Detecting large biomolecules from high-salt solutions by fused-droplet electrospray ionization mass spectrometry

Characterization of proteins by ambient mass spectrometry

Proteins play important roles in living systems and are topics of many fundamental and applied research projects. With the introduction of electrospray ionization and matrix-assisted laser desorption/ionization for analysis of biomacromolecules in the late 1980s, mass spectrometry has become an important tool for characterization of proteins. Characterization of proteins in raw samples by these mass spectrometric techniques, however, usually requires extensive sample pretreatment. Ambient ionization techniques are new mass spectrometric techniques that allow direct analysis of samples with no or little sample preparation. Can these techniques facilitate or even eliminate sample preparation for mass spectrometric analysis of proteins? Apart from sample preparation, do these techniques offer any new features for characterization of proteins as compared with conventional ESI or MALDI? Recent advances in characterization of proteins by ambient mass spectrometry are summarized and commented in this article.

Dual nano-electrospray for probing solution interactions and fast reactions of complex biomolecules

A novel nano-electrospray emitter has been developed containing two separated channels running throughout the length of the emitter. The emitters have been fabricated from "theta-shaped" borosilicate capillaries. Loading of different solutions into the two different channels opens up the possibility to study short timescale interactions within a Taylor cone common to both channels. The common Taylor cone constitutes an extremely small "mixing volume" of the order of femtolitres. The products of electrospray from the dual-channel emitters have been analysed by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Results are presented for interactions of vancomycin with diacetyl-L-lysyl-D-alanyl- D-alanine and interactions of vancomycin with deuterated vancomycin. On the basis of these results, it is concluded that, during electrospray, specific non-covalent adducts have been formed and that there have been exchange reactions involving making and breaking of covalent bonds.

Sequential and exhaustive ionization of analytes with different surface activity by probe electrospray ionization

The probe electrospray ionization (PESI) is an ESI-based ionization technique that generates electrospray from the tip of a solid metal needle. In this work, mass spectra for the single-shot PESI were measured as a function of time for a mixture of several analytes with different surface activity values. It was found that the analytes were elecrosprayed in the order of their surface activity. For example, detergent and protein were detected separately and respectively at the first and last stages of electrospray, for a mixed sample of 10(-3) M Triton X100 and 10(-5) M cytochrome c. For human breast cancer tissue, at first proteins such as α and β chains of hemoglobin, were observed as the dominant ions, but just before the liquid droplet on the needle was depleted only lipids were observed, meaning that PESI has the advantage of the suppression effect with analytes being detected separately in the order of their surface activity values.

Air flow assisted ionization for remote sampling of ambient mass spectrometry and its application

Ambient ionization methods are an important research area in mass spectrometry (MS) analysis. Under ambient conditions, the gas flow and atmospheric pressure significantly affect the transfer and focusing of ions. The design and implementation of air flow assisted ionization (AFAI) as a novel and effective, remote sampling method for ambient mass spectrometry are described herein. AFAI benefits from a high extracting air flow rate. A systematic investigation of the extracting air flow in the AFAI system has been carried out, and it has been demonstrated not only that it plays a role in the effective capture and remote transport of charged droplets, but also that it promotes desolvation and ion formation, and even prevents ion fragmentation during the ionization process. Moreover, the sensitivity of remote sampling ambient MS analysis was improved significantly by the AFAI method. Highly polar and nonpolar molecules, including dyes, pharmaceutical samples, explosives, drugs of abuse, protein and volatile compounds, have been successfully analyzed using AFAI-MS. The successful application of the technique to residue detection on fingers, large object analysis and remote monitoring in real time indicates its potential for the analysis of a variety of samples, especially large objects. The ability to couple this technique with most commercially available MS instruments with an API interface further enhances its broad applicability.

