Liquid Secondary Ion Mass Spectrometry Research Articles

Investigation of ion-pair precipitates of selected alkoxylates and complex salts of specific metal cations by liquid secondary ion mass spectrometry.

Liquid secondary ion (LSI) mass spectra of ion-pair precipitates obtained for Triton X-100 with strontium, lead, cadmium and mercury tetraphenylborates and for selected butoxylene-ethoxylene monoalkyl ethers with barium tetraiodobismuthate(III) are discussed. On the basis of LSI mass spectra, recorded in both positive and negative modes, the formulae of the ion-pair precipitates were determined. On the basis of B/E mass spectra, the fragmentation routes of [M - H + Ba](+) ions for butoxylene-ethoxylene monoalkyl ether complexes of barium and [M - H + Cd](+) ions for the Triton X-100 complex of cadmium are proposed.

Ionized aniline and its distonic radical cation isomers

A multi-sector tandem mass spectrometer fitted with a radiofrequency (rf)-only quadrupole collision cell (Qcell) has allowed fast and unambiguous identification of dehydroanilinium ions, the distonic isomers of ionized aniline. These ions were prepared by collisional deiodonation of protonated iodoanilines previously generated by chemical ionization (CI) or even better by liquid secondary ion mass spectrometry (LSIMS) conditions. Ab initio quantum chemical calculations were also used to probe the protonation of aniline and halogeno-anilines, X–C 6H 4–NH 2 ( X = F , Cl, Br and I) and the relative stabilities of ionized aniline and its distonic radical cation isomers. Proton affinities (PAs) of anilines at nitrogen and carbon sites were found rather dependent on the nature and the position of the substituents.

Loss of isocyanic acid from the internal oxadiazole ring of protonated molecules of some 2,5-diaryl-1,3,4-oxadiazoles.

The fragmentation pattern of some protonated 2,5-diaryl-1,3,4-oxadiazoles is discussed. An unusual decomposition consisting of elimination of the isocyanic acid molecule from the internal oxadiazole ring was found. This fragmentation pathway was deduced on the basis of B/E linked scan mass spectra of metastable ions with liquid secondary ion mass spectrometry as the ionization method and also of low-energy CID mass spectra where electrospray was used as the ionization technique. High resolution measurements were also performed.

Ferrocene-containing carbohydrate dendrimers.

Aliphatic amines, incorporating one or three (branched) acylated beta-D-glucopyranosyl residues, were coupled with the acid chloride of ferrocenecarboxylic acid and with the diacid chloride of 1,1'-ferrocenedicarboxylic acid to afford four dendrimer-type, carbohydrate-coated ferrocene derivatives in good yields (54-92%). Deprotection of the peracylated beta-D-glucopyranosyl residues was achieved quantitatively by using Zemplén conditions, affording four water-soluble ferrocene derivatives. When only one of the two cyclopentadienyl rings of the ferrocene unit is substituted, strong complexes are formed with beta-cyclodextrin in H2O, as demonstrated by liquid secondary ion mass spectrometry (LSIMS), 1H NMR spectroscopy, electrochemical measurements, and circular dichroism spectroscopy. Molecular dynamics calculations showed that the unsubstituted cyclopentadienyl ring is inserted through the cavity of the toroidal host in these complexes. The electrochemical behavior of the protected and deprotected ferrocene-containing dendrimers was investigated in acetonitrile and water, respectively. The diffusion coefficient decreases with increasing molecular weight of the compound. The potential for oxidation of the ferrocene core, the rate constant of heterogeneous electron transfer, and the rate constant for the energy-transfer reaction with the luminescent excited state of the [Ru(bpy)3]2+ complex (bpy = 2,2'-bipyridine) are strongly affected by the number (one or two) of substituents and by the number (one or three) of carbohydrate branches present in the substituents. These effects are assigned to shielding of the ferrocene core by the dendritic branches. Electrochemical evidence for the existence of different conformers for one of the dendrimers in aqueous solution was obtained.

Mass spectrometric studies of 2-aryl-5-acetylthiazole derivatives.

Electron ionisation mass spectrometry was usefully used to characterize structurally 2-aryl-5-acetylthiazole derivatives in the gas phase. The compounds show characteristic fragmentation pathways depending on the chemical nature of the substituent at position 2, consisting mainly in the cleavage of both the 1,2- and 3,4-bonds of the thiazole ring. Liquid secondary ion mass spectrometry was applied to study the effects of protonation on the gas-phase unimolecular reactions of this class of compound. Tandem mass spectrometric experiments, carried out on molecular and protonated molecular ions, and also on fragment ions produced in the source, allowed the elucidation of gas-phase decompositions of low-internal energy ions.

