Electrospray ionization mass spectrometry in the study of biomolecular non-covalent interactions

Biodereplication of Antiplasmodial Extracts: Application of the Amazonian Medicinal Plant Piper coruscans Kunth

Improved methodological tools to hasten antimalarial drug discovery remain of interest, especially when considering natural products as a source of drug candidates. We propose a biodereplication method combining the classical dereplication approach with the early detection of potential antiplasmodial compounds in crude extracts. Heme binding is used as a surrogate of the antiplasmodial activity and is monitored by mass spectrometry in a biomimetic assay. Molecular networking and automated annotation of targeted mass through data mining were followed by mass-guided compound isolation by taking advantage of the versatility and finely tunable selectivity offered by centrifugal partition chromatography. This biodereplication workflow was applied to an ethanolic extract of the Amazonian medicinal plant Piper coruscans Kunth (Piperaceae) showing an IC50 of 1.36 µg/mL on the 3D7 Plasmodium falciparum strain. It resulted in the isolation of twelve compounds designated as potential antiplasmodial compounds by the biodereplication workflow. Two chalcones, aurentiacin (1) and cardamonin (3), with IC50 values of 2.25 and 5.5 µM, respectively, can be considered to bear the antiplasmodial activity of the extract, with the latter not relying on a heme-binding mechanism. This biodereplication method constitutes a rapid, efficient, and robust technique to identify potential antimalarial compounds in complex extracts such as plant extracts.

Polysaccharide nanoparticles for oral controlled drug delivery: the role of drug-polymer and interpolymer interactions

Introduction: The oral route still represents the most popular way of administering drugs; nowadays oral administration faces new challenges, in particular with regards to the delivery of APIs that are poorly absorbed and sensitive to degradation such as macromolecules and biotechnological drugs. Nanoparticles are promising tools for the efficient delivery of these drugs to the gastrointestinal tract. Areas covered:Approaches and techniques for the formulation of drugs, with particular focus on the preparation of polysaccharide nanoparticles obtained by non-covalent interactions. Expert opinion:Polysaccharide-based nanoparticulate systems offer the opportunity to address some of the issues posed by biotechnological drugs, as well as by small molecules, with problems of stability/intestinal absorption, by exploiting the capability of the polymer to establish non-covalent bonds with functional groups in the chemical structure of the API. This area of research will continue to grow, provided that these drug delivery technologies will efficaciously be translated into systems that can be manufactured on a large scale under GMP conditions. Industrial scale-up represents the biggest obstacle to overcome in view of the transformation of very promising results obtained on lab scale into medicinal products. To do that, an effort toward the simplification of the process and technologies is necessary.

A mass spectrometric stochastic dynamic diffusion approach to selective quantitative and 3D structural analyses of native cyclodextrins by electrospray ionization and atmospheric pressure chemical ionization methods

The paper addressed shortcoming with highly precise and selective 3D structural analysis of native cyclodextrins in mixture using ions observable at low m/z-region by ESI- and APCI-mass spectrometry. Because of, the quantitative and structural analyses of CDs, in general, are vexed by a set of complications. The study outlines our own stochastic dynamic approaches to the latter issues based on new model relations, quantifing the measurable MS outcome intensity. They introduce the so-called stochastic dynamic mass spectrometric diffusion "D_SD" parameter, exhibiting high accuracy, precision, sensitivity and selectivity, respectively. It is linearly connected with the so-called quantum chemical diffusion parameter "D_QC" according to Arrhenius's theory. The most important upshot is that statistical parameters r = 0.99₆₃₉-0.99₉₈₁ has been obtained correlating between D_SD and D_QC parameters. Therefore, we determine high accurately 3D molecular and electronic structures of analytes by mass spectrometry. Fragment peaks at m/z 313, 279, 272, 252, 231, 214, 198, 171, 158 and 141 are examined. Mixtures of CDs and monomeric and acyclic oligomer carbohydrates glucose (1), sucrose (2), raffinose (3), melezitose (4) and cellotriose (5) are also studied. Our method is able to account precisely for the effect of the temperature under ESI- and APCI-MS conditions, as well. Correlative analysess between D_SD parameters of ESI- and APCI-MS measurements under different temperatures is also shown. Chemometric tests are used. Another important results and conclusions, among others, to draw from this research are: (i) excellent linear correlation between D_SD and D_QC parameters of r = 0.99₆₃₆ is found looking at common ions at m/z 141, 158 and 171, belonging to 2-formyl-3,4-dihydroxy-pyranylium, 4,5,6-trihydroxy-6H-pyran-2-carbaldehyde and 3,4,5-trihydroxy-6-oxo-6H-pyran-2-ylmethylidyne-oxonium ions. Thus, we distinguish precisely between the last structure and 3-formyl-4,5-dihydroxy-2,7-dioxa-8-oxonia-bicyclo[4.2.0]octa-1(8),3,5-triene cation. In the case of ion at m/z 141 subtle electronic effects are distinguished between the mentioned structure and the charged 3,4-dihydroxy-6H-pyran-2-carbaldehyde one. The method determines precisely very similar structurally poly-OH-substituted derivatives. Because of, (ii) absolute reproducibility (r = 1) of D_SD parameters of ESI-MS spectra is obtained studying the shown in point (i) MS peaks of β-CD between j^th and j^th numbers of experiments. The statistical equation is Dⁱ_SD = (0.51 ± 3.1.10^-5) × D^j_SD; (iii) the APCI- and ESI-MS provide identical results studying common MS ions of CDs and the correlation between D^APCI_SD and D^ESI_SD parameters excludes from error, due to, experiment; and (iv) The correlation between theory and experiment accounting for the factor temperature within our model equations shows r = 0.98₂₈ looking at the MS peaks at m/z 313 280, 279, 274 and 252, respectively. The effect of the temperature under both ESI- and APCI-MS conditions on the 3D molecular and electronic structures of CDs is precisely studied, respectively.

