcal toxins in food

Biomolecular interaction analysis mass spectrometry (BIA-MS) was applied to detection of bacterial toxins in food samples. This two-step approach utilizes surface plasmon resonance (SPR) to detect the binding of the toxin(s) to antibodies immobilized on a surface of a sensor chip. SPR detection is then followed by identification of the bound toxin(s) by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Staphylococcal enterotoxin B (SEB) was readily detected in milk and mushroom samples at levels of 1 ng/ml. In addition, non-specific binding of food components to the immobilized antibody and to the sensor chip surface was detected. To evaluate the applicability of BIA-MS in the analysis of materials containing multiple toxic components, sample containing both SEB and toxic-shock syndrome toxin-1 was analyzed. Both toxins were successfully and simultaneously detected through the utilization of multiaffinity sensor chip surfaces.

Surface plasmon resonance-based immunoassays

Surface plasmon resonance (SPR) has been successfully incorporated into an immunosensor format for the simple, rapid, and nonlabeled assay of various biochemical analytes. Proteins, complex conjugates, toxins, allergens, drugs, and pesticides can be determined directly using either natural antibodies or synthetic receptors with high sensitivity and selectivity as the sensing element. Immunosensors are capable of real-time monitoring of the antigen-antibody reaction. A wide range of molecules can be detected with lower limits ranging between 10(-9) and 10(-13) mol/L. Several successful commercial developments of SPR immunosensors are available and their web pages are rich in technical information. This review highlights many recent developments in SPR-based immunoassay, functionalizations of the gold surface, novel receptors in molecular recognition, and advanced techniques for sensitivity enhancement. Furthermore, it describes the challenge of current problems and provides some insights toward the future technologies.

Practical considerations in BIA/MS: optimizing the biosensor-mass spectrometry interface

Biomolecular interaction analysis mass spectrometry (BIA/MS) is a multiplexed analytical technique that utilizes a unique combination of surface plasmon resonance (SPR) and matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) for the detection and analysis of small amounts of proteins residing in complex biological systems. In order to achieve high sensitivity during BIA/MS, certain experimental parameters and sequences of events need to be optimized and maintained. Immobilized ligand density, flow rate and biosensor control (in SPR-BIA) and matrix choice and application (in MALDI-TOF MS) have significant influence on the final outcome of the BIA/MS analysis and, consequently, need to be optimized and carefully controlled. In addition, chip washing and cutting are essential in converting the SPR-active sensor chips into target surfaces amenable to MALDI-TOF MS. Reviewed here are the prerequisites for successfully interfacing SPR-BIA with MALDI-TOF MS.

Electrospray ionization and matrix-assisted laser desorption ionization mass spectrometry. Emerging technologies in biomedical sciences

Tremendous progress in biomedical sciences has been made possible in part by recent advances in bioanalytical methods, in particular biological mass spectrometry. Since the introduction of electrospray ionization mass spectrometry (ESI-MS) in 1984 and matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) in 1988, the field of bioanalytical mass spectrometry has seen rapid growth. In concert with separation techniques such as capillary electrophoresis and high performance liquid chromatography, mass spectrometry allows characterization of a large array of small organic molecules, peptides, proteins, oligonucleotides, and RNA fragments. Thus, substantially more expedient and definitive determination of molecular weight is now possible by mass spectrometric analysis. In this commentary, general descriptions of ESI- and MALDI-MS are presented. Furthermore, several recent developments and applications in addressing difficult biological problems are discussed.

Biosensor chip mass spectrometry: a chip-based proteomics approach

Rapid advances in genomic sequencing, bioinformatics, and analytical instrumentation have created the field of proteomics, which at present is based largely on two-dimensional electrophoresis (2-DE) separation of complex protein mixtures and identification of individual proteins using mass spectrometry. These analyses provide a wealth of data, which upon further evaluation leads to many questions regarding the structure and function of the proteins. The challenge of answering these questions create a need for high-specificity approaches that may be used in the analysis of biomolecular recognition events and interacting partners, and thereby places great demands on general protein characterization instrumentation and the types of analyses they need to perform. Over the past five years we have been actively involved in interfacing two general, instrumental techniques, surface plasmon resonance-biomolecular interaction analysis (SPR-BIA) and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, into a single concerted approach for use in the functional and structural characterization of proteins. Reviewed here is the recent progress made using biomolecular interaction analysis - mass spectrometry (BIA-MS) in the detailed characterization of proteins and protein-protein interactions and the development of biosensor chip mass spectrometry (BCMS) as a new chip-based proteomics approach.

Recent advancements in surface-enhanced laser desorption/ionization-time of flight-mass spectrometry

The overall history and recent advances in surface enhanced laser desorption/ionization-time of flight-mass spectrometry (SELDI-TOF-MS) technology is reviewed herein. Fundamentals of SELDI-TOF analysis are presented while drawing comparisons with other laser-based mass spectrometry techniques. The application of SELDI-TOF-MS to functional genomics and biomarker discovery is discussed and exemplified by elucidating a biomarker candidate for prostatic carcinoma. Finally, a short discussion regarding future SELDI requirements and developments is supplied.