Evidence for ionization-related conformational differences of peptide ions in a quadrupole ion trap

The differences in boundary-activated dissociation (BAD) onsets have been investigated for peptide ions that were generated by two different ionization techniques, nanoflow electrospray ionization (nanoESI) and liquid secondary-ion mass spectrometry (LSIMS). BAD onsets of these ions were determined to compare the relative internal energies of the ions. Protonated peptide ions formed by nanoESI had lower BAD onsets than ions formed by LSIMS. The BAD onsets of peptides derivatized to have a fixed charge on the N-terminus also were lower for those generated by nanoESI than those generated by LSIMS. The BAD onsets of ions formed by nanoESI did not change with the variation of collisional cooling periods after gating ions into the ion trap and after isolating them prior to dissociation, indicating that the ions formed by the two ionization techniques would not adopt the same energy distributions. It is proposed that the ions formed by the two techniques differ in secondary structure, and the LSIMS ions are collisionally cooled to a lower local minimum along the potential energy surface than the nanoESI ions. Ions formed by both techniques show the same dissociation patterns, so assuming the absolute energy required for dissociation is the same, the LSIMS ions possess a higher critical energy of dissociation. This leads to the observation of the higher BAD onset.

Formation of cryptophanes from their precursors as viewed by liquid secondary ion mass spectrometry.

The formation of cryptophane-A (C1) and the deuterated cryptophanes C2-C6 from their respective precursors P1-P6 in a mass spectrometer ion-source was evidenced by liquid secondary ion mass spectrometry (LSIMS). Mass-analyzed ion kinetic energy experiments performed on the precursor molecular ions suggested that cryptophane formation occurred mainly in the liquid-matrix before desorption rather than in the gas phase. In addition, we observed that the presence of cations, such as lithium or sodium ions, inhibited the formation of the cryptophane molecular ions. In the light of these results we used the LSIMS technique to investigate the formation of the new cryptophanes C7-C13. All the data collected support the idea that a direct comparison can be made between these experimental findings and chemistry in solution.

Complexation of transition metal ions M+ (M = Fe, Co, Ni, Cu, Zn, Ag) by [2.n]paracyclophane-enes (n = 3, 4, 5, 6) in the gas phase: effect of the molecular cavity on the complexation capability

Complex formation in the gas phase between transition metal ions M+ = Fe+, Co+, Ni+, Cu+, Zn+, and Ag+ and dimethyl[2.n]paracyclophane-enes 1–4(n = 3–6) has been studied by secondary ionization mass spectrometry (SIMS) and tandem mass spectrometry. With the exception of Zn+, complex formation was observed if dry mixtures of salts of a transition metal and a cyclophane were bombarded with a 30 keV primary Cs+ ion beam in a SIMS ion source. Using liquid SIMS, abundant signals of a complex [M(cyclophane)]+ were only obtained for Ag+ and NBA as liquid matrix. Besides a large signal of M+ and a significant signal of [M(cyclophane)]+ all SIMS mass spectra contained a distinct peak of the molecular ion [cyclophane]·+ and a series of small peaks of hydrocarbon fragment ions. Dimeric adduct ions [M(cyclophane)2]+, typical of gas phase complexation of transition metal ions and planar aromatic hydrocarbons, are only detected in the mass spectra of mixtures of 1 and salts of Co or Ag. It is concluded from the experimental results that complex formation occurs in the gas phase of the SIMS ion source by ion/molecule reaction between the sputtered metal ion M+ and the neutral cyclophane evaporating from the heated target holder in competition with charge transfer. By this model the efficiency of complex formation between a certain metal ion and cyclophane can be estimated from the intensity ratio r = [M(cyclophane)]+/[cyclophane]·+. From this ratio and the results of separate experiments, in which two M+ compete in complex formation with cyclophane 3 or in which a mixture of cyclophanes is used for complexation of Ag+, it is shown that complex formation increases in the series Fe+ < Co+ ≈ Ni+ < Cu+ ≪ Ag+ and in the series 1 < 2 < 3 ≈ 4. The selectivity in the series of cyclophanes as well as the lack of formation of dimeric complexes [M(cyclophane)2]+ concurs with the formation of “in” complexes (IC) or “side-on” complexes (SC), in which the metal ion is more or less buried in the cavity of the cyclophane ligand. This is corroborated by collision-induced dissociation experiments, which show—with the exception of Ag+ complexes—intense losses of small hydrocarbon fragments and/or extensive decomposition of the complex ions, but no major dissociation into the components, and by semiempirical AM1 analysis of the structures of the complexes.