Quantification of Protein-Ligand Interactions by Laser Electrospray Mass Spectrometry

Laser electrospray mass spectrometry (LEMS) measurement of the dissociation constant (K_d) for hen egg white lysozyme (HEWL) and N,N',N″-triacetylchitotriose (NAG₃) revealed an apparent K_d value of 313.2 ± 25.9 μM for the ligand titration method. Similar measurements for N,N',N″,N″'-tetraacetylchitotetraose (NAG₄) revealed an apparent K_d of 249.3 ± 13.6 μM. An electrospray ionization mass spectrometry (ESI-MS) experiment determined a K_d value of 9.8 ± 0.6 μM. In a second LEMS approach, a calibrated measurement was used to determine a K_d value of 6.8 ± 1.5 μM for NAG₃. The capture efficiency of LEMS was measured to be 3.6 ± 1.8% and is defined as the fraction of LEMS sample detected after merging with the ESI plume. When the dilution is factored into the ligand titration measurement, the adjusted K_d value was 11.3 μM for NAG₃ and 9.0 μM for NAG₄. The calibration method for measuring K_d developed in this study can be applied to solutions containing unknown analyte concentrations. Graphical Abstract.

A Critical Review of the Characterization of Polyphenol-Protein Interactions and of Their Potential Use for Improving Food Quality

BACKGROUND

Interest in protein-phenol interactions in biological systems has grown substantially in recent decades.

METHODS

The interest has focused largely on food systems in response to reports on the prominent roles of phenolic compounds in nutrition and health.

RESULTS

Phenolic compounds can have both favourable and adverse nutritional effects. Polyphenols are widely known for their antioxidant, anti-inflammatory, anticancer and antiaging properties; however, they have also been ascribed anti-nutritional effects resulting from interactions with some proteins and enzymes. Interactions between proteins and polyphenols can additionally influence food quality by altering some physical-chemical and sensory properties of foods. These effects may be useful to develop new products in food science and technology provided the nature of physical-chemical interactions between proteins and phenols is accurately elucidated. In this paper, we review the different possible modes of interaction between selected food proteins and phenolic compounds.

CONCLUSION

Existing knowledge on the mechanisms behind polyphenol-protein reactions, the structures of the resulting products and their potential uses is reviewed.

Probing chirality recognition of protonated glutamic acid dimers by gas-phase vibrational spectroscopy and first-principles simulations

We characterize stereospecific aspects of homochiral and heterochiral dimers of glutamic acid by infrared spectroscopy and first-principles molecular dynamics simulations.

Ion suppression effect in desorption electrospray ionization and electrospray ionization mass spectrometry

RATIONALE

Although it is claimed that desorption electrospray ionization (DESI) causes less ion suppression effects than electrospray ionization (ESI), a related investigation with quantification measurement of ion suppression effects is absent. Herein, a comparative analysis of ion suppression effects between DESI and ESI was conducted, with the aiming of quantitatively studying the ion suppression effect.

METHODS

CTVA mixtures, a constant concentration of tioconazole with varied concentrations of atenolol, were analyzed by ESI and DESI. The ion suppression effect was characterized by the signal loss of tioconazole in the mixture compared to the signal intensity of tioconazole without interference (denoted as (I_single - I_mixture )/I_single ). According to the variations in the experimental conditions (such as flow rate, solvent composition, substrate material, capillary inner diameter, sheath gas velocity), ion suppression effects in DESI and ESI were compared.

RESULTS

With the increasing flow rate, the ion suppression effect in DESI became weaker, while the opposite trend was obtained for ESI. As for capillary inner diameter, a smaller inner diameter resulted in weaker ion suppression effects in DESI and ESI. The solvent composition affected the ion suppression effect, and the PTFE substrate presented the weakest ion suppression effect among the five substrate materials.

CONCLUSIONS

Though the ion suppression effect in DESI and ESI was shown to relate to experimental conditions, DESI had less effect than ESI under the same experimental conditions in most cases. Moreover, DESI displayed stronger matrix-tolerant ability than ESI which is also attributed to the weaker ion suppression effect.

