DNA analysis using an electrospray scanning mobility particle sizer

Exploring the manganese-dependent interaction between a transcription factor and its corresponding DNA: insights from gas-phase electrophoresis on a nES GEMMA instrument

Manganese ion homeostasis is vital for bacteria and is achieved via manganese-dependent transcription factors. Manganese mediation of transcription factor attachment to the corresponding oligonucleotide sequences can be investigated, e.g. via electrophoretic mobility shift assays (EMSA). Formation of specific biocomplexes leads to differences in the migration pattern upon gel electrophoresis. Focusing on electrophoresis in the gas-phase, applying a nano electrospray gas-phase electrophoretic mobility molecular analyzer (nES GEMMA) also known as nES differential mobility analyzer (nES DMA), and on transcription factors (MntR proteins) from Bacillus subtilis and Mycobacterium tuberculosis, we took interest in the gas-phase electrophoresis of the corresponding biospecific complexes. We compared nES GEMMA, separating analytes in the nanometer regime (a few to several hundred nm in diameter) in the gas-phase in their native state according to particle size, to EMSA data. Indeed we were able to demonstrate manganese-mediated attachment of MntR to target genomic sequences with both analytical techniques. Despite some inherent pitfalls of the nES GEMMA method like analyte/instrument surface interactions, we were able to detect the target complexes. Moreover, we were able to calculate the molecular weight (MW) of the obtained species by application of a correlation function based on nES GEMMA obtained data. As gas-phase electrophoresis also offers the possibility of offline hyphenation to orthogonal analysis techniques, we are confident that nES GEMMA measurements are not just complementary to EMSA, but will offer the possibility of further in-depth characterization of biocomplexes in the future.

Absolute protein quantification based on calibrated particle counting using electrospray-differential mobility analysis

The traceability of in vitro diagnostics or drug products is based on the accurate quantification of proteins. In this study, we developed an absolute quantification approach for proteins. This method is based on calibrated particle counting using electrospray-differential mobility analysis (ES-DMA) coupled with a condensation particle counter (CPC). The absolute concentration of proteins was quantified with the observed protein particle number measured with ES-DMA-CPC, and the detection efficiency was determined by calibrators. The measurement performance and quantitative level were verified using two certificated reference materials, BSA and NIMCmAb. The linear regression fit for the detection efficiency values of three reference materials and one highly purified protein (myoglobin, BSA, NIMCmAb and fibrinogen) indicated that the detection efficiency and the particle size distribution of these proteins exhibited a linear relationship. Moreover, to explore the suitability of the detection efficiency-particle size curve for protein quantification, the concentrations of three typical proteinaceous particles, including two high molecular weight proteins (NIST reference material 8671 and D-dimer) and one protein complex (glutathione S-transferase dimer), were determined. This work suggests that this calibrated particle counting method is an efficient approach for nondestructive, rapid and accurate quantification of proteins, especially for measuring proteinaceous particles with tremendous size and without reference standards.

nES-DMA with Charge-reduction based on Soft X-ray Radiation: Analysis of a Recombinant Monoclonal Antibody

Due to the fast growing importance of monoclonal antibodies in biomedical research, bioanalytics and human therapy, sensitive, fast and reliable methods are needed to monitor their production, target their characteristics, and for their final quality control. Application of a nano electrospray (nES) with soft X-ray radiation (SXR) based charge reduction and differential mobility analysis (DMA, aka nano electrospray gas-phase electrophoretic mobility molecular analysis, nES GEMMA) allows the size-separation and detection of macromolecules and (bio-)nanoparticles from a few nm up to several hundreds of nm in diameter in a native-like environment. The current study focuses on the analysis of a 148 kDa recombinant monoclonal antibody (rmAb) with the above mentioned instrumental setup and applying an universal detector, i.e. a water-based condensation particle detector (CPC). Next to the intact rmAb, its aggregates and fragment products after digestion with IdeS protease were analyzed. Additionally, influence of temperature treatment and pH variation on the stability of the rmAb was monitored. In this context, changes in electrophoretic mobility diameter (EMD) values, peak shape, and signal intensity based on particle numbers were of interest. Molecular weights calculated by application of a correlation derived from respective standard protein compounds were compared to mass spectrometric values and were found to be in good accordance. To conclude, we demonstrate that nES-DMA is a valuable tool in the characterization and quality control of rmABs.

The charge reduction rate for multiply charged polymer ions via ion-ion recombination at atmospheric pressure

The charge reduction of multiply charged macromolecular ions via recombination with small ions in the gas phase is commonly employed to modulate the charge on macromolecules prior to mass spectrometric and mobility analyses. We employ a recently developed continuum-Molecular Dynamics (MD) calculation approach to determine the recombination rate coefficient of multiply charged (1 to 7 excess positive charged) polyethylene glycol ions (mass of 4600 Da) with smaller singly charged anions, modeled as NO₂^- ions. The continuum-MD approach accounts explicitly for the influence of the background gas on the recombination process, accounts explicitly for ion translational, vibrational, and rotational motion, and enables recombination rate coefficient calculation in nitrogen near atmospheric pressure, wherein neither low pressure nor high pressure recombination theories are strictly applicable. Continuum-MD simulations yield recombination rate coefficients near 3.9 × 10^-14 m³ s^-1 for singly charged ions, increasing to 3.0 × 10^-11 m³ s^-1 for the +7 ion. Pre-existing collision rate coefficient expressions for rigid ions are found to be within a factor of 2-5 of calculations for all charge states, but their use requires knowledge of an appropriate collision distance, which is not well-defined for flexible polymer ions. Continuum-MD-inferred rate coefficients are incorporated into a model of charge reduction, and the charge state distribution versus anion concentration determined with it is compared to charge reduction measurements made via atmospheric pressure differential mobility analysis. Good agreement is observed between simulations and experiments; although results are extremely sensitive to the recombination rate coefficients, experimental results are bound by models utilizing rates within a factor of 2 (0.5-2.0×) of the continuum-MD rates.