Structural characterisation of flavonoids and flavonoid-O-glycosides extracted from Genista tenera by fast-atom bombardment tandem mass spectrometry.

Genista tenera is a plant native to the Madeira Island (Portugal). From the ethanol extract of its powdered aerial parts, two flavones, three isoflavones and one 7-O-glucosyl isoflavone were isolated. A mass spectrometric study of these compounds was performed using liquid secondary ion mass spectrometry (LSIMS) in combination with high-energy collision-induced dissociation (CID) and tandem mass spectrometry (MS/MS). Characteristic fragmentation patterns were observed in all the investigated compounds; the loss of small neutral species from the protonated molecules was useful for identifying the presence of specific functional groups in the A- and B-rings. In order to help to establish the proposed structures, NMR and UV studies were also performed.

ChemInform Abstract: Inertness of the [Re6Se5Cl3]5+ Cluster Core to Substitution by OH‐ in Organic Solutions: Synthesis, Structural and Liquid Secondary Ion Mass Spectroscopy Characterization of K(H2O)2[Re6Se5Cl9] and (n‐Bu4N)[Re6Se5Cl9] and the Crystal Structure of (n‐Bu4N)2[Re6Se6Cl8

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Structural Elucidation of Eighteen Cerebrosides from Holothuria Coronopertusa in a Complex Mixture by High-Energy Collision-Induced Dissociation of [M + Li]+ Ions

Eighteen cerebrosides were detected in admixture from the sea cucumber Holothuria coronopertusa. Their structures have been established on the basis of liquid-secondary ion mass spectrometry (LSIMS)experiments. The collision-induced dissociation (CID)of the lithiated molecules ([M + Li]+)led to diagnostic fragment ions which were further identified by tandem mass spectrometry (MS/MS). In the present study, the combination of LSIMS as ionization mode and high-energy MS/MS analysis appears to be a very powerful method for the rapid analysis of unresolved mixtures of complex molecules.

Diastereoselective Formation of Host–Guest Complexes between a Series of Phosphate-Bridged Cavitands and Alkyl- and Arylammonium Ions Studied by Liquid Secondary-Ion Mass Spectrometry

Diastereoselective Formation of Host–Guest Complexes between a Series of Phosphate-Bridged Cavitands and Alkyl- and Arylammonium Ions Studied by Liquid Secondary-Ion Mass Spectrometry

Diastereoselective formation of host-guest complexes between a series of phosphate-bridged cavitands and alkyl- and arylammonium ions studied by liquid secondary-ion mass spectrometry.

The reactions occurring between a new class of cavitands that carry up to four dioxaphosphocin binding units and alkyl- and arylammonium ions was investigated by liquid secondary-ion mass spectrometry (LSIMS). As the cavitands existed as distinct diastereomers with different spatial orientation of their binding groups, these geometrical differences proved to have a dramatic influence on their chemical properties, including their ability to form host-guest complexes. In practice, only the cavitands that carry at least three P=O groups oriented toward the inside of the cavity were demonstrated to be strong ligands toward organic ammonium ions, whereas those with only two converging binding groups (either adjacent or opposite in the cavitand structure) still formed host-guest complexes, but they were much weaker. Adjacent binding sites proved to be more effective in interacting with organic ammonium ions than those lying in opposite positions. The isomers with no converging P=O groups did not act as molecular receptors. Even the isomer with one group oriented toward the inside of the cavity did not form host-guest complexes, as the absence of synergistic hydrogen bonding made the interaction from inside the cavity energetically equivalent (or even less favorable) to the outside binding. The presence in the cavitand structure of substituents with an electron-donating character proved to increase the proton affinity of the P=O binding groups and, consequently, their binding energy. The strong proton affinity of the cavitands led to the formation of stable host-guest complexes, as confirmed by the collisionally activated dissociation experiments. Effects of steric hindrance were weak, at least for the cavitands with three converging P=O groups. This confirmed that the cavity has a wide and readily accessible opening. The relative complexation constants were measured for the various guests, yielding scales of relative affinity toward each cavitand. These relative constants may represent thermodynamic values referred to the matrix used in LSIMS experiments, namely 3-nitrobenzyl alcohol (NBA), provided that kinetically controlled selvedge processes are negligible. Absolute complexation constants could not be obtained on account of the unknown pH and the protonation constant in the NBA matrix.