Direct Determination of Site-Specific Noncovalent Interaction Strengths of Proteins from NMR-Derived Fast Side Chain Motional Parameters

A novel approach to accurately determine residue-specific noncovalent interaction strengths (ξ) of proteins from NMR-measured fast side chain motional parameters (O_axis²) is presented. By probing the environmental sensitivity of side chain conformational energy surfaces of individual residues of a diverse set of proteins, the microscopic connections between ξ, O_axis², conformational entropy (S_conf), conformational barriers, and rotamer stabilities established here are found to be universal among proteins. The results reveal that side chain flexibility and conformational entropy of each residue decrease with increasing ξ and that for each residue type there exists a critical range of ξ, determined primarily by the mean side chain conformational barriers, within which flexibility of any residue can be reversibly tuned from highly flexible (with O_axis² ∼ 0) to highly restricted (with O_axis² ∼ 1) by increasing ξ by ∼3 kcal/mol. Beyond this critical range of ξ, both side chain flexibility and conformational entropy are insensitive to ξ. The interrelationships between conformational dynamics, conformational entropy, and noncovalent interactions of protein side chains established here open up new avenues to probe perturbation-induced (for example, ligand-binding, temperature, pressure) changes in fast side chain dynamics and thermodynamics of proteins by comparing their conformational energy surfaces in the native and perturbed states.

Investigation of non-covalent complexations of Ca(II) and Mg(II) ions with insulin by using electrospray ionization mass spectrometry

RATIONALE

Insulin is a peptide hormone secreted by pancreatic β-cells. Ca(II) and Mg(II) ions play an important role in the secretion of insulin. There is no study about a direct complexation of Ca(II) or Mg(II) with insulin and their equilibrium constants. Electrospray ionization mass spectrometry (ESI-MS) is a practical method for the monitoring of non-covalent complexes such as Ca(II)-insulin and Mg(II)-insulin. Here, the equilibrium constants of Ca(II)-insulin and Mg(II)-insulin non-covalent complexes have been calculated after ESI-MS measurements in aqueous solutions.

METHODS

The effects of pH, competitive binding, ion exchange, and Na(I) and K(I) ions on Ca(II)-insulin and Mg(II)-insulin complexation have been examined by measuring by ESI-MS. The dissociation equilibrium constants (K1 and K2 ) of Ca(II)-insulin and Mg(II)-insulin complexes were calculated from the binomial graph derived from the ESI-MS normalized peak intensities. The MS/MS spectra of the complexes have been examined.

RESULTS

The dissociation equilibrium constants were found to K1 : 1.29 × 10(-4)  M and K2 : 9.69 × 10(-4)  M for the Ca(II)-insulin complexes, and K1 : 1.37 × 10(-4)  M and K2 : 9.12 × 10(-4)  M for Mg(II)-insulin complexes. Ca(II) ions have higher complexation capability with insulin than Mg(II) ions.

CONCLUSIONS

The binding equilibrium constants of Ca(II)- and Mg(II)-insulin non-covalent complexes have been determined successfully by ESI-MS. Ca(II) and Mg(II) ions are involved in the insulin secretion by forming non-covalent complexes. Copyright © 2016 John Wiley & Sons, Ltd.

Revealing Higher Order Protein Structure Using Mass Spectrometry

The development of rapid, sensitive, and accurate mass spectrometric methods for measuring peptides, proteins, and even intact protein assemblies has made mass spectrometry (MS) an extraordinarily enabling tool for structural biology. Here, we provide a personal perspective of the increasingly useful role that mass spectrometric techniques are exerting during the elucidation of higher order protein structures. Areas covered in this brief perspective include MS as an enabling tool for the high resolution structural biologist, for compositional analysis of endogenous protein complexes, for stoichiometry determination, as well as for integrated approaches for the structural elucidation of protein complexes. We conclude with a vision for the future role of MS-based techniques in the development of a multi-scale molecular microscope. Graphical Abstract ᅟ.

Infrared Spectroscopy of Mobility-Selected H+-Gly-Pro-Gly-Gly (GPGG)

We report the first results from a new instrument capable of acquiring infrared spectra of mobility-selected ions. This demonstration involves using ion mobility to first separate the protonated peptide Gly-Pro-Gly-Gly (GPGG) into two conformational families with collisional cross-sections of 93.8 and 96.8 Å(2). After separation, each family is independently analyzed by acquiring the infrared predissociation spectrum of the H(2)-tagged molecules. The ion mobility and spectroscopic data combined with density functional theory (DFT) based molecular dynamics simulations confirm the presence of one major conformer per family, which arises from cis/trans isomerization about the proline residue. We induce isomerization between the two conformers by using collisional activation in the drift tube and monitor the evolution of the ion distribution with ion mobility and infrared spectroscopy. While the cis-proline species is the preferred gas-phase structure, its relative population is smaller than that of the trans-proline species in the initial ion mobility drift distribution. This suggests that a portion of the trans-proline ion population is kinetically trapped as a higher energy conformer and may retain structural elements from solution.

Studies of the complexation behavior of tetramorpholinylo-PNP-lariat ether with Ag(i), Ca(ii), Cd(ii), Cu(ii) and Pb(ii) using Electrospray Ionization Mass Spectrometry

The identification of complexes containing PNP-lariat ether with Ag(i), Ca(ii), Cu(ii), Cd(ii) and Pb(ii) by ESI-MS technique is presented. The tandem mass spectra of these complexes were done in order to evaluate the stability of the different types of the complexes formed.