Microfluidic condensation nanoparticle counter using water as the condensing liquid for assessing individual exposure to airborne nanoparticles

We present a compact and inexpensive detection system that can accurately measure the number concentration of nanoparticles (NPs; particles smaller than 100 nm) in real-time for assessing individual exposure to airborne NPs in various environments. Our system is based on the condensation nucleation light scattering technique and uses water as the condensing liquid, which solves the self-contamination issues that affect most portable NP detection systems. Our system comprises two units: a microfluidic condensation chip for growing NPs into water droplets and a miniature optical detector for singly counting grown droplets. To effectively minimize the size and cost of our system, droplets are grown on a single chip according to the semiconductor manufacturing process. To use water as the condensing liquid, a super-hydrophilic wick (i.e., Cu micropillar array coated with CuO nanowires) is monolithically integrated into the chip. Simulations were performed to verify the method of generating supersaturated water vapor. Quantitative experiments using NaCl and Ag NPs revealed that our system grew NPs larger than 9.3 nm into 2.25 μm diameter water droplets and could count individual droplets over an extremely wide concentration range (0.021-105 N cm-3) with high accuracy. This outstanding performance allowed our system to resolve the daily pattern of the NP concentration along a metropolitan commuting street with strong agreement compared to the reference instrument. Because our system uses water, it can accurately monitor individual exposure to NPs in various kinds of environments, including multiuse facilities such as elementary schools and hospitals.

Virus-like particle size and molecular weight/mass determination applying gas-phase electrophoresis (native nES GEMMA)

(Bio-)nanoparticle analysis employing a nano-electrospray gas-phase electrophoretic mobility molecular analyzer (native nES GEMMA) also known as nES differential mobility analyzer (nES DMA) is based on surface-dry analyte separation at ambient pressure. Based on electrophoretic principles, single-charged nanoparticles are separated according to their electrophoretic mobility diameter (EMD) corresponding to the particle size for spherical analytes. Subsequently, it is possible to correlate the (bio-)nanoparticle EMDs to their molecular weight (M_W) yielding a corresponding fitted curve for an investigated analyte class. Based on such a correlation, (bio-)nanoparticle M_W determination via its EMD within one analyte class is possible. Turning our attention to icosahedral, non-enveloped virus-like particles (VLPs), proteinaceous shells, we set up an EMD/M_W correlation. We employed native electrospray ionization mass spectrometry (native ESI MS) to obtain M_W values of investigated analytes, where possible, after extensive purification. We experienced difficulties in native ESI MS with time-of-flight (ToF) detection to determine M_W due to sample inherent characteristics, which was not the case for charge detection (CDMS). nES GEMMA exceeds CDMS in speed of analysis and is likewise less dependent on sample purity and homogeneity. Hence, gas-phase electrophoresis yields calculated M_W values in good approximation even when charge resolution was not obtained in native ESI ToF MS. Therefore, both methods-native nES GEMMA-based M_W determination via an analyte class inherent EMD/M_W correlation and native ESI MS-in the end relate (bio-)nanoparticle M_W values. However, they differ significantly in, e.g., ease of instrument operation, sample and analyte handling, or costs of instrumentation.

Assessment of Capacity to Capture DNA Aerosols by Clean Filters for Molecular Biology Experiments

Experimental contamination by exogenous DNA is a major issue in molecular biological studies for data quality and its management. We herein assessed DNA aerosols for the risk of contamination and tested the capacity of clean air filters to trap and remove DNA aerosols. DNA aerosols were generated by atomizing a DNA solution and introduced into a laminar flow clean air unit. Capture and detection performed upstream and downstream of the clean air unit showed that a significant fraction (>99.96%) of introduced molecules was trapped and removed by the filter. Although DNA aerosols appear to be an avoidable source of exogenous contamination, a clearer understanding and careful experimental procedures are needed in order to perform contamination-free, high-quality molecular biology experiments.

Size and molecular weight determination of polysaccharides by means of nano electrospray gas-phase electrophoretic mobility molecular analysis (nES GEMMA)