Electrospray droplet exposure to gaseous acids for the manipulation of protein charge state distributions

The exposure of electrospray droplets to acid vapors can significantly affect protein charge state distributions (CSDs) derived from unbuffered solutions. Such experiments have been conducted by leaking acidic vapors into the counter-current nitrogen drying gas of an electrospray interface. On the basis of changes in protein CSDs, protein folding and unfolding phenomena are implicated in these studies. Additionally, noncovalently bound complexes are preserved, and transient intermediates are observed, such as high charge state ions of holomyoglobin. CSDs of proteins containing disulfide bonds shift slightly, if at all, with acid vapor leak-in, but when these disulfide bonds are reduced in solution, charge states higher than the number of basic sites (Lys, Arg, His, and N-terminus) are observed. Since there is no observed change in the CSD of buffered proteins exposed to acidic vapors, this novel multiple charging phenomenon is attributed to a pH effect. Thus, this acid vapor leak-in approach can be used to reverse "wrong-way-round" nanoelectrospray conditions by altering solution pH in the charged droplets relative to the pH in bulk solution. In general, the exposure of electrospray droplets to acidic vapors provides means for altering protein CSDs independent of bulk unbuffered solution pH.

Contribution of liquid-phase and gas-phase ionization in extractive electrospray ionization mass spectrometry of primary amines

In this study, we investigated how binary mixtures of compounds influence each other's signal intensity in electrospray ionization (ESI), extractive electrospray ionization (EESI) and secondary electrospray ionization (SESI) experiments. The experiments were conducted using a series of homologous primary amines (from 1-butyl to 1- decylamine). In every experiment, two of the amines were present, and all 21 possible combinations were measured with EESI, ESI and SESI as ionization sources. Except for the volatility, which decreases with increasing molecular weight, the physico-chemical properties of the amines are very similar, so that the intensity ratio obtained in each experiment provides information about discrimination effects occurring during the ionization process. The results show that for the relatively volatile compounds investigated, the EESI ionization mechanism resembles the SESI-like gas-phase charge transfer more than ESI-like analyte ionization in solution. In addition, almost no discrimination effects were observed in the spectra obtained in EESI experiments. Quantitative EESI experiments with nonylamine as internal standard showed that EESI is capable of providing both more accurate and more precise results than SESI and ESI.

Study on reactions of long-lived phenoxathiin radical cation with aliphatic alcohols, phenol and phenyl halides in ambient condition by fused-droplet electrospray ionization mass spectrometry

he reactions of phenoxathiin radical cations with diverse organic compounds in ambient conditions were realized by using fused-droplet electrospray ionization mass spectrometry. In the investigation, the phenoxathiin radical cation was prepared by electrospray ionization. The reactants included aliphatic alcohols, phenol and phenyl halides and the reaction studies showed the unique reactivity the of phenoxathiin radical cation towards neutral organic compounds in ambient conditions, which has not been revealed in previous studies.

Fast detection of volatile organic compounds from bacterial cultures by secondary electrospray ionization-mass spectrometry

We propose a novel application of secondary electrospray ionization-mass spectrometry (SESI-MS) as a real-time clinical diagnostic tool for bacterial infection. It is known that volatile organic compounds (VOCs), produced in different combinations and quantities by bacteria as metabolites, generate characteristic odors for certain bacteria. These VOCs comprise a specific metabolic profile that can be used for species or serovar identification, but rapid and sensitive analytical methods are required for broad utility. In this study, the VOC profiles of five bacterial groups from four genera, Pseudomonas aeruginosa, Staphylococcus aureus, Escherichia coli, Salmonella enterica serovar Typhimurium, and Salmonella enterica serovar Pullorum, were characterized by SESI-MS. Thirteen compounds were identified from these bacterial cultures, and the combination of these VOCs creates a unique pattern for each genus. In addition, principal component analysis (PCA) was applied for the purpose of species or serovar discrimination. The first three principal components exhibit a clear separation between the metabolic volatile profiles of these five bacterial groups that is independent of the growth medium. As a first step toward addressing the complexity of clinical application, in vitro tests for mixed cultures were conducted. The results show that individual species or serovars in a mixed culture are identifiable among a biological VOC background, and the ratios of the detected volatiles reflect the proportion of each bacterium in the mixture. Our data confirm the utility of SESI-MS in real-time identification of bacterial species or serovars in vitro, which, in the future, may play a promising clinical role in diagnosing infections.