Evaluation of intermolecular association of glycosaminoglycan oligosaccharides using nanoelectrospray ionization mass spectrometry

This study examines the non-covalent interactions between glycosaminoglycan (GAG) oligosaccharides using nanoelectrospray ionization mass spectrometry (nanoESI-MS). It is the first time that interactions between oligosaccharides have been observed using MS. The importance of interactions between GAGs has recently attracted much interest because they are related to biological functions. For instance, hyaluronic acid (HA) is known to associate with chondroitin sulfates (CSs), although the details of the interaction remain unclear. In general, non-covalent interactions between glycans are too weak to detect by general means. In this work, we applied nanoESI-MS with high sensitivity, which is widely used to observe non-covalent interactions, to investigate the interaction between HA and CSs. HA and CS oligosaccharides are used to discuss the results in a simplified manner. Our approach is aimed at interpreting the behavior of GAG polysaccharides from the information obtained using the oligosaccharides. HA and CS tetrasaccharides were demonstrated to associate to form heterodimer ions that were easily detected using nanoESI-MS. We also determined the stoichiometry of the interaction and calculated the K(d) values of the interactions between HA and CS tetrasaccharides. How these structures affect the strength and stability of the non-covalent complexes is discussed. Further study of the interactions between HA and CS oligosaccharides will clarify the biological meaning of the coexistence of HA and CS in body fluids and tissues.

Study on the noncovalent interactions of saikosaponins and cytochrome c by electrospray ionization mass spectrometry

RATIONALE

The study of interactions between protein and pharmaceutical molecules including natural extracts has become of increasing interest in biological and biomedical research. An investigation of the interaction between saikosaponins and cytochrome c (Cyt c) by electrospray ionization mass spectrometry (ESI-MS) is described in this study. Saikosaponins are found in Bupleurum falcatum (a flowering plant), and they are glycosides that consist of saccharides and the sapogenins of triterpenoids.

METHODS

Seven model molecules of saccharides and triterpenes, namely maltose (Mal II), maltotriose (Mal III), raffinose (Raf), and stachyose (Sta), glycyrrhetinic acid (Gly), ursolic acid (Urs) and oleanic acid (Ole), were chosen to perform a series of ESI-MS control experiments for the exploration of the interaction groups in saikosaponins with Cyt c. The dissociation constants of detected noncovalent complexes were determined by using a direct ESI-MS assay.

RESULTS

We have observed in the ESI mass spectra the formation of Cyt c complexes with saikosaponins a and c, and these saccharides, with 1:1 and 1:2 stoichiometry. Our results showed that no complex ions of triterpenes and Cyt c were detected in the ESI-MS and similar Kd values were obtained for the Cyt c complexes of saikosaponins and saccharides. This demonstrates that the glycosyl moiety in the saikosaponins is the effective interaction group with Cyt c. We propose that saikosaponins and saccharides interact with Cyt c by hydrogen bonds. The binding affinity of these six ligands with Cyt c is shown to be in the order Ssa > Ssc > Raf, Mal III > Sta ≥ Mal II.

CONCLUSIONS

The ESI-MS methodology presented in this study enables us to investigate the interactions of saikosaponins with Cyt c, and allows the direct determination of binding constants. These results could guide further research for providing insights into the structure-binding relationship of ligands with Cyt c.

Large molecule bioanalysis using Q-TOF without predigestion and its data processing challenges

Background: Recent developments in LC–MS have turned it into a viable and valid alternative to ligand-binding assays. Large molecule bioanalysis by LC–MS is generally performed by tryptic digestion, purification and detection of one or more small signature peptides. High-resolution MS instruments offer quantification of intact small proteins or peptides and are able to increase the selectivity, while maintaining sensitivity. Results: Unlike multiple reaction monitoring assays, several factors affecting data processing were presented and the optimal parameters to consider during quantification method building were also demonstrated. MUC5AC-13 (MW 1709.8 Da), human hepcidin/LEAP-1 (MW 2797.4 Da), porcine calcitonin (MW 3604 Da) and chicken lysozyme (MW 14.3 kDa) were selected as model compounds and the possibility of intact peptide and small protein quantification, without tryptic digestion, was demonstrated. Conclusion: Selectivity and sensitivity were improved using different scan modes, such as TOF-MS and TOF-MS/MS.

Electrospray ionization mass spectrometry: a technique to access the information beyond the molecular weight of the analyte

The Electrospray Ionization (ESI) is a soft ionization technique extensively used for production of gas phase ions (without fragmentation) of thermally labile large supramolecules. In the present review we have described the development of Electrospray Ionization mass spectrometry (ESI-MS) during the last 25 years in the study of various properties of different types of biological molecules. There have been extensive studies on the mechanism of formation of charged gaseous species by the ESI. Several groups have investigated the origin and implications of the multiple charge states of proteins observed in the ESI-mass spectra of the proteins. The charged analytes produced by ESI can be fragmented by activating them in the gas-phase, and thus tandem mass spectrometry has been developed, which provides very important insights on the structural properties of the molecule. The review will highlight recent developments and emerging directions in this fascinating area of research.