Size, size distribution and molecular weight (MW) determination of nanoparticles and that are for example large polymers, are of great interest and pose an analytical challenge. In this context, nano electrospray gas-phase electrophoretic mobility molecular analysis (nES GEMMA) is a valuable tool with growing impact. Separation of single-charged analytes according to their electrophoretic mobility diameter (EMD) starting from single-digit EMDs up to several hundred nm diameters is possible. In case of spherical analytes, the EMD corresponds to the dry nanoparticle size. Additionally, the instrument is capable of number-based, single-particle detection following the recommendation of the European Commission for nanoparticle characterization (2011/696/EU). In case an EMD/MW correlation for a particular compound class (based on availability of well-defined standards) exists, a nanoparticle's MW can be determined from its EMD. In the present study, we focused on nES GEMMA of linear and branched, water-soluble polysaccharides forming nanoparticles and were able to obtain spectra for both analyte classes regarding single-charged species. Based on EMDs for corresponding analytes, an excellent EMD/MW correlation could be obtained in case of the branched natural polymer (dextran). This enables the determination of dextran MWs from nES GEMMA spectra despite high analyte polydispersity and in a size/MW range, where classical mass spectrometry is limited. EMD/MW correlations based on linear (pullulans, oat-ß-glucans) polymers were significantly different, possibly indicating challenges in the exact MW determination of these compounds by, for example, chromatographic and light scattering means. Despite these observations, nES GEMMA of linear, monosaccharide-based polymers enabled the determination of size and size-distribution of such dry bionanoparticles.

nES GEMMA Analysis of Lectins and Their Interactions with Glycoproteins - Separation, Detection, and Sampling of Noncovalent Biospecific Complexes

In order to better understand biological events, lectin-glycoprotein interactions are of interest. The possibility to gather more information than the mere positive or negative response for interactions brought mass spectrometry into the center of many research fields. The presented work shows the potential of a nano-electrospray gas-phase electrophoretic mobility molecular analyzer (nES GEMMA) to detect weak, noncovalent, biospecific interactions besides still unbound glycoproteins and unreacted lectins without prior liquid phase separation. First results for Sambucus nigra agglutinin, concanavalin A, and wheat germ agglutinin and their retained noncovalent interactions with glycoproteins in the gas phase are presented. Electrophoretic mobility diameters (EMDs) were obtained by nES GEMMA for all interaction partners correlating very well with molecular masses determined by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) of the individual molecules. Moreover, EMDs measured for the lectin-glycoprotein complexes were in good accordance with theoretically calculated mass values. Special focus was laid on complex formation for different lectin concentrations and binding specificities to evaluate the method with respect to results obtained in the liquid phase. The latter was addressed by capillary electrophoresis on-a-chip (CE-on-a-chip). Of exceptional interest was the fact that the formed complexes could be sampled according to their size onto nitrocellulose membranes after gas-phase separation. Subsequent immunological investigation further proved that the collected complex actually retained its native structure throughout nES GEMMA analysis and sampling. Graphical Abstract ᅟ.

Ultra-High Resolution Ion Mobility Separations Utilizing Traveling Waves in a 13 m Serpentine Path Length Structures for Lossless Ion Manipulations Module

We report the development and initial evaluation of a 13 m path length Structures for Lossless Manipulations (SLIM) module for achieving high resolution separations using traveling waves (TW) with ion mobility (IM) spectrometry. The TW SLIM module was fabricated using two mirror-image printed circuit boards with appropriately configured RF, DC, and TW electrodes and positioned with a 2.75 mm intersurface gap. Ions were effectively confined in field-generated conduits between the surfaces by RF-generated pseudopotential fields and moved losslessly through a serpentine path including 44 "U" turns using TWs. The ion mobility resolution was characterized at different pressures, gaps between the SLIM surfaces, and TW and RF parameters. After initial optimization, the SLIM IM-MS module provided about 5-fold higher resolution separations than present commercially available drift tube or traveling wave IM-MS platforms. Peak capacity and peak generation rates achieved were 246 and 370 s(-1), respectively, at a TW speed of 148 m/s. The high resolution achieved in the TW SLIM IM-MS enabled, e.g., isomeric sugars (lacto-N-fucopentaose I and lacto-N-fucopentaose II) to be baseline resolved, and peptides from an albumin tryptic digest were much better resolved than with existing commercial IM-MS platforms. The present work also provides a foundation for the development of much higher resolution SLIM devices based upon both considerably longer path lengths and multipass designs.

Reversible and Irreversible Mechanical Damaging of Large Double-Stranded DNA upon Electrospraying

Electrohydrodynamic spraying (or electrospaying, ES) of DNA solutions is an attractive technique for applications in mass spectrometry, in microarray fabrication, and in generation of DNA nanoaerosols. Here we report how ES affects DNA structure and evaluate possible ways to reduce DNA damage upon ES. It is shown that under any ES conditions, linear λ-phage DNA is subjected to intensive rupture producing a mixture of fragments. In addition to such fragmentation, notable reversible changes in the DNA structure were revealed by a slight increase in DNA electrophoretic mobility. The degree of fragmentation was shown to decrease with decreased DNA length and with increased flow rate through the ES capillary. Fragments shorter than 5 kbp did not show any notable damage upon ES. Both experimental data and theoretical estimations of the forces acting on DNA during ES indicate that DNA is damaged by mechanical forces, and the damage takes place in the vicinity of the Taylor cone tip, presumably due to the high shear stress or/and viscous drag forces operating there. Condensation of λ-DNA with hexamminecobalt(III) ions completely protected it from any damage upon ES.

Electrospray differential mobility analysis for nanoscale medicinal and pharmaceutical applications

Nanoscale characterization tools hold the potential to overcome long-standing medicinal and pharmaceutical challenges. For example, electrospray differential mobility analysis (ES-DMA) is an emerging tool that rapidly provides label-free multimodal size distributions for proteins and particles from ~1 nm to <500 nm with subnanometer precision. Here we critically review the contributions of this tool to medicine, pharmaceutical practice, and pharmaceutical production. Our review critically evaluates, first, the use of ES-DMA for diagnostic strategies that detect and quantify lipoproteins, bacterial infections, viruses and amyloid fibrillation and then focuses on ES-DMA's contribution to treatment strategies that employ tailored virus-like particles as vaccines and decorated nanoparticle vectors for gene delivery. Our review also highlights ES-DMA's contribution to viral clearance and antibody aggregation and potential as a process analytical technology (PAT).