Real-time reaction monitoring by probe electrospray ionization mass spectrometry

Probe electrospray ionization (PESI) is a modified version of the electrospray ionization (ESI), where the capillary for sampling and spraying is replaced by a solid needle. High tolerance to salts and direct ambient sampling are major advantages of PESI compared with conventional ESI. In this study, PESI-MS was used to monitor some biological and chemical reactions in real-time, such as acid-induced protein denaturation, hydrogen/deuterium exchange (HDX) of peptides, and Schiff base formation. By using PESI-MS, time-resolved mass spectra and ion chromatograms can be obtained reproducibly. Real-time PESI-MS monitoring can give direct and detailed information on each chemical species taking part in reactions, and this is valuable for a better understanding of the whole reaction process and for the optimization of reaction parameters. PESI-MS can be considered as a potential tool for real-time reaction monitoring due to its simplicity in instrumental setup, direct sampling with minimum sample preparation and low sample consumption.

Ambient mass spectrometry: bringing MS into the "real world"

Mass spectrometry has recently undergone a second contemporary revolution with the introduction of a new group of desorption/ionization (DI) techniques known collectively as ambient mass spectrometry. Performed in an open atmosphere directly on samples in their natural environments or matrices, or by using auxiliary surfaces, ambient mass spectrometry (MS) has greatly simplified and increased the speed of MS analysis. Since its debut in 2004 there has been explosive growth in the applications and variants of ambient MS, and a very comprehensive set of techniques based on different desorption and ionization mechanisms is now available. Most types of molecules with a large range of masses and polarities can be ionized with great ease and simplicity with the outstanding combination of the speed, selectivity, and sensitivity of MS detection. This review describes and compares the basis of ionization and the concepts of the most promising ambient MS techniques known to date and illustrates, via typical analytical and bioanalytical applications, how ambient MS is helping to bring MS analysis deeper than ever into the "real world" open atmosphere environment--to wherever MS is needed.

Ambient ionization mass spectrometry

Mass spectrometric ionization methods that operate under ambient conditions and require minimal or no sample pretreatment have attracted much attention in such fields as biomedicine, food safety, antiterrorism, pharmaceuticals, and environmental pollution. These technologies usually involve separate ionization and sample-introduction events, allowing independent control over each set of conditions. Ionization is typically performed under ambient conditions through use of existing electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) techniques. Rapid analyses of gas, liquid, and solid samples are possible with the adoption of various sample-introduction methods. This review sorts different ambient ionization techniques into two main subcategories, primarily on the basis of the ionization processes, that are further differentiated in terms of the approach used for sampling.

What can we learn from ambient ionization techniques?

Ambient mass spectrometry-mass spectrometric analysis with no or minimal effort for sample preparation-has experienced a very rapid development during the last 5 years, with many different methods now available for ionization. Here, we review its range of applications, the hurdles encountered for its quantitative use, and the proposed mechanisms for ion formation. Clearly, more effort needs to be put into investigation of matrix effects, into defining representative sampling of heterogeneous materials, and into understanding and controlling the underlying ionization mechanisms. Finally, we propose a concept to reduce the number of different acronyms describing very similar embodiments of ambient mass spectrometry.

Probing the limits of liquid droplet laser desorption mass spectrometry in the analysis of oligonucleotides and nucleic acids