Hydrolysis of glycine-containing elastin pentapeptides by LasA, a metalloelastase from Pseudomonas aeruginosa.

Pseudomonas aeruginosa is an opportunistic pathogen that causes severe infections in vulnerable hosts. It may produce various virulence factors including proteases. Among them, LasA possesses both elastolytic and staphylolytic (hydrolysis of pentaglycine cross-links in the cell wall peptidoglycan) activities. To understand if its elastolytic activity results from a preference for glycine-rich substrates, we studied its ability to hydrolyse the 65 pentapeptides of human tropoelastin containing at least three glycines. As demonstrated by capillary electrophoresis (CE), 22 of these peptides were hydrolysed by LasA, generally at a single peptide bond and the catalytic ratio kcat/KM was determined for most of them. The highest value was obtained for LGGGA, 59 +/- 9 min(-1) x mmol(-1) x L. The specificity of hydrolysis was elucidated by CE, liquid secondary ion mass spectrometry and, in some cases, collision activated dissociation-mass analysis of ion kinetic energy. The preferred cleavage sites are GG and GA peptide bonds, the sequence GG(cleavage site)A being especially sensitive to hydrolysis. Both positions P2 and P'2 must be occupied for hydrolysis and the presence of an amino acid in P3 (but not in P'3) significantly increases the catalytic ratio. Considering these results, about 30 GGX sequences (X: G, A or Y) of human tropoelastin could be susceptible to LasA elastolysis.

Sequences and Antimycoplasmic Properties of Longibrachins LGB II and LGB III, Two Novel 20-Residue Peptaibols fromTrichodermalongibrachiatum

Longibrachins are members of the class of natural Aib-containing peptides designated as peptaibols. Six longibrachins, LGA I-IV and LGB II and III, were purified from a Trichoderma longibrachiatum strain by a procedure employing several chromatography steps including reversed-phase HPLC. The amino acid sequence determination was based on a combination of liquid secondary ion mass spectrometry (LSIMS) and two-dimensional 1H and 13C NMR spectroscopy. Longibrachins are 20-residue peptaibols with a C-terminal phenylalaninol and either neutral (LGA; Gln18) or acidic (LGB; Glu18) character. Longibrachins LGB II and III have novel sequences. Both longibrachins LGA and LGB show significant bactericidal activity against mycoplasmas (Acholeplasma, Mycoplasma, and Spiroplasma), with minimal inhibitory concentrations in the range 1.56-12.5 microM (3-25 micrograms/mL), and also perturb the permeability of membrane bilayers. Longibrachin LGA IV is the most potent of the presently known 18-20-residue peptaibols. The antimicrobial and membrane-perturbing properties of longibrachins, which are described here for the first time, were shown to be correlated.

Acidic glycerol lipids of Trichomonas vaginalis and Tritrichomonas foetus.

The isolation and characterization of acidic lipids from both Trichomonas vaginalis and Tritrichomonas foetus have been carried out using radiolabeling, a combination of high performance liquid and thin layer chromatographic techniques, and mass spectrometry. Unique among the eukaryotes, these organisms produce phosphatidylglycerols and O-acyl phosphatidylglycerol-like compounds. In this study, the molecular weight distributions of the phosphatidylglycerols and acyl phosphatidylglycerols were determined by negative-ion liquid secondary ionization mass spectrometry (LSIMS) and the fatty acyl groups within each molecular species were assessed by collision-induced decomposition tandem mass spectrometry (CID MS/MS). Both species were found to contain primarily oleic acid in the sn-2 position. The lipids of T. vaginalis had approximately equal amounts of C16 and C18 in the sn-1 position, with varying degrees of unsaturation, especially in the C18 species. The T. foetus lipids had C18 almost exclusively, but also varied in the unsaturation. Other acidic lipids included inositol phosphosphingolipids and inositol diphosphosphingolipids.