FROM THE CLINICAL EDITOR

Electrospray differential mobility analysis is an emerging nanotechnology-based tool with potential clinical utility in the detection and quantification of lipoproteins, glycoproteins, viruses, amyloids, bacterial infections. Its contribution to treatment strategies and pharmaceutical production is also discussed in this comprehensive review.

Analytical approaches for size and mass analysis of large protein assemblies

Analysis of the size and mass of nanoparticles, whether they are natural biomacromolecular or synthetic supramolecular assemblies, is an important step in the characterization of such molecular species. In recent years, electrospray ionization (ESI) has emerged as a technology through which particles with masses up to 100 MDa can be ionized and transferred into the gas phase, preparing them for accurate mass analysis. Here we review currently used methodologies, with a clear focus on native mass spectrometry (MS). Additional complementary methodologies are also covered, including ion-mobility analysis, nanomechanical mass sensors, and charge-detection MS. The literature discussed clearly demonstrates the great potential of ESI-based methodologies for the size and mass analysis of nanoparticles, including very large naturally occurring protein assemblies. The analytical approaches discussed are powerful tools in not only structural biology, but also nanotechnology.

Characterization and deposition of various light-harvesting antenna complexes by electrospray atomization

Photosynthetic organisms have light-harvesting complexes that absorb and transfer energy efficiently to reaction centers. Light-harvesting complexes (LHCs) have received increased attention in order to understand the natural photosynthetic process and also to utilize their unique properties in fabricating efficient artificial and bio-hybrid devices to capture solar energy. In this work, LHCs with different architectures, sizes, and absorption spectra, such as chlorosomes, Fenna-Matthews-Olson (FMO) protein, LH2 complex, and phycobilisome have been characterized by an electrospray-scanning mobility particle-sizer system (ES-SMPS). The size measured by ES-SMPS for FMO, chlorosomes, LH2, and phycobilisome were 6.4, 23.3, 9.5, and 33.4 nm, respectively. These size measurements were compared with values measured by dynamic light scattering and those reported in the literature. These complexes were deposited onto a transparent substrate by electrospray deposition. Absorption and fluorescence spectra of the deposited LHCs were measured. It was observed that the LHCs have light absorption and fluorescence spectra similar to that in solution, demonstrating the viability of the process.

Physical analysis of virus particles using electrospray differential mobility analysis

This review critically examines an emerging tool to measure viral clearance from biomanufacturing streams, monitor assembly of viruses and virus-like particles, rapidly identify viruses from biological milieu, assay virus neutralization, and prepare bionanoconjugates for bacterial detection. Electrospray differential mobility analysis (ES-DMA) is a tool of choice to simultaneously determine viral size and concentration because it provides full multimodal size distributions with subnanometer precision from individual capsid proteins to intact viral particles. The review contrasts ES-DMA to similar tools and highlights expected growth areas including at-line process sensing as a process analytical technology (PAT), bioseparating as a distinct unit operation, monitoring viral reactions, and interrogating virus-host protein interactions.

Size determination of hyaluronan using a gas-phase electrophoretic mobility molecular analysis

Hyaluronan (HA) is a linear non-sulfated polysaccharide mainly found in the extracellular matrix. The size of HA can vary from a few disaccharides up to at least 25,000 units, reaching molecular weights of 10 10(3) kDa. HA has many biological functions, and both its size and tissue concentration play an important role in many physiological and pathological processes. It is relatively easy to determine the HA concentration using enzyme-linked binding protein assays, but the molecular weight of HA has so far been shown to be a more challenging task to measure. Here, we present a method for size determination of HA using gas-phase electrophoretic mobility molecular analysis (GEMMA), which utilizes the electrophoretic mobility of molecules in air to estimate the molecular weight of the analyte. We show that this method gives reliable molecular weight estimations of HA in the range of 30-2400 kDa, which covers almost its whole biological range. The average measuring time for one GEMMA spectrum is between 5 and 10 min using only 6 pg of HA. In addition, the peak area in a GEMMA spectrum can be used to estimate the HA concentration in the sample. The high sensitivity and small sample volumes make GEMMA an excellent tool for both size determinations and estimation of concentration of samples with low HA concentration, as is the case for HA extracted from small tissue samples.

A scanning frequency mode for ion cyclotron mobility spectrometry

A new operational mode for an ion cyclotron mobility spectrometry instrument is explored as a possible means of performing high-resolution separations. The approach is based on oscillating fields that are applied to segmented regions of a circular drift tube. Ions with mobilities that are resonant with the frequency of field application are transmitted while nonresonant species are eliminated. An ion mobility spectrum is obtained by scanning the drift field application frequency. The approach is demonstrated by examining mixtures of ions produced by electrospraying the substance P peptide, as well as a mixture of tryptic peptides obtained by enzymatic digestion of cytochrome c. Drift field application frequency scans of substance P peptide ions show that it is possible to separate [M+2H](2+) ions, and compact and elongated forms of [M+3H](3+) ions. The resolution of different ions is related to the number of cycles for the analysis. At high cycle numbers (>50 3/4 or a drift length of 9242.03 cm) values of the resolving power can exceed 300 with a maximum resolving power of ∼400. The ability to tune the resolving power of a mobility-based separation by varying the ion cycle number has substantial analytical utility.