In the present work we demonstrate the advantages of LILBID mass spectrometry (laser-induced liquid bead ion desorption) in the analysis of nucleic acids and large oligonucleotides. For established methods like matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI), the mass analysis of oligonucleotides or of noncovalent oligonucleotide-protein complexes, in particular of very large ones, still represents a considerable challenge either due to the lack of native solutions or nonspecific adduct formation or due to a reduced salt tolerance or a high charge state of the ions. With LILBID, oligonucleotides, solvated in micro-droplets of aqueous buffer at certain pH and ion strength, are brought into the gas phase by laser ablation. We show that our method is able to detect single- and double-stranded oligonucleotides with high softness, demonstrated by the buffer dependence of the melting of a duplex. The absolute sensitivity is in the attomole range concomitant with a total analyte consumption in the femtomole region. The upper mass limit of oligonucleotides still detected with good signal-to-noise ratio with LILBID is the 1.66 MDa plasmid pUC19. With DNA ladders from short duplexes with sticky ends, we show that LILBID correctly reflects the relative thermodynamic stabilities of the ladders. Moreover, as an example for a specific DNA-protein complex we show that a NF-kappaB p50 homodimer binds sequence specifically to its match DNA. In summary we demonstrate that LILBID, although presently performed only with low mass resolution, due to these advantages, is an alternative mass spectrometric method for the analysis of oligonucleotides in general and of specific noncovalent nucleic acid-protein complexes in particular.

Generation of multiply charged peptides and proteins by radio frequency acoustic desorption and ionization for mass spectrometric detection

The design and implementation of a radio frequency acoustic desorption ionization (RADIO) source has been demonstrated for the analysis of multiply charged peptides and proteins. One muL aliquots of melittin, BNP-32, and ubiquitin ( approximately 1 mug of analyte) were deposited onto a quartz crystal microbalance (QCM) electrode before radio frequency actuation for desorption. Continuous electrospray parallel to/above the sampling surface enabled the ionization of desorbed species. Detection by a hybrid linear ion trap Fourier transform ion cyclotron resonance mass spectrometer confirmed the intact and dissociated species observed during MS and MS/MS experiments, respectively.

Construction of a versatile high precision ambient ionization source for direct analysis and imaging

The design and construction of a high precision ambient ionization source matrix-assisted laser desorption electrospray ionization (MALDESI) are described in full detail, including a complete parts list. The computer controlled high precision motion control system and high repetition rate Explorer laser are demonstrated during MALDESI-FT-ICR analysis of peptides and proteins ranging from 1 to 17 kDa. The high stability ionization source platform described herein demonstrates both the advantages of the new MALDESI source and versatility for application to numerous desorption and ionization techniques.

Established and emerging atmospheric pressure surface sampling/ionization techniques for mass spectrometry

The number and type of atmospheric pressure techniques suitable for sampling analytes from surfaces, forming ions from these analytes, and subsequently transporting these ions into vacuum for interrogation by MS have rapidly expanded over the last several years. Moreover, the literature in this area is complicated by an explosion in acronyms for these techniques, many of which provide no information relating to the chemical or physical processes involved. In this tutorial article, we sort this vast array of techniques into relatively few categories on the basis of the approaches used for surface sampling and ionization. For each technique, we explain, as best known, many of the underlying principles of operation, describe representative applications, and in some cases, discuss needed research or advancements and attempt to forecast their future analytical utility.

Application of probe electrospray to direct ambient analysis of biological samples

Recently, we have developed probe electrospray ionization (PESI) that uses a solid needle. In this system, the probe needle moves up and down along the vertical axis by a motor-driven system. At the highest position of the probe needle, electrospray is generated by applying a high voltage. In this study, we applied PESI directly to biological samples such as urine, mouse brain, mouse liver, salmon egg, and fruits (orange, banana, etc.). Strong ion signals for almost all the samples were obtained. The amount of liquid sample picked up by the needle is as small as pL or less, making PESI a promising non-invasive technique for detecting biomolecules in living systems such as cells. Therefore, PESI may be useful as a versatile and ready-to-use semi-online analytical tool in the fields of medicine, pharmaceuticals, agriculture, food science, etc.