Possible conformational change within the desolvated and cationized sBBI/trypsin non-covalent complex during the collision-induced dissociation process

Electrospray ionization mass spectrometry (ESI-MS) has become an analytical technique widely used for the investigation of non-covalent protein-protein and protein-ligand complexes due to the soft desolvation conditions that preserve the stoichiometry of the interacting partners. Dissociation studies of solvated or desolvated complexes (in the source and in the collision cell, respectively) allow access to information on protein conformation and localization of the metal ions involved in protein structure stabilization and biological activity. The complex of bovine trypsin and small soybean Bowman-Birk inhibitor (sBBI) was studied by ESI-MS to determine changes occurring within the complex during its transfer from droplets to the gas phase independently of the ion polarity. Under collision-induced dissociation (CID) conditions, unexpected binding of the Ca(2+) ion (cofactor of native trypsin) to the inhibitor molecule was observed within the desolvated sBBI/trypsin/Ca(2+) complex (with a 1:1:1 stoichiometry). This formal gas-phase migration of the calcium ion from trypsin to the inhibitor may be related to conformational rearrangements in the solvent-free and likely collapsed complex. However, under conditions leading to the increase in complex charge state, the appearance of the cationized trypsin molecule was detected during complex dissociation, thus reflecting different pathways of the evolution of complex conformation.

Measuring copper and zinc superoxide dismutase from spinal cord tissue using electrospray mass spectrometry

Metals are key cofactors for many proteins, yet quantifying the metals bound to specific proteins is a persistent challenge in vivo. We have developed a rapid and sensitive method using electrospray ionization mass spectrometry to measure Cu,Zn superoxide dismutase (SOD1) directly from the spinal cord of SOD1-overexpressing transgenic rats. Metal dyshomeostasis has been implicated in motor neuron death in amyotrophic lateral sclerosis (ALS). Using the assay, SOD1 was directly measured from 100 μg of spinal cord, allowing for anatomical quantitation of apo, metal-deficient, and holo SOD1. SOD1 was bound on a C(4) Ziptip that served as a disposable column, removing interference by physiological salts and lipids. SOD1 was eluted with 30% acetonitrile plus 100 μM formic acid to provide sufficient hydrogen ions to ionize the protein without dislodging metals. SOD1 was quantified by including bovine SOD1 as an internal standard. SOD1 could be measured in subpicomole amounts and resolved to within 2 Da of the predicted parent mass. The methods can be adapted to quantify modifications to other proteins in vivo that can be resolved by mass spectrometry.

Combined electrospray-SMPS and SR-SAXS investigation of colloidal silica aggregation. Part I. Influence of starting material on gel morphology

The slow aggregation of monodisperse, polydisperse, and preaggregated silica nanoparticles was studied with an electrospray-scanning mobility particle sizer (ES-SMPS) and time-resolved synchrotron radiation-small-angle X-ray scattering (SR-SAXS). Aggregation was induced by varying the NaCl concentration to obtain a fixed gelation time of ∼40 min. The combination of these techniques provides a unique tool to monitor and resolve the aggregate development in detail. The monodisperse spherical particles were converted to dimers, trimers, and eventually larger clusters as the aggregation proceeded, while the polydisperse spherical particles formed large clusters at an early stage. The initial particle shape and polydispersity had profound effects on the morphology of the aggregates; spherical primary particles produced compact spherical clusters, whereas the preaggregated dispersions formed open, elongated aggregates. All dispersions produced gels that contained free primary particles well past the point of gelation. The stability of the aggregates and the gel morphology were interpreted by relating to the structure of porous gel layers around the particles.

Observation of symmetric denaturation of hemoglobin subunits by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) has been used to characterize the denaturation of porcine hemoglobin (Hb) induced by solvent changes. This work provides evidence for the symmetric nature of Hb denaturation and demonstrates that heme losses from α- and β-monomers occur in parallel, in response to the addition of acid and organic co-solvents in solution. When subject to one of the following solution conditions (pH 3.2-4.0 or 15-30% acetonitrile-water or 30-45% methanol-water solution), α- and β-globins undergo symmetric dissociation to release the heme groups, which is detected by ESI-MS. Circular dichroism (CD) and fluorescence spectroscopy (FS) data show that the acid-induced and organic solvent-induced heme release, as observed in the mass spectra, can probably be ascribed to different aspects of the conformational changes taking place in the protein. The acidity of the solvent has a significant effect on the secondary structure, whereas organic content level in solution (15-30% acetonitrile or 30-45% methanol) tends to destroy the tertiary structure of Hb globins, both leading to release of the heme from each subunit.