Using nanoelectrospray ion mobility spectrometry (GEMMA) to determine the size and relative molecular mass of proteins and protein assemblies: a comparison with MALLS and QELS

The determination of protein assembly size and relative molecular mass is currently of great importance in biochemical analysis. In particular, the technique of nanoelectrospray (nES) with a gas-phase electrophoretic mobility molecular analyzer (GEMMA) has received increased attention for such measurements. However, in order for the GEMMA technique to gain broader acceptance in protein analysis, it must be further evaluated and compared with other established bioanalytical techniques. In the present study, nES-GEMMA was evaluated for the analysis of a set of protein and protein complexes involved in the Sec and the bacterial type III secretion pathway of enteropathogenic Escherichia coli bacteria. The same set of proteins, isolated and purified using standard biochemical protocols, were also analyzed using multi-angle laser light scattering (MALLS) and quasi-elastic light scattering (QELS), following size exclusion chromatography. This allowed for direct comparisons between the three techniques. It was found that nES-GEMMA, in comparison to the more established MALLS and QELS techniques, offers several complementary advantages. It requires considerably less amount of material, i.e., nanogram vs. milligram amounts, and time per sample analysis, i.e., few minutes vs. tens of minutes. Whereas the determined size and relative molecular mass are similar between the compared methods, the electrophoretic diameters determined using nES-GEMMA seem to be systematically smaller compared to the hydrodynamic diameter derived by QELS. Some of the GEMMA technique disadvantages include its narrow dynamic range, limited by the fact that at elevated protein concentrations there is increased potential for the occurrence of nES-induced oligomers. Thus, it is preferred to analyze dilute protein solutions because non-specific oligomers are less likely to occur whereas biospecific oligomers remain detected. To further understand the formation of nES-oligomers, the effect of buffer concentration on their formation was evaluated. Also, nES-GEMMA is not compatible with all the buffers commonly used with MALLS and QELS. Overall, however, the nES-GEMMA technique shows promise as a high-throughput proteomics/protein structure tool.

Parallel differential mobility analysis for electrostatic characterization and manipulation of nanoparticles and viruses

The electrophoretic mobility of charged, airborne nanoparticles (NPs) or macromolecules and their specific complexes opens new avenues for their analysis and handling. The newly developed parallel differential mobility analyzer in combination with an electrostatic particle sampler enables not only the characterization of bio-NPs, but even their sampling while preserving their bioactivity (e.g., the enzyme activity of galactosidase). Precondition for the applicability of this technique is a well-defined charging status of the NPs in question. This charge conditioning can be achieved by means of a radioactive source, Po-210, even if the yield in terms of charged particles is low for sub-20-nm particles and the aging of the source influences the size spectra measured. Nevertheless, this technique enables size-defined sampling and enrichment, combined with real-time measurement of the size of both NPs and viruses. Furthermore, it allows determination of the number of attached biospecific antibodies, thereby providing information about the surface coverage of viruses by antibodies.

Determination of cross sections by overtone mobility spectrometry: evidence for loss of unstable structures at higher overtones

Overtone mobility spectrometry (OMS) is examined as a means of determining the collision cross sections for multiply charged ubiquitin and substance P ions, as well as for singly charged rafinose and melezitose ions. Overall, values of collision cross section measured by OMS for stable ion conformations are found to be in agreement with values determined by conventional ion mobility spectrometry (IMS) measurements to within ∼1% relative uncertainty. The OMS spectra for ubiquitin ions appear to favor different conformations at higher overtones. We propose that the changes in the distributions as a function of the overtone region in which they are measured arise from the elimination of ions that undergo structural transitions in the drift regions. Kinetics simulations suggest that structural transitions occurring on the order of a few ms and resulting in an ∼4% change in ion collision cross sections are detected by OMS measurements. The unique method of distinguishing ion mobilities with OMS reveals these structural transitions which are not readily apparent from traditional IMS measurements.

Flow interface for charge-reduced electrospray of nanoparticle solutions

A charge reduction (CR) interface for electrospray ionization that permits simultaneous analysis of nanoparticle solutions by multiple detection methods was characterized. In the direct infusion configuration, a constant flow of analyte solution undergoes electrospray ionization (ESI). The charged aerosol is sampled directly into the atmospheric pressure inlet of a quadrupole time-of-flight mass spectrometer and into a CR device followed by a differential mobility analyzer (DMA) and condensation particle counter (CPC). In the plug injection configuration, analyte solution is injected into a liquid chromatograph. The effluent is split to an evaporative light scattering detector (ELSD) and the ESI interface. The charged aerosol is then sampled through the CR device directly into the CPC. Performance characteristics of the two configurations were studied with sucrose and protein solutions. When a liquid flow rate in the low microliter per minute range was used, the reconstructed droplet size distribution from the ESI interface had an average diameter of 184 nm with a geometric standard deviation of 1.4. For the first configuration, the linear working range was wider for ESI-MS than the CR device-DMA-CPC. For the second configuration, the detection efficiency, defined as the fraction of molecules flowing through the ESI interface that are ultimately detected by the CPC, was on the order of 10(-6). Simultaneous measurements with the ELSD and CPC were consistent with analyte molecular size and may provide a means of estimating the size of unknown particles.