Infrared laser-assisted desorption electrospray ionization mass spectrometry

We have used an infrared laser for desorption of material and ionization by interaction with electrosprayed solvent. Infrared laser-assisted desorption electrospray ionization (IR LADESI) mass spectrometry was used for the direct analysis of water-containing samples under ambient conditions. An ion trap mass spectrometer was modified to include a pulsed Er:YAG laser at 2.94 microm wavelength coupled into a germanium oxide optical fiber for desorption at atmospheric pressure and a nanoelectrospray source for ionization. Analytes in aqueous solution were placed on a stainless steel target and irradiated with the pulsed IR laser. Material desorbed and ablated from the target was ionized by a continuous stream of charged droplets from the electrosprayed solvent. Peptide and protein samples analyzed using this method yield mass spectra similar to those obtained by conventional electrospray. Blood and urine were analyzed without sample pretreatment to demonstrate the capability of IR LADESI for direct analysis of biological fluids. Pharmaceutical products were also directly analyzed. Finally, the role of water as a matrix in the IR LADESI process is discussed.

Calibrant delivery for mass spectrometry

This article describes a means of sampling ions that are created at a location remote from the primary ion source used for mass spectral analysis. Such a source can be used for delivery of calibrant ions on demand. Calibrant ions are sprayed into an atmospheric pressure chamber, at a position substantially removed from the sampling inlet. A gas flow sweeps the calibrants towards the sampling inlet, and a new means for toggling the second ion beam into the instrument can be achieved with the use of a repelling field established by an electrode in front of the sampling inlet. The physical separation of two or more sources of ions eliminates detrimental interactions due to gas flows or fields. When using a nanoflow electrospray tip as the primary ion source, the potential applied to the tip completely repels calibrant ions and there is no compromise in terms of electrospray performance. When calibrant ions are desired, the potential applied to the nanoflow electrospray tip is lowered for a short period of time to allow calibrant ions to be sampled into the instrument, thus providing a means for external calibration that avoids the typical complications and compromises associated with dual spray sources. It is also possible to simultaneously sample ions from multiple ion beams if necessary for internal mass calibration purposes. This method of transporting additional ion beams to a sampling inlet can also be used with different types of atmospheric pressure sources such as AP MALDI, as well as sources configured to deliver ions of different polarity.

Desorption electrospray ionization of aerosol particles

We have applied desorption electrospray ionization to aerosol particles. Ions were formed from aerosols by merging suspended dry particles with an electrospray of solvent in a modified ion trap mass spectrometer. Dry aerosol particles were generated using a fluidized bed powder disperser and directed toward the inlet of the mass spectrometer. A nanospray source was used to create a spray of solvent droplets directed at the inlet and at a right angle with respect to the aerosol. Ions generated by the interaction of the particles and electrospray were transferred into the ion trap mass spectrometer. Using this method, pure samples of caffeine and erythromycin A were analyzed. In addition, commonly available food and drug powders including instant cocoa powder, artificial sweetener and ibuprofen were analyzed.

Using electrospray-assisted pyrolysis ionization/mass spectrometry for the rapid characterization of trace polar components in crude oil, amber, humic substances, and rubber samples

We describe the use of electrospray-assisted pyrolysis ionization/mass spectrometry (ESA-Py/MS) to selectively ionize trace polar compounds that coexist with large amounts of nonpolar hydrocarbons in crude oil, amber, humic substances, and rubber samples. Samples of different origins are distinguished rapidly by their positive ion ESA-Py mass spectra without prior separation or chemical pretreatment. During ESA-Py analysis, the samples in their solid or liquid states were pyrolyzed at 590, 630 or 940 degrees C using a commercial Curie-point pyrolysis probe. The gaseous pyrolysates were transferred into a glass reaction cell. The polar compounds (M) in the pyrolysates were then ionized by electrospray ionization (ESI), yielding protonated molecules (MH+). Although the major components of the pyrolysates are nonpolar hydrocarbons, their lack of functional groups that can receive a proton in the ESA-Py source results in no hydrocarbon ion signals being produced; thus, the positive ions detected in ESA-Py mass spectra all result from trace polar components in the pyrolysates.

