Observation of triply charged metal ion clusters by electrospray and laser spray

Structures and Dissociation Products of Ce/Peptide Complexes: Competition between Coordination and Charge Delocalization

Structures of [Ce(GGG)]³⁺ and [Ce(GGG ? H)]²⁺ have been investigated by DFT calculations. The two lowest-energy structures of the triply charged metal complex have the peptide in either the iminol or conventional zwitterionic form, and these ions have almost identical energies. In the doubly charged complex, the iminol and charge-solvated structures are the best structures on the potential energy surface, but the latter is favored. In both iminol structures, the metal ion coordinates to the iminol oxygen rather than to the nitrogen, unlike in previously reported iminol-containing complexes. Triply charged [Ce(peptide)]³⁺ complexes are fragile and not easily isolated in a mass spectrometer, whereas the doubly charged [Ce(peptide ? H)]²⁺ complexes are more robust. Here, we studied the fragmentations of 37 [Ce(peptide ? H)]²⁺ and 30 [Ce(peptide)(peptide ? H)]²⁺ complexes and the results are systematically summarized. Losses of CO and/or H₂O are the most commonly observed fragmentation channels for [Ce(peptide ? H)]²⁺ complexes and these dissociation pathways are modeled by DFT calculations. For [Ce(peptide)(peptide ? H)]²⁺ complexes the neutral peptide plays the role of a solvent molecule but, unlike in the dissociations of [Ce(CH₃CN)(peptide ? H)]²⁺ complexes, the loss of the solvent molecule is not observed. Instead, fragmentation occurs by cleavage of the second amide bond of the solvating peptide molecule.

Formation and Characterization of Zr4+ Stabilized by Neutral Tridentate Ligands in the Gas Phase

Ligated tetrapositive metal ions are rare gas-phase species which tend to form complexes with lower charges due to the high 4th ionization energies of metals. We report the observation of tetrapositive Zr(TMPDA)₃⁴⁺ and Zr(TMOGA)₃⁴⁺ complexes in the gas phase by electrospray ionization of Zr(ClO₄)₄/TMPDA and Zr(ClO₄)₄/TMOGA mixtures. The Zr⁴⁺ center in both complexes is coordinated by nine atoms from three neutral diamide ligands forming nine-coordinate twisted tricapped trigonal prismatic geometry on the basis of DFT calculations. Collision-induced dissociation of both complexes resulted in the loss of protonated ligands to form tripositive Zr(TMPDA)(TMPDA-H)³⁺ and Zr(TMOGA)(TMOGA-H)³⁺ products which retain the IV oxidation state of zirconium at the cost of charge reduction from 4+ to 3+ of the whole complexes. The very high 4th ionization energy of zirconium (34.34 eV) makes tetrapositive zirconium complex the most challenging tetracation to be stabilized against charge reduction in the gas phase to date. Graphical abstract ᅟ.

Observation of dimethylaminoethyl methacrylate-myoglobin binding reaction using laser spray mass spectrometry

Covalent bonds are often created by a reaction between chemicals and protein before causing various adverse effects in a cell. Dimethylaminoethyl methacrylate (DMAEMA), which has moderate toxicity, causes skin inflammation and throat irritation. For this study, we investigated a reaction mechanism between myoglobin and (DMAEMA) using a new analytical tool developed at our laboratory: laser spray mass spectrometry technique. It was found that initially DMAEMA was added to the amino group of protein by the Michael addition mechanism; the added DMAEMA was hydrolyzed to methacrylic acid using an autocatalytic system. The results of this study indicate the feasibility of the laser spray technique in analyses of reaction dynamics.

Measurement of sugars using the laser spray technique with a gold capillary

A gold (Au) capillary has higher thermal conductivity than a stainless steel capillary and can withstand capillary over-heating induced by high CO(2) laser irradiation (over 2.5 W) better than a stainless steel capillary. For this study, a laser spray using an Au capillary was applied for the detection of sugars. The signal of cationized compounds [M+Na](+) can be detected with higher sensitivity than with conventional laser sprays using high laser power (over 2.7 W). Using 3.5 W of laser power, the signal intensity is 15 times higher than the maximum value with stainless steel (2.3 W) in a 10(-5) M maltose aqueous solution. It is considered that almost all the water molecules evaporate by laser irradiation, which is impossible to achieve using a stainless steel capillary.