Sucrose clusters exhibiting a magic number in dilute aqueous solutions

We report clustering of sucrose molecules in dilute aqueous solutions, based on measurements of the electrical mobility spectrum of singly charged airborne clusters produced by electrospraying the solution. The spectrum contains peaks with a smooth envelope, except for one peak which has about twice the amplitude of the envelope. The mobility at this peak is found to be somewhat higher than would be expected based on the rest of the sequence, suggesting a more compact structure for this cluster. This "magic" peak is prominent for basic, but not acidic, solutions. It is argued that these observations demonstrate the existence of clusters of many sizes in the solution prior to electrospraying, including the "magic" cluster. A simplified model is presented which reproduces the observed features.

Direct determination of lipoprotein particle sizes and concentrations by ion mobility analysis

BACKGROUND

Current methods for measuring the concentrations of lipoprotein particles and their distributions in particle subpopulations are not standardized. We describe here and validate a new gas-phase differential electrophoretic macromolecular mobility-based method (ion mobility, or IM) for direct quantification of lipoprotein particles, from small, dense HDL to large, buoyant, very-low-density lipoprotein (VLDL).

METHODS

After an ultracentrifugation step to remove albumin, we determined the size and concentrations of lipoprotein particles in serum samples using IM. Scan time is 2 min and covers a particle range of 17.2-540.0 A. After scanning, data are pooled by totaling the particle number across a predetermined size range that corresponds to particular lipoprotein subclasses. IM results were correlated with those of standard methods for cholesterol and apolipoprotein analysis.

RESULTS

Intra- and interassay coefficients of variation for LDL particle size were <1.0%. The intra- and interassay variation for LDL and HDL particle subfraction measurements was <20%. IM-measured non-HDL correlated well with apolipoprotein B (r = 0.92).

CONCLUSIONS

The IM method provides accurate, reproducible, direct determination of size and concentration for a broad range of lipoprotein particles. Use of this methodology in studies of patients with cardiovascular disease and other pathologic states will permit testing of its clinical utility for risk assessment and management of these conditions.

Colloidal nanoparticle analysis by nanoelectrospray size spectrometry with a heated flow

A nanoelectrospray-furnace-scanning mobility particle spectrometer (SMPS) system was developed which is capable of rapidly and accurately measuring the size distributions of colloidal nanoparticles. Many colloidal suspensions require the use of nonvolatile surfactants to stabilize the suspensions. Nonvolatile materials coat colloidal particles and form residue particles during the electrospray process; thus, SMPS measurements are normally inaccurate for colloidal particles. Here, a tubular furnace reactor is used to evaporate residue nanoparticles and coatings, which allows for correct nanoparticle size measurement. Test suspensions of silver, gold, and SiO(2) nanoparticles were measured in the electrospray-furnace-SMPS system. SMPS measurements and field emission scanning electron microscopy measurements were in excellent agreement for all test suspensions. High-temperature heating in the furnace was used to evaporate the nanoparticles themselves, which shifted the nanoparticle size spectra to smaller sizes, allowing for thermal analysis to be performed on colloidal suspensions in addition to size measurement.

Charge Reduced Electrospray Size Spectrometry of Mega- and Gigadalton Complexes: Whole Viruses and Virus Fragments

The ability to analyze and identify large macromolecular complexes whose molecular weight is beyond the analyzable range of mass spectrometry is of great interest. The size of such complexes makes them suitable for analysis via mobility size spectrometry. In this work, charge reduced electrospray size spectrometry was used for the analysis of bacteriophage viruses with total molecular masses ranging from 3.6 MDa up to the gigadalton range. The electrospray source used was operated in "cone jet" mode with a mean droplet diameter of 170.56 nm. Bacteriophage MS2 was found to have a mobility diameter of 24.13 +/- 0.06 nm and remain highly viable after the electrospray process. Larger bacteriophages T2 and T4 have lengths greater than the diameter of the electrospray jet and droplets; thus, they could not be completely enclosed and were found to fragment at the virus capsid head-tail noncovalent interface during either the jet formation or jet breakup process. No viable T2 or T4 virions were detectable after being electrosprayed. While the exact mechanism of fragmentation could not be determined, it is proposed here that macromolecular fragmentation at noncovalent interfaces occurs due to mechanically and electrically induced stresses during jet formation and jet breakup. Bacteriophage T4 capsid heads were found to be statistically significantly larger than bacteriophage T2 capsid heads, with a mean peak diameter of 88.32 +/- 1.02 nm for T4 and 87.03 +/- 0.18 nm for T2. While capsid head fragments were detectable, tail and tail-fiber fragments could not be detected by size spectrometric analysis. This is attributed to the fact that the contractile tails of bacteriophage T2 and T4 virions mechanically deform to a varying degree while confined within the smaller jet and droplets. Further evidence of contractile tail deformation during the electrospray process was found by measuring the size spectrum of bacteriophage lambda, which has a noncontractile tail. Bacteriophage lambda had two distinct peaks in its size spectrum, one corresponding to the capsid head and the other corresponding to the tail fragment. Size spectrometry was also used for rapid quantification of virus concentrations, thus demonstrating its full capabilities in the analysis of large macromolecular complexes.