Characterization of the chemical components on the surface of different solids with electrospray-assisted laser desorption ionization mass spectrometry

In this study we demonstrate that electrospray-assisted laser desorption ionization (ELDI) mass spectrometry (MS) can be used to rapidly characterize major chemical components on the surfaces of different solids under ambient conditions. The major chemical components in (a) dried milks with different fat contents, (b) different color-regions of a painting, (c) the thin coating on a compact disc, (d) drug tablets, and (e) porcine brain tissue were rapidly characterized as protonated molecules [M+H](+) or sodiated molecules [M+Na](+) by ELDI-MS with minimum sample pretreatment. The ionized ions of synthetic polymer and dye standards were detected directly from dried sample solutions using either positive or negative ion mode. Further structural information for the FD&C Red dye was obtained through tandem mass spectrometric (MS/MS) analysis using an ion trap mass analyzer attached to the ELDI source.

Direct Protein Detection from Biological Media through Electrospray-Assisted Laser Desorption Ionization/Mass Spectrometry

We report here using a novel technology-electrospray-assisted laser desorption ionization (ELDI)/mass spectrometry-for the rapid and sensitive detection of the major proteins that exist in dried biological fluids (e.g., blood, tears, saliva, serum), bacterial cultures, and tissues (e.g., porcine liver and heart) under ambient conditions. This technique required essentially no sample pretreatment. The proteins in the samples were desorbed using a pulsed nitrogen laser without the assistance of an organic matrix. The desorbed protein molecules were then post-ionized through their fusion into the charged solvent droplets produced from the electrospray of an acidic methanol solution; electrospray ionization (ESI) proceeded from the newly formed droplets to generate the ESI-like protein ions. This new ionization approach combines some of the features of electrospray ionization with those of matrix-assisted laser desorption ionization (MALDI), that is, sampling of a solid surface with spatial resolution, generating ESI-like mass spectra of the desorbed proteins, and operating under ambient conditions.

Characterization of Synthetic Polymers by Electrospray-Assisted Pyrolysis Ionization-Mass Spectrometry

Rapid characterization of synthetic polymers based on the differences in the appearance of the polar pyrolysate ions was achieved by electrospray-assisted pyrolysis ionization-mass spectrometry. The pyrolytical products produced by a commercial Curie point pyroprobe were conducted to the tip of a capillary, where charged methanol droplets were generated continuously by electrospray. Polar components in the pyrolysates may react directly with the protonated methanol ions or fuse with the charged methanol droplets; electrospray ionization proceeds from the fused droplets to generate protonated analyte ions. The mass spectra obtained through this approach were used to rapidly distinguish the polymer standards that differ in the nature of building units, degrees of polymerization, and copolymerization coefficients.

Eliminating the Interferences from TRIS Buffer and SDS in Protein Analysis by Fused-Droplet Electrospray Ionization Mass Spectrometry

Multiply charged protein ions were detected from the solutions containing a high concentration of tris(hydroxymethyl) aminomethane buffer (TRIS) and sodium dodecyl sulfate (SDS) using fused-droplet electrospray ionization mass spectrometry (FD-ESI/MS). The sample aerosols were generated at ambient temperature with a pneumatic nebulizer commonly used to produce sample aerosols in an atmospheric pressure chemical ionization (APCI) source. The aerosols were carried by nitrogen gas to the tip of a capillary where charged methanol droplets had been continuously generated by electrospraying an acidic methanol solution. The neutral sample aerosols then fused with the charged methanol droplets and electrospray ionization proceeded from the newly formed fused droplets to generate multiply charged protein ions. Because of its low solubility in methanol, TRIS molecules (concentration as high as 1 M) were efficiently excluded from the newly formed droplets and the protein ion signals were detected and observed in the mass spectra. To remove the interferences from SDS, equal moles of positively charged cetyltrimethylammonium bromide (CTAB) was added into the SDS containing sample solution to form the dodecyl sulfate-cetyltrimethylammonium ion pair (DS-CTA). The DS-CTA ion pair has a low polarity and solubility in methanol and is excluded from the fused droplet. Protein ions were still detected from the solution containing 10(-2) M of SDS.