Micropreparative ligand fishing with a cuvette-based optical mirror resonance biosensor

We have previously demonstrated the role of an optical biosensor (BIAcore 2000) as a specific detector to monitor chromatographic fractions during the purification and characterisation of ligands for orphan biomolecules. We have now extended this application to perform micropreparative ligand fishing directly on the sensor surface using an automated cuvette-based optical biosensor (Iasys Auto+) equipped with a high-capacity carboxymethyldextran surface (surface area 16 mm2). Using a F(ab)2' fragment of the A33 monoclonal antibody as bait, we have recovered microgram quantities of essentially homogeneous A33 ligand from the sensor surface in a form suitable for subsequent sensitive and specific down stream analysis (micropreparative HPLC, sodium dodecyl sulphate-polyacrylamide gel electrophoresis and Western blotting). The design of the cuvette-based system facilitates recovery of desorbed material from the constrained workspace in small volumes at high concentration. The use of on-surface detection allows the surface viability to be continuously monitored and permits direct quantitation of both bound and recovered material.

Biotechnology match making: screening orphan ligands and receptors

To date there has been a considerable amount of interest and success in the pharmaceutical industry in the discovery of drug targets and diagnostics via genomic technologies, namely DNA sequencing, mutation/polymorphism detection and expression monitoring of mRNA. As the ultimate targets for the majority of these methods are actually proteins, more and more emphasis has been placed upon protein-based methods in an effort to define the function of proteins discovered by genomic technologies. One of the most challenging areas of drug target discovery facing researchers today is the search for novel receptor-ligand pairs. Database mining techniques in conjunction with other computational methods are able to identify many novel sequences of putative receptors, but the ability to similarly identify the receptor's natural ligand is not possible by these methods. The past few years have seen an increase in methodology and instrumentation focused on the ability to discover and characterize protein-protein interactions, as well as receptor-ligand pairs. Significant advances have been made in the areas of instrumentation (biosensors and fluorescent plate readers) as well as methodologies relating to phage/ribosome display and library construction.

Methods of screening combinatorial libraries using immobilized or restrained receptors

The screening of combinatorial libraries for compounds with high affinity toward drug receptors is currently a major center of attention. We describe methods recently developed for library screening that involve "constrained" receptors (either immobilized onto a surface or restrained to a compartment by some physical means). These include affinity selection chromatography, ultrafiltration assays, the scintillation proximity assay, a variety of interfacial optical techniques (surface plasmon resonance and its relatives, among others), the quartz crystal microbalance, the jet ring cell, and new interferometric assays using porous silicon to immobilize the receptor. We note some trends in assay development involving assays of membrane-bound complexes, and the coupling of two analytical methods to expand the assay resolution.

Instrumental biosensors: new perspectives for the analysis of biomolecular interactions

The use of instrumental biosensors in basic research to measure biomolecular interactions in real time is increasing exponentially. Applications include protein-protein, protein-peptide, DNA-protein, DNA-DNA, and lipid-protein interactions. Such techniques have been applied to, for example, antibody-antigen, receptor-ligand, signal transduction, and nuclear receptor studies. This review outlines the principles of two of the most commonly used instruments and highlights specific operating parameters that will assist in optimising experimental design, data generation, and analysis.

Advances in surface plasmon resonance biomolecular interaction analysis mass spectrometry (BIA/MS)

Ongoing, worldwide efforts in genomic and protein sequencing, and the ability to readily access corresponding sequence databases, have emphatically driven the development of high-performance bioanalytical instrumentation capable of characterizing proteins and protein-ligand interactions with great accuracy, speed and sensitivity. Two such analytical techniques have arisen over the past decade to play key roles in the characterization of proteins: surface plasmon resonance biomolecular interaction analysis (SPR-BIA) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF). SPR-BIA is used in the real-time investigation of biomolecular recognition events, and is thereby capable of providing details on the association and dissociation kinetics involved in the interaction, information ultimately leading to the determination of dissociation constants involved in the event. MALDI-TOF is used in the structural characterization, identification and sensitive detection of biomolecules. Although the two techniques have found many independent uses in bioanalytical chemistry, the combination of the two, to form biomolecular interaction analysis mass spectrometry (BIA/MS), enables a technique of analytical capabilities greater than those of the component parts. Reviewed here are issues of concern critical to maintaining high-levels of performance throughout the multiplexed analysis, as well as examples illustrating the potential analytical capabilities of BIA/MS.

The use of bioreactive probes in protein characterization

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF) has in the past decade found routine use in the biological sciences. With this use has evolved several mass spectrometric-based methods directed at the intricate investigation of biomolecular structure and function. One such methodology involves the enzymatic modification of a protein prior to the mass spectrometric readout of the resulting products. The enzyme-modification/mass spectrometric approach has a definite use in a number of applications, including: the verification/identification of protein sequence, elucidation of post-translational modifications, the investigation of protein higher-order structure, and even the characterization of the modifying enzyme. To avoid the potentials of sample loss and autolytic interferences in the mass spectrum, mass spectrometer targets can be covalently derivatized with enzymes for use in the characterization procedures. The enzymatically active, or bioreactive, probes are used by application of the analyte to the activated surface, followed by application of a suitable MALDI matrix and mass analysis from the surface of the probe. Limited transfer and handling steps eliminate sample losses, and surface-tethered enzymes (and autolytic fragments) are prohibited from interfering with analytical signals in the mass spectra. In addition, the probes are rapid and easy to use. Reviewed here are issues of concern during the manufacture and use of the bioreactive probes, and application of the probes to investigate protein structure and function.