Aerosol size standards in the nanometer size range

A technique for generating charged aerosols of polystyrene (pSty) with narrow size distributions has been developed. It is based on electrospraying commercial narrow mass standards of pSty dissolved in l-methyl-2-pyrrolidone (NMP) seeded with the newly synthesized salt dimethyl ammonium formate. This salt imparts a much larger electrical conductivity than previously known NMP electrolytes, leading to higher quality sprays with greatly reduced attachment of impurities. Controlling the solute concentration enables forming polystyrene particles containing from one up to more than ten single polystyrene molecules, whereby 4 mass standards with molecular weights from 9200 up to 96,000 g/mol yield particles covering densely the diameter range from 3 to 11 nm. Combined mobility and mass measurement with a differential mobility analyzer and a mass spectrometer in tandem are carried out with a pSty sample 9200 amu in molecular weight. They fix directly the mass versus mobility relation near 9200 amu, and indirectly for the other standards and their clusters. The apparent particle density resulting from mobility versus mass data agrees with the bulk density of the polymer, indicating that the particles are dense and spherical. Although these standards have been studied only in gaseous suspension, their injection in liquids such as water where pSty is insoluble should keep them spherical.

Oligomers of ERBB3 have two distinct interfaces that differ in their sensitivity to disruption by heregulin

ErbB receptors associate in a ligand-dependent or -independent manner, and overexpression of epidermal growth factor receptor (ErbB1) or ErbB2 results in ligand-independent activation. Ligand-independent activation is poorly understood, and dimerization alone is not sufficient for activation. ErbB receptors also form higher order oligomers, but the mechanism of oligomer formation and their contribution to signaling are not known. The kinase-deficient ErbB3 as well as its extracellular domains are particularly prone to ligand-independent oligomerization, and oligomers are destabilized by binding of the ligand heregulin. In contrast, ligand binding facilitates heterodimerization with ErbB2 and is expected to stabilize an extended conformation of the ErbB3 extracellular domain (ECD) in which the dimerization interface is exposed. In the absence of ligand, ErbB3 can adopt a closed conformation that is held together by an intramolecular tether. We used a constitutively extended form of the ErbB3-ECD to analyze the conformation of the ECD in oligomers and the mechanism of oligomer disruption by heregulin. The extended conformation of the ECD forms oligomers more readily, suggesting the crystallographically defined dimer interface is one of the interfaces involved in oligomerization. Heregulin destabilizes oligomeric complexes but not dimers, which are neither stabilized nor disrupted by ligand binding, indicating a distinct second interface in oligomers of ErbB3. Cross-linking and activation studies on membrane-embedded ErbB3/ErbB2 chimeras confirm this dual effect of heregulin. Most of the ErbB3-ECD on the cell surface is apparently kept in an open conformation through oligomerization, and the resulting oligomers adopt a conformation representing a state of reduced activity.

Optical detection methods for mass spectrometry of macroions

Detection of macroions has been a challenge in the field of mass spectrometry. Conventional ionization-based detectors, relying on production and multiplication of secondary electrons, are restricted to detection for charged particles of m/z < 1 x 10(6). While both energy-sensitive and charge-sensitive detectors have been developed recently to overcome the limitation, they are not yet in common use. Photon-sensitive detectors are suggested to be an alternative, with which detection of macroions (or charged particles) by either elastic light scattering (ELS) or laser-induced fluorescence (LIF) has been possible. In this article, we provide a critical review on the developments of novel optical detection methods for mass spectrometry of macroions, including both micron-sized and nano-sized synthetic polymers as well as high-mass biomolecules. Design and development of new spectrometers making possible observations of the mass spectra of macroions with sizes in the range of 10-10(3) nm or masses in the range of 1-10(6) MDa are illustrated. The potential and promise of this optical approach toward macroion detection with high efficiency are discussed in practical aspects.

Mass Analysis of Water-Soluble Polymers by Mobility Measurement of Charge-Reduced Ions Generated by Electrosprays

Aqueous solutions of poly(ethylene glycol) (PEG) in a 10 mM ammonium acetate buffer are electrosprayed, and the maximum charge state on the resulting gas-phase ions is reduced to unity using a radioactive source. The mobility distribution of these charged particles is then measured in air in a differential mobility analyzer of unusually high resolution. The relation Z(m) between the mobility Z of a polymer molecule and its mass m is determined by means of narrowly distributed PEG mass standards. The molecular weight range of available standards is extended by generating clusters containing from one up to six molecules of the primary PEG standard. The mass at the peak of the distribution of the lowest standard (PEG-4k) is determined by MALDI mass spectrometry and agrees with the manufacturer's value and previous MALDI literature data. The masses for the 50K and 120K standards are found to differ by 8.6 and 6.6%, respectively, from the manufacturer's value. Using known relationships, the particle diameter d of the ions is calculated from the measured mobility. Plots of d versus m(1/3) give straight lines over the full mass range studied (4000-700 000 Da, particle diameter from 3 to 12 nm), indicating that these PEG particles are indeed spherical and have a density rho independent of size. The slope of the d versus m(1/3) curve provides a density rho = 1.25 g/cm(3), close to the known bulk density, rho(PEG) = 1.21 g/cm(3).