Inertness of the [Re6Se5Cl3]5+ cluster core to substitution by OH- in organic solutions: synthesis, structural and liquid secondary ion mass spectroscopy characterization of K(H2O)2[Re6Se5Cl9] and (n-Bu4N)[Re6Se5Cl9] and the crystal structure of (n-Bu4N)2[Re6Se6Cl8

When ethanol solutions of K[Re6Se5Cl9] are allowed to crystallize, the compound formulated K(H2O)2[Re6Se5Cl9] (1) is obtained. Extensive characterization, which includes chemical analysis, X-ray structure determination and liquid secondary ion mass spectrometry (LSIMS), supports this formulation, which differs from that proposed earlier, K[Re6Se5(OH)2Cl7]·H2O. This conclusively demonstrates that no substitution of an inner μ-core halogen by OH− occurs within the cluster core. This is discussed and shown to be fully consistent with the present well-documented chemistry, which requires that substitutions of halogen by divalent elements only are allowed. The paper is complemented by the crystal structures of (n-Bu4N)[Re6Se5Cl9] and (n-Bu4N)2[Re6Se6Cl8], which had not yet been properly reported.

An alternative approach to the synthesis of a recombinant galanin-like peptide (GALP)

GALP is a novel galanin-like peptide that has been isolated from porcine hypothalamus in our Research Division. In the present study, a large quantity of porcine GALP (pGALP) was synthesized with a high degree of purity using recombinant DNA technology. A basic fibroblast growth factor (bFGF) mutein, CS23, was used as a fusion partner and pGALP was released by chemical cleavage with cyanogen bromide (BrCN). Using various criteria such as HPLC, SDS–PAGE, amino acid analysis, amino acid sequence, relative-molecular-mass measurement by liquid secondary-ion mass spectrometry, as well as biological activity, the purified molecule was shown to be pGALP and was found to have the same biological activity as a chemically synthesized standard. The pGALP obtained here will be useful for elucidation of its physiological role in vivo. Furthermore, CS23ML76, a newly prepared bFGF mutein, was found to be more appropriate than CS23 as a fusion partner in this procedure.

Stabilization of a tyrosine O-sulfate residue by a cationic functional group: formation of a conjugate acid-base pair.

Sulfated tyrosine [Tyr(SO3H)]-containing peptides showed characteristic peak patterns in their liquid secondary-ion mass spectrometry (LSIMS) spectra. Protonated molecules were desulfated more easily than their deprotonated counterparts. Therefore, the stabilities of the Tyr(SO3H) residues were well-reflected by peak patterns in their positive-ion spectra. These intrinsic peak patterns were investigated by comparing the behavior of each Tyr(SO3H) residue in acidic solution. As the peptide chain was lengthened and the number of cationic functional groups increased, the peak representing the [MH]+ of a Tyr(SO3H)-containing peptide became more prominent than that representing the desulfated [MH-SO3]+. These alterations in peptide structure also increased the stability of the Tyr(SO3H) residue in acidic solution. Based on the desulfation mechanism of an aryl monosulfate, we predicted that intramolecular cationic functional groups would stabilize Tyr(SO3H) residues by forming conjugate acid-base pairs (or salt bridges) both in the gaseous phase and in acidic solution. In accordance with this theory, Arg residues would take primary responsibility for this self-stabilization within Tyr(SO3H)-containing peptides. Moreover, a long peptide backbone was expected to have a weak protective effect against desulfation of the [MH]+ in the gaseous phase. Tyr(SO3H) residues were also stabilized by adding an external basic peptide containing multiple Arg residues. Formation of such intermolecular acid-base pairs was demonstrated by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) which detected conjugated peptide ions. The energetically favorable formation of conjugate acid-base pairs prompted by Tyr(SO3H) residues might be a driving force for protein folding and protein-protein interaction.

Orientation of membrane-bound melittin studied by a combination of HPLC and liquid secondary ion mass spectrometry (LSIMS).

Combining two analytical techniques, HPLC and liquid secondary ion mass spectrometry, the orientation of liposomal membrane-bound melittin was analyzed through its trypsin-digested products. We found that trypsin can access all proteolytic sites of the membrane-bound melittin when the liposomes have no transmembrane potential, whereas the proteolytic site near the N terminus of melittin is blocked when the liposomes have a negative transmembrane potential. The results suggest that the negative transmembrane potential may induce the melittin molecules to insert into the membrane perpendicularly, whereas melittin lies flat on the membrane surface in the absence of a negative potential.