Thermal unfolding of proteins probed by laser spray mass spectrometry

The stability and conformational changes of cytochrome c (cyt c) at different temperatures and pH have been well examined so far by using various analytical methods. We have found that laser spray mass spectrometry enables much faster and more convenient monitoring of those changes of cyt c compared with other methods. The results correlated well with circular dichroism (CD) experiments under relatively acidic conditions, which destabilize the protein. Laser spray mass spectra of cyt c at various pH were obtained at different levels of laser power. Bimodal charge-state distributions of the protein were observed in laser spray mass spectra, indicating the two-state model of structural change; the lower charges correspond to the folded state, the higher charges to the unfolded state. Based on this result, the presumed denaturation curve of the protein was plotted as a function of laser power, and laser power by which 50% of the protein was assumed to be denatured, E50%, as obtained at each pH. We also examined the melting temperatures, Tm, of cyt c at various values of pH by using CD spectroscopy. The correlation coefficient between E50% and Tm for cyt c was 0.999, demonstrating an excellent correlation. Furthermore, laser spray analysis of ubiquitin, which is found to be more thermally stable than cyt c, gave a higher E50% than cyt c. These results indicate that laser spray mass spectrometry can be an extremely convenient method for probing thermal stabilities and dynamic conformational changes of proteins with subtle structural differences caused by slight changes in pH.

Study on chemical speciation in aluminum chloride solution by ESI-Q-MS

AlCl(3) solution was analyzed at concentrations from 0.02 to 100 mM using an electrospray ionization quadrupole mass spectrometer (ESI-Q-MS), and the dissolution state of aluminum ions is discussed. Results obtained using ESI-Q-MS were consistent with those obtained using (27)Al nuclear magnetic resonance ((27)Al NMR). Aluminum species existed mainly as positively charged monomeric aluminum hydroxide coordinated with several water molecules in solution. The complexation of chloride ions by aluminum ions differed between the positive and negative ion modes. Chemical reactions that partially modified chemical forms of species through ESI-Q-MS measurement were also observed. In the same aluminum chloride solution, using ESI-TOF-MS and ESI-Q-MS/MS studies, the disagreement of the reports is discussed. It is concluded that ESI-TOF-MS might show also the gas-phase reaction in the mass spectrometer but the dissolution state of aluminum species can be shown by ESI-Q-MS.

Investigation of molecular interaction within biological macromolecular complexes by mass spectrometry

The advent of electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) has accelerated structural studies of biological macromolecular complexes. At present, mass spectrometry can provide accurate mass values not only of individual biological macromolecules but also of their assemblies. Furthermore, it can also give information on the interface sites of the biological macromolecular complexes. The present article focuses on the role of mass spectrometry in the investigation of biological molecular interactions, such as protein-protein, protein-DNA, and protein-ligand interactions, which play essential roles in various biological events.

Structural and Functional Characterization of Biological Macromolecules by Mass Spectrometry

Mass spectrometry has widely been used as a tool for identification of proteins in the research fields of biochemistry and clinical chemistry because it can provide accurate information on molecular masses of biological molecules with a small amount of sample in a short time. If mass spectrometry is properly used, it can also give information on the tertiary structure or on the molecular interactions of biological macromolecules. The present paper focuses on the role of mass spectrometry as a tool for the investigation on the tertiary structure of proteins and on the biological molecular interactions that play essential roles in various biological events.

Stability analysis for double-stranded DNA oligomers and their noncovalent complexes with drugs by laser spray

Laser spray, which is a newly developed ionization technique, can characterize the stability of noncovalent complexes in the solution phase. By using this advantage, laser spray has been applied to probe the intrinsic stability of double-stranded DNA (dsDNA) sequences and their binding affinities with various drugs in the solution phase. Systematic experiments were carried out using six 16-mer and three 22-mer dsDNA oligomers, together with the complexes of the 16-mer dsDNA with minor groove binders: berenil, Hoechst 33342, DAPI, and netropsin. Dissociation curves for each dsDNA or each complex were plotted as a function of laser power. The laser power (E50%), where 50% of each dsDNA or each complex was dissociated, was compared with its melting temperature (Tm) determined by UV spectroscopy. Linear correlations between E50% and Tm were obtained not only for the dsDNA oligomers (correlation factor r = 0.9835) but also for the 16-mer dsDNA complexes with minor groove binders (r = 0.9966). In addition, laser spray has successfully clarified the binding affinities of a 16-mer dsDNA with two intercalators: daunomycin and nogalamycin. In the case of the dsDNA-daunomycin complex, by changing the molar ratio of dsDNA : drug from 1 : 1 to 1 : 5, the concentration-dependent stability of the complex was confirmed by laser spray. The present results demonstrate that laser spray mass spectrometry can be a powerful and convenient method to investigate the relative binding affinities of dsDNA-ligand complexes in the solution phase, which could be applied to the early stage of high-throughput screening of drugs targeting for dsDNA.