Mass Distribution Measurement of Water-Insoluble Polymers by Charge-Reduced Electrospray Mobility Analysis

1-Methyl-2-pyrrolidone (NMP) seeded with 5% trifluoroacetic acid is identified as a singular buffer, polar enough to produce fine electrospray drops, yet having excellent solubility for many industrial polymers such as polystyrene (PSR) and poly(methyl methacrylate) (PMMA). Four PSR mass standards (M = 9.2, 34.5, 68, and 170 kDa) with narrow mass distributions are electrosprayed from their solutions in this buffer. The high charge on the resulting ions is reduced to unity with a radioactive source, whereby their electrical mobility distributions, determined by a differential mobility analyzer, yield unambiguously their size distribution. Each standard produces (at high solution concentration) several mobility peaks associated with the formation of particles containing from one to six polymer molecules, used to establish a relation Z(M) between electrical mobility Z and polymer mass. Within the indeterminacy given by inaccuracies in the nominal masses of the standards, this relation indicates that the polymers form spherical balls with a density close to the bulk density of polystyrene, as seen previously with poly(ethylene glycol) chains. Good mobility spectra from the same buffer are also obtained for PMMA (M = 49 kDa). Because NMP is less conductive and contains more involatile impurities than common aqueous buffers, the electrospray ions formed tend to carry a small contaminant crust, which distorts the inferred mass distribution unless a high spray quality is achieved.

Charge reduction electrospray mass spectrometry

A new mass spectrometric technique, charge reduction electrospray mass spectrometry (CREMS), allowing the analysis of complex mixtures of biological molecules is described. The charge state of ions produced by electrospray ionization may be reduced in a controlled manner to yield predominantly singly charged ions through reactions with bipolar (i.e., both positively and negatively charged) ions generated using a 210Po alpha particle source. The electrospray-generated multiply charged ions undergo charge reduction in a "neutralization chamber" positioned before the entrance nozzle to the mass spectrometer. The ions are detected using a commercial orthogonal electrospray time-of-flight mass spectrometer, although the neutralization chamber can be adapted to virtually any mass analyzer. The CREMS results obtained exhibit a signal intensity drop-off with increasing oligonucleotide size similar to that observed with matrix-assisted laser desorption/ionization mass spectrometry. Proton-transfer reactions were found to be responsible for reducing charge on proteins and oligonucleotides in both positive and negative ion mode.

Nucleotide exchange in genomic DNA of rat hepatocytes using RNA/DNA oligonucleotides. Targeted delivery of liposomes and polyethyleneimine to the asialoglycoprotein receptor

Chimeric RNA/DNA oligonucleotides have been shown to promote single nucleotide exchange in genomic DNA. A chimeric molecule was designed to introduce an A to C nucleotide conversion at the Ser365 position of the rat factor IX gene. The oligonucleotides were encapsulated in positive, neutral, and negatively charged liposomes containing galactocerebroside or complexed with lactosylated polyethyleneimine. The formulations were evaluated for stability and efficiency in targeting hepatocytes via the asialoglycoprotein receptor. Physical characterization and electron microscopy revealed that the oligonucleotides were efficiently encapsulated within the liposomes, with the positive and negative formulations remaining stable for at least 1 month. Transfection efficiencies in isolated rat hepatocytes approached 100% with each of the formulations. However, the negative liposomes and 25-kDa lactosylated polyethyleneimine provided the most intense nuclear fluorescence with the fluorescein-labeled oligonucleotides. The lactosylated polyethyleneimine and the three different liposomal formulations resulted in A to C conversion efficiencies of 19-24%. In addition, lactosylated polyethyleneimine was also highly effective in transfecting plasmid DNA into isolated hepatocytes. The results suggest that both the liposomal and polyethyleneimine formulations are simple to prepare and stable and give reliable, reproducible results. They provide efficient delivery systems to hepatocytes for the introduction or repair of genetic mutations by the chimeric RNA/DNA oligonucleotides.

Analysis of a 3.6-MDa hexagonal bilayer hemoglobin from Lumbricus terrestris using a gas-phase electrophoretic mobility molecular analyzer

The recent successful use of electrospray gas-phase electrophoretic mobility molecular analysis (GEMMA) to separate globular proteins (mass 6 to 670 kDa) and the excellent correlation found between the electrophoretic mobility diameter (EMD), or Millikan diameter, and the protein mass (S. L. Kaufman et al., 1996, Anal. Chem. 68, 1895-1904; 1996, Anal. Chem. 68, 3703), prompted the examination of a large protein complex, the 3.6-MDa, heteromultimeric, hexagonal bilayer hemoglobin (Hb) and its subunits from the earthworm Lumbricus terrestris. The native Hb had an EMD of 25.7 nm and the products of its dissociation at pH >8 and <5 were resolved into peaks with EMDs of 10.5, 6.3, 5.0, and 4.2 nm, identified as a dodecamer of globin chains ([a+b+c]3d3, 213 kDa), the disulfide-bonded trimer of globin chains ([a+b+c], 52.7 kDa), all the linker chains (L1, 27.5 kDa; L2, 32.1 kDa; L3, 24.9 kDa; L4, 24. 1 kDa), and the monomer subunit (chain d, 17 kDa), respectively. Reassembly of the Hb complex was observed on restoring the pH from >8 to 7. The EMDs and the masses of the Hb and its subunits are in excellent agreement with the correlation found earlier, under the assumption of nearly spherical shape with an effective density around 0.7 g/cm3. GEMMA also provided a profile of the Hb completely dissociated in 0.1% SDS; its deconvolution permitted a quantitative determination of the subunit stoichiometry, providing a globin to linker ratio of 3 to 1.