Synthesis and physicochemical characterization of Gd-C4-thyroxin-DTPA, a potential MRI contrast agent. Evaluation of its affinity for human serum albumin by proton relaxometry, NMR diffusometry, and electrospray mass spectrometry

Gd-C(4)-thyroxin-DTPA, a potential MRI contrast agent, was synthesized from Gd-DTPA and thyroxine, which interacts strongly with human serum albumin (HSA). It was characterized in water by its relaxometric properties and its stability versus zinc transmetalation. The affinity of the complex for HSA was studied by using three different methods: proton relaxometry, NMR diffusometry, and electrospray mass spectrometry. From the results, it appears that Gd-C(4)-thyroxin-DTPA exhibits a relatively high relaxivity (r(1) = 9.01 s(-1) mM(-1) at 1.5 T and 310 K), a good stability versus zinc transmetalation, and a strong interaction with HSA (K(a) approximately 10,000 M(-1) with two binding sites). The kinetics of the exchange between the bound and the free form of the complex was evaluated by the NMR diffusometry technique. Competition experiments have allowed the assignment of the chelate's binding site on HSA.

Characterization of conformational changes and noncovalent complexes of myoglobin by electrospray ionization mass spectrometry, circular dichroism and fluorescence spectroscopy

Electrospray ionization mass spectrometry (ESI-MS) was employed to monitor the heme release and the conformational changes of myoglobin (Mb) under different solvent conditions, and to observe ligand bindings of Mb. ESI-MS, complemented by circular dichroism and fluorescence spectroscopy, was used to study the mechanism of acid- and organic solvent-induced denaturation by probing the changes in the secondary and the tertiary structure of Mb. The results obtained show that complete disruption of the heme-protein interactions occurs when Mb is subjected to one of the following solution conditions: pH 3.2-3.6, or solution containing 20-30% acetonitrile or 40-50% methanol. Outside these ranges, Mb is present entirely in its native state (binding with a heme group) or as apomyoglobin (i.e. without the heme). Spectroscopic data demonstrate that the denaturation mechanism of Mb induced by acid may be significantly different from that by the organic solvent. Low pH reduces helices in Mb, whereas certain organic content level in solution results in the loss of the tertiary structure. ESI-MS conditions were established to observe the H(2)O- and CO-bound Mb complexes, respectively. H(2)O binding to metmyoglobin (17,585 Da), where the heme iron is in the ferric oxidation state, is observed in ESI-MS. CO binding to Mb (17,595 Da), on the other hand, can be only observed after the heme iron is reduced to the ferrous form. Therefore, ESI-MS combined with spectroscopic techniques provides a useful means for probing the formation of ligand-binding complexes and characterizing protein conformational changes.

Electrospray deposition of biomolecules

This chapter describes the basic physics underlying the newly emerging technique of electrospray deposition (ESD) as applied to biological macromolecules. Fabrication of protein films and microarrays are considered as the most important applications of this technology. All the major stages in the ESD process (solution electrification, formation of a cloud of charged microdroplets, transformation of microdroplets into ions and charged clusters, deposition, and neutralization) are discussed to reveal the physical processes involved, such as space charge effects, dissipation of energy upon landing and neutralization mechanisms. Fundamentals of ESD are presented together with a discussion of potential practical problems in realizing ESD through dielectric masks. Retention of structure and functional properties of protein molecules in ESD-fabricated films and microarrays is discussed in detail.

Evaluation of binding selectivities and affinities of platinum-based quadruplex interactive complexes by electrospray ionization mass spectrometry

The quadruplex binding affinities and selectivities of two large pi-surface Pt(II) phenanthroimidazole complexes, as well as a smaller pi-surface platinum bipyridine complex and a larger Ru(II) complex, were evaluated by electrospray ionization mass spectrometry. Circular dichroism (CD) spectroscopy was used to determine the structures of various quadruplexes and to study the thermal denaturation of the quadruplexes in the absence and presence of the metal complexes. In addition, chemical probe reactions with glyoxal were used to monitor the changes in the quadruplex conformation because of association with the complexes. The platinum phenanthroimidazole complexes show increased affinity for several of the quadruplexes with elongated loops between guanine repeats. Quadruplexes with shorter loops exhibited insubstantial binding to the transition metal complexes. Similarly binding to duplex and single strand oligonucleotides was low overall. Although the ruthenium-based metal complex showed somewhat enhanced quadruplex binding, the Pt(II) complexes had higher quadruplex affinities and selectivities that are attributed to their square planar geometries. The chemical probe reactions using glyoxal indicated increased reactivity when the platinum phenanthroimidazole complexes were bound to the quadruplexes, thus suggesting a conformational change that alters guanine accessibility.

Gas-phase behavior of noncovalent transmembrane segment complexes

Specific helix oligomerization between transmembrane segments (TMSs) is often promoted by motifs like GxxxG. Disruption of this motif in the transmembrane segments of vesicular stomatitis virus G-protein and of glycophorin A results in a reduced dimerization level studied by in vivo systems like ToxR. This paper reports the influence of sequence motifs like GxxxG in solution and the gas phase.The transmembrane segments may behave differently in the gas and liquid phase, because of the absence of surrounding solvent molecules in the gas phase. Comparison of experiments depending on peptide properties performed in the gas and liquid phase discloses that the peptides retain 'some memory' of their liquid-phase structure in the gas phase. A direct correlation has been found between helicity in solution as determined by circular dichroism and dimerization in the gas phase monitored by electrospray mass spectrometry. These results show that a proper folding in solution is required for oligomerization.On the other hand, sequence-specific oligomerization depending on the GxxxG motif was not observed with the mass spectrometric detection. Further on, neither concentration-dependent complex studies nor studies regarding complex stability in the gas phase - via collision-induced dissociation (CID) - led to sequence-specific differences.Finally, the findings show that in mass spectrometric measurements noncovalent interactions of studied TMSs is rather more dependent on the secondary structure and proper folding than on their primary structure.