Formation of hydrated triply charged metal ions from aqueous solutions using nanodrop mass spectrometry

Forming hydrated clusters containing triply charged metal ions is challenging due to the competing process of dissociation by forming the metal hydroxide with one less net charge and a protonated water molecule. It is demonstrated for the first time that it is possible to form such clusters using a method we call "nanodrop mass spectrometry". Clusters of the form [M(H(2)O)(n)](3+), where M = Ce, Eu, and La, are generated using electrospray ionization and are mass analyzed in a Fourier-transform ion cyclotron resonance mass spectrometer with an ion cell cooled to -140 °C. Clusters containing trivalent La with n ranging from 16 to over 160 can be readily produced. These clusters are stable at this temperature for many seconds, enabling all standard methods to probe structure and reactivity of these unusual species. Photodissociation experiments on extensively hydrated clusters of trivalent lanthanum using resonant infrared radiation indicate that a minimum of 17 water molecules is necessary to stabilize these trivalent clusters under the low-energy ion excitation conditions and long time frame of these experiments. These results indicate that a minimum droplet size of approximately a nanometer is necessary for these trivalent species to survive intact. This suggests that elemental speciation of trivalent metal ions from aqueous solutions should be possible using nanodrop mass spectrometry.

Electrospray mass spectrometry (ESI-MS) in the study of metal-ligand solution equilibria

In the 20 years, since the introduction of electrospray mass spectrometry (ESI-MS), the use of this technique in various fields of inorganic, organometallic, and analytical chemistry has been steadily increasing. In this study, the application of ESI-MS to the study of metal-ligand solution equilibria is reviewed (till 2004 included). In a first section, advantages and drawbacks of ESI-MS in this type of application are described. Subsequently, a list of ca. 300 studies is reported, in which ESI-MS was used to give number and stoichiometry of the species at equilibrium, or also to estimate their stability constants. All studies are classified according to the metal ions under examination. Other related applications, such as host-guest interactions and metal ion-protein binding studies, are briefly reviewed as well.

Evaluation of binding affinity of protein-mutant DNA complexes in solution by laser spray mass spectrometry

We have applied laser spray mass spectrometry developed by Hiraoka et al. to investigate the binding affinity of protein-mutant DNA complexes. The results were compared with our previous data of collision-induced dissociation (CID) experiments using electrospray ionization mass spectrometry (ESI-MS). Systematic experiments were carried out on the complexes of the c-Myb DNA binding domain (c-Myb DBD) bound to eight kinds of 16- or 22-mer point mutant double-stranded DNA (dsDNA), whose solution K(d) values are different in the range from 10(-9) M to 10(-7) M. The dissociation curve as a function of laser power was plotted for each complex, and the laser power where 50% of complex was dissociated (E(50%)) in population was obtained. The correlation coefficient between E(50%) and the relative binding free-energy change (DeltaDeltaG) of each complex formation in solutions was 0.9808, which is much better than the coefficient obtained by the previous ESI-CID experiments that was 0.859. In addition, complexes of the c-Myb DBD with five other mutant dsDNA were also examined to confirm that laser spray can be used to estimate the K(d) values of a DNA-protein complex in solutions if an appropriate calibration curve is available. In the process of laser spray, dissociations of these noncovalent complexes occur in solutions, but not in the gas phase. This differs greatly from ESI-CID. Laser spray mass spectrometry has been found to be better than ESI-CID in evaluating binding affinity of a protein to various mutant DNA.

Denaturation of lysozyme and myoglobin in laser spray

In laser spray, the tip of an electrospray capillary is irradiated with a continuous CO(2) laser beam. Here, we report results from a modified laser spray method that employs a relatively low laser irradiance level. With a laser power of approximately 2 W and a focal spot size ( approximately 0.3 mm), which covered the entire front surface of the electrospray capillary, the irradiance was approximately 3 x 10(3) W/cm(2). This resulted in a quiescent and smooth vaporization of aqueous solutions. This "evaporation-mode" laser spray method yielded the best results so far obtained in our laboratory with laser-irradiated electrospray, producing higher and more stable signals. The method was applied to the analysis of aqueous solutions of lysozyme and myoglobin. Mass spectra were obtained as a function of laser power from 0 W (electrospray) to approximately 2 W. The spray generated at the tip of the stainless steel capillary was observed with a CCD camera. With increase of laser power, the droplets in the spray became finer and the Taylor cone became progressively smaller. The strongest ion signals were recorded when the sample solution protruded only slightly from the tip of the capillary. A broadening of the lysozyme charge-state distribution, attributable to protein unfolding, was observed with a laser power of 2 W. No denaturation of myoglobin took place up to a laser power of 1.6 W. However, a sudden onset of denaturation was observed at 1.8 W as a broadening of the myoglobin charge distribution and the appearance of apo-myoglobin peaks. These findings demonstrate that laser spray is capable of dissociating the noncovalent complexes selectively without breaking covalent bonds.