Electrospray-assisted laser desorption/ionization mass spectrometry for direct ambient analysis of solids

A new method of electrospray-assisted laser desorption/ionization (ELDI) mass spectrometry, which combines laser desorption with post-ionization by electrospray, was applied to rapid analysis of solid materials under ambient conditions. Analytes were desorbed from solid metallic and insulating substrata using a pulsed nitrogen laser. Post-ionization produced high-quality mass spectra characteristic of electrospray, including protein multiple charging. For the first time, mass spectra of intact proteins were obtained using laser desorption without adding a matrix. Bovine cytochrome c and an illicit drug containing methaqualone were chosen in this study to demonstrate the applicability of ELDI to the analysis of proteins and synthetic organic compounds.

2003 Fred Beamish Award Lecture — Exploring the dynamics of biological systems by mass spectrometry

This review describes the use of electrospray ionization mass spectrometry (ESI-MS) in conjunction with on-line rapid mixing techniques. This combination, termed "time-resolved" ESI-MS, provides a powerful approach for studying solution-phase reactions on timescales as short as a few milliseconds. Of particular interest is the application of this technique for monitoring protein folding reactions. Time-resolved ESI-MS can provide detailed information on structural changes of the polypeptide chain, while at the same time probing the occurrence of noncovalent ligand–protein interactions. Especially when used in combination with hydrogen–deuterium pulse labeling, these measurements yield valuable structural information on short-lived folding intermediates. Similar approaches can be used to monitor the dynamics of proteins under equilibrium conditions. Another important application of time-resolved ESI-MS are mechanistic studies on enzyme-catalyzed processes. These reactions can be monitored under presteady-state conditions, without requiring artificial chromophoric substrates or radioactive labeling. We also discuss the use of ESI-MS for monitoring noncovalent ligand–protein interactions by diffusion measurements. In contrast to conventional MS-based techniques, this approach does not rely on the preservation of noncovalent interactions in the gas phase. It appears that diffusion measurements by ESI-MS could become an interesting alternative to existing methods for the high throughput screening of compound libraries in the context of drug discovery.Key words: reaction intermediate, rapid mixing, kinetics, protein conformation, protein function.

Analysis of food proteins and peptides by chromatography and mass spectrometry

The research topics and the analytical strategies dealing with food proteins and peptides are summarized. Methods for the separation and purification of macromolecules of food concern by both high-performance liquid chromatography (HPLC) on conventional packings and perfusion HPLC are examined. Special attention is paid to novel methodologies such those based on multi-dimensional systems that comprise liquid-phase based protein separation, protein digestion and mass spectrometry (MS) analysis of food peptide and proteins. Recent applications of chromatography and MS-based techniques for the analysis of proteins and peptides in food are discussed.

Protein-folding kinetics and mechanisms studied by pulse-labeling and mass spectrometry

The "protein-folding problem" refers to the question of how and why a denatured polypeptide chain can spontaneously fold into a compact and highly ordered conformation. The classical description of this process in terms of reaction pathways has been complemented by models that describe folding as a biased conformational diffusion on a multidimensional energy landscape. The identification and characterization of short-lived intermediates provide important insights into the mechanism of folding. Pulsed hydrogen/deuterium exchange (HDX) methods are among the most powerful tools for studying the properties of kinetic intermediates. Analysis of pulse-labeled proteins by mass spectrometry (MS) provides information that is complementary to that obtained in nuclear magnetic resonance (NMR) studies; NMR data represent an average of entire protein ensembles, whereas MS can detect co-existing protein species. MS-based pulse-labeling experiments can distinguish between folding scenarios that involve parallel pathways, and those where folding is channeled through obligatory intermediates. The proteolytic digestion/MS technique provides spatially resolved information on the HDX pattern of folding intermediates. This method is especially important for proteins that are too large to be studied by NMR. Although traditional pulsed HDX protocols are based on quench-flow techniques, it is also possible to use electrospray (ESI) MS to analyze the reaction mixture on-line and "quasi-instantaneously" after labeling. This approach allows short-lived protein conformations to be studied by their HDX level, their ESI charge-state distribution, and their ligand-binding state. Covalent labeling of free cysteinyl residues provides an alternative approach to pulsed HDX experiments. Another promising development is the use of synchrotron X-rays to induce oxidation at specific sites within a protein for studying their solvent accessibility during folding.