Aggregation of nanosized colloidal silica in the presence of various alkali cations investigated by the electrospray technique

The slow aggregation process of a concentrated silica dispersion (Bindzil 40/220) in the presence of alkali chlorides (LiCl, NaCl, KCl, RbCl, and CsCl) was investigated by means of mobility measurements. At intervals during the aggregation, particles and aggregates were transferred from the liquid phase to the gas phase via electrospray (ES) and subsequently size selected and counted using a scanning mobility particle sizer (SMPS). This method enables the acquisition of particle and aggregate size distributions with a time resolution of minutes. To our knowledge, this is the first time that the method has been applied to study the process of colloidal aggregation. The obtained results indicate that, independent of the type of counterion, a sufficient dilution of the formed gel will cause the particles to redisperse. Hence, the silica particles are, at least initially, reversibly aggregated. The reversibility of the aggregation indicates additional non-DLVO repulsive steric interactions that are likely due to the presence of a gel layer at the surface. The size of the disintegrating aggregates was monitored as a function of the time after dilution. It was found that the most stable aggregates were formed by the ions that adsorb most strongly on the particle surface. This attractive effect was ascribed to an ion-ion correlation interaction.

Investigation of monovalent and bivalent enantioselective molecular recognition by electrospray ionization-mass spectrometry and tandem mass spectrometry

In this work is described the investigation of bivalent versus monovalent enantioselective molecular recognition in the context of enantioselective separations. Electrospray ionization-mass spectrometry (ESI-MS) and tandem mass spectrometry (MS/MS) are used for evaluating enantioselective systems through the measurement of (1) relative solution-phase binding constants via titration and (2) relative gas-phase binding via collision threshold dissociation. In HPLC, a cinchonane-type chiral stationary phase (CSP) based on tert.-butylcarbamoylquinine provides vastly increased retention and enantioselectivity for separation of bivalent versus monovalent alkoxy-benzoyl-N-blocked leucine enantiomers. The bivalent enantiomers are able to span and simultaneously interact with multiple interaction sites on the CSP surface, leading to enhanced separation. ESI-MS titration measurements also show an increased avidity for binding between bivalent selector and bivalent selectand, compared with the monovalent system. However, enhanced enantioselectivities measured in HPLC for the bivalent system cannot be reproduced by MS due to inherent mechanistic differences. Assumed discrepancies in relative response factors also give rise to systematic errors which are discussed. The results of MS/MS gas-phase experiments show that enantioselectivity is essentially lost in the absence of solvation, but that dissociation thresholds can provide a measure of relative dissociation energy in the bivalent interaction system compared to the monovalent counterpart. Such measurements may prove useful and efficient in better understanding multivalent interactions, in line with current theoretical considerations of effective concentrations and ion trap effects. This is the first application of mass spectrometric methods for assessing increased avidity of binding in multivalent enantioselective molecular recognition.

Theoretical investigations of the dissociation of charged protein complexes in the gas phase

A series of calculations, varying from simple electrostatic to more detailed semi-empirical based molecular dynamics ones, were carried out on charged gas phase ions of the cytochrome c(') dimer. The energetics of differing charge states, charge partitionings, and charge configurations were examined in both the low and high charge regimes. As well, preliminary free energy calculations of dissociation barriers are presented. It is shown that one must always consider distributions of charge configurations, once protein relaxation effects are taken into account, and that no single configuration dominates. All these results also indicate that in the high charge limit, the dissociation of protein complex ions is governed by electrostatic repulsion from the net charges, the consequences of which are enumerated and discussed. There are two main trends deriving from this, namely that charges will move so as to approximately maintain constant surface charge density, and that the lowest barrier to dissociation is the one that produces fragment ions with equal charges. In particular, it is shown that the charge-to-mass ratio of a fragment ion is not the key physical parameter in predicting dissociation products. In fact, from the perspective of the division of total charge, many dissociation pathways reported to be "asymmetric" in the literature should be more properly labelled as "symmetric" or "near-symmetric". The Coulomb repulsion model assumes that the timescale for charge transfer is faster than that for protein structural changes, which in turn is faster than that for complex dissociation.

Comprehensive investigation of the non-covalent binding of MRI contrast agents with human serum albumin

Three techniques, electrospray mass spectrometry, ultrafiltration, and proton relaxometry, are compared in the context of the quantitative analysis of non-covalent binding between human serum albumin (HSA) and MRI contrast agents. The study of the affinity by proton relaxometry reveals the association constant and the number of interaction sites assuming that all sites are identical and independent. Ultrafiltration was adapted for the study of paramagnetic complexes. This technique confirmed the results obtained by relaxometry. Electrospray mass spectrometry, an original method able to study non-covalent binding because of its soft ionization process that allows for the survival of weak binding, provides qualitative and quantitative results. Electrospray mass spectrometry confirmed the affinity measured by proton relaxometry and ultrafiltration. This technique requires very small amounts of products and directly gives the stoichiometry of the association, information not easily obtained by classic techniques. Nevertheless, proton relaxometry remains a useful and mandatory technique for determining the enhancement of the relaxation subsequent to the binding although it demands large amounts of compounds. It is to be pointed out that even if the three techniques lead to a similar ranking of the affinity of the contrast agents for HSA, the absolute values of the association constants disagree as a result of the difference in the experimental conditions (presence of salt, native protein or desalted one, approximations in the fitting of the data, liquid or gas phases).