The effect of the presence of foreign salts on the formation of gaseous ions for electrospray and laser spray

The effect of the presence of foreign salts (NaCl, aerosol OT, tetra-n-hexylammonium bromide, and CH3COONH4) on the formation of gaseous ions for electrospray (ES) and laser spray (LS) was studied in the positive and negative modes of operations. The ion signals for amino acids show sudden decrease with the concentration of foreign salts greater than 10(-5) M for both ES and LS. When the surface-active counter ions were added, the signal intensities showed a marked decrease for both ES and LS. This may be due to the enrichment of the surface-active counter ions on the surface of the charged droplets. When CH3COONH4 was added to an aqueous solution of 10(-6) M lysozyme chloride, an increase of the signal intensities for (lysozyme+nH)n+ and a decrease in the values of n were observed. The decrease in n may be due to the salt formation of (lysozyme+nH)n+ with the negative acetate ion leading to the reduction of positive charges.

Molecular recognition and supramolecular chemistry in the gas phase

Supramolecular chemistry, in particular, the fields of molecular recognition and self-assembly, profit much from the development of soft ionization techniques and advanced methods for mass analysis and gas-phase chemistry. Vice versa, weakly bonded architectures and host-guest complexes represent a veritable challenge for the mass spectrometrist, leading to further development of methods and techniques. This review describes the state-of-the-art in this field, and includes topics such as the effects of solvation on meta binding to crown ethers, chiral discrimination of guests by chiral hosts, the elucidation of the secondary structure of self assembled complexes, and the mechanistic pathways of self assembly or the fragmentations of supramolecular complexes in the gas phase.

High-sensitivity negative-ion laser spray for liquid chromatography/mass spectrometry

A negative-ion mode laser spray interface for use in liquid chromatography/mass spectrometry (LC/MS) has been developed and investigated. The laser spray gave sensitivities that were orders of magnitude better than the negative-ion electrospray for all samples investigated with the exception of sugars. Fragment ions, HSO(-4)and SO(-3), were formed from the laser-sprayed aqueous solution of cholesterol 3-sulfate sodium salt. This suggests that structural information may be obtained directly from the laser-spray mass spectra for thermally labile compounds.

Electrospray mass spectrometry of hydrophobic compounds using dimethyl sulfoxide and dimethylformamide as solvents

The use of dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) as solvents in electrospray ionization (ESI) is suggested for the analysis of hydrophobic compounds. Its use was shown to overcome solubility problems and resulted in good quality electrospray spectra of protected hydrophobic peptides, sugars and other hydrophobic compounds. Intense protonated and/or sodiated molecules were formed in positive ion mode while negative ion mode resulted in [M + 95](-) ions from DMSO and [M + Cl](-) ions from DMF in cases where no significant molecular ion related peaks could be observed applying commonly used protic solvents such as methanol or acetonitrile. Deuterium labeling (d6-DMSO), high resolution experiments and tandem mass spectrometric measurements showed that the methanesulfonic acid (MSA), present in DMSO as a common impurity, is responsible for the formation of protonated molecules in positive ion mode and for methane sulfonate anion adducts [M + 95](-) in negative ion mode.

A comparative study of laser spray and electrospray

A comparative study of electrospray and laser spray has been undertaken from various aspects. In general, laser spray gave stronger ion signals than electrospray, for solutions with the sample concentration of </=10(-5)M. For an aqueous solution of arginine, ion intensities of the cluster ions H(+)(arginine)(n) with larger n were observed by laser spray than by electrospray. This may be due to the enrichment of the sample concentration by the selective vaporization of the solvent from the meniscus of the sample solution effusing out of the stainless steel capillary by the laser irradiation. For an H(2)O/CH(3)OH (1:1, v/v) solution of 10(-5) M Arg-Lys-Arg-Ala-Arg-Lys-Glu, laser spray gave stronger ion signals than electrospray while the most highly charged ion observed was [M + 4H](4+) for laser spray and [M + 5H](5+) for electrospray. Laser spray was found to be the most suitable for the analyses of aqueous solutions. Copyright 2000 John Wiley & Sons, Ltd.