Frontal Analysis Capillary Electrophoresis Hyphenated to Electrospray Ionization Mass Spectrometry for the Characterization of the Antithrombin/Heparin Pentasaccharide Complex

The interaction of proteins with polysaccharides represents a major and challenging topic in glycobiology, since such complexes mediate fundamental biological mechanisms. A new strategy based on the hyphenation of frontal analysis capillary electrophoresis (FACE) with electrospray ionization mass spectrometry (ESIMS) is reported for the characterization of protein/carbohydrate complexes. While most of the previously reported CE-MS experiments were performed using capillary electrophoresis in zone format, we report for the first time CE-MS experiments in which CE was performed in frontal analysis (FACE-MS). We showed that the frontal mode offered a better sensitivity than zone mode and was well suited for the CE-MS coupling. This FACE-MS coupling was applied to the analysis of the complex between antithrombin and the sulfated pentasaccharide reproducing the antithrombin-binding sequence in heparin. The mixture of coincubated antithrombin and heparin pentasaccharide was continuously injected into the capillary, and the electrophoretic separation of the free and bound forms of the protein was achieved. The intact noncovalent complex antithrombin/heparin pentasaccharide was detected on-line by ESIMS in positive ionization mode and in nondenaturing sheath liquid conditions. The complex stoichiometry was determined from the mass measurement of the complex. In addition, the characterization of the sulfated pentasaccharide ligand dissociated from the complex was performed in negative ionization mode using a denaturing sheath liquid, allowing the determination of its molecular mass and sulfation features. This FACE-ESIMS strategy opens the way to ligand fishing experiments performed on heterogeneous carbohydrate mixtures and subsequent characterization of specifically bound carbohydrates.

Investigation of the affinity and selectivity of avian prion hexarepeat peptides for physiological divalent metal ions

To further the understanding of the biological importance of metal-binding by avian prion proteins, we have investigated the affinity and selectivity of peptides Hx1 [Ac-HNPGYP-nh] and Hx2 [Ac-NPGYPHNPGYPH-nh] with a range of physiological metals via electrospray ionization mass spectrometry and tyrosine fluorescence emission spectroscopy. Both the hexamer Hx1 and the "dimer" peptide Hx2 bind only one equivalent of Cu(II), although only the latter peptide binds copper with significant affinity (Hx1 K(d)=150+/-35 microM; Hx2 K(d)=1.07+/-0.78 microM, pH 7.0 in 3-(N-morpholino)propanesulfonic acid (MOPS) buffer). Both peptides are selective for Cu(II) over divalent Ca, Co, Mg, Mn, Ni, and Zn. Cyclic voltammetry was used to estimate Cu(II/I) solution potentials at pH 6.8, which were very similar for the two peptides (CuHx1 E degrees'=+350 mV, CuHx2 E degrees'=+320 mV vs. normal hydrogen electrode). These results suggest similar binding modes for the two peptides, and relative stabilization of Cu(I) relative to similar His-Gly-rich peptides in the literature.

Quantitative determination of lysozyme-ligand binding in the solution and gas phases by electrospray ionisation mass spectrometry

Affinity constants for the binding of a range of substrate and non-substrate oligosaccharides to hen egg white lysozyme were determined by direct observation of the protein.ligand complexes using electrospray ionisation mass spectrometry (ESI-MS) with a chip-based nano-ESI source. The values obtained for a series of beta-1,4-N-acetylglucosamine oligomers (NAGn) were found to be in good agreement with those determined by fluorescence measurement. Oligomers of alpha-1,4-glucose (Glcn), which are believed to bind to lysozyme non-specifically, exhibited a 10(6)- to 10(8)-fold lower affinity for the enzyme. Lysozyme.NAGn complexes displayed an increase in Ka from n=2 to n=4, but then reached a plateau. In contrast non-specific lysozyme.Glcn complexes showed no such trend. Determination of gas-phase complex stability was achieved by quantitative collision-induced dissociation (CID) and infrared multiphoton dissociation (IRMPD) measurements. The collision energy (Ec50) or laser power (IRMPD50) required to dissociate precursor ions to 50% of their original intensity was determined for lysozyme.NAGn and Glcn complexes using the [M+8H]8+ charge state. An excellent correlation between trends in Ka and gas-phase stability was seen for NAGn oligomers bound to lysozyme, whereas no such relationship was observed with the non-specific, weaker lysozyme.Glcn complexes. These results illustrate that ESI-MS can be used to quantify the interactions between lysozyme and oligosaccharides in both the solution and gas phase and that measurement of gas-phase complex stability by CID or IRMPD can provide information about specific solution binding events.