Collision-induced Dissociation Experiments Research Articles

Theoretical Study of the Dissociation Energy of First-Row Metallocenium Ions.

The bond dissociation energy of a series of metallocenium ions, i.e., the energy difference of the reaction MCp2(+) → MCp(+) + Cp· (with M = Ti, V, Cr, Mn, Fe, Co, and Ni), was studied by means of multiconfigurational perturbation theory (CASPT2, RASPT2, NEVPT2) and restricted coupled cluster theory (CCSD(T)). From a comparison between the results obtained from these different methods, and a detailed analysis of their treatment of electron correlation effects, a set of MCp(+)-Cp binding energies are proposed with an accuracy of 5 kcal/mol. The computed results are in good agreement with the experimental data measured by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy but disagree with the more recent threshold collision-induced dissociation (TCID) experiments.

Gas-phase ions produced by freezing water or methanol for analysis using mass spectrometry.

Introducing water or methanol containing a low concentration of volatile or nonvolatile analyte into an inlet tube cooled with dry ice linking atmospheric pressure and the first vacuum stage of a mass spectrometer produces gas-phase ions even of small proteins that can be detected by mass spectrometry. Collision-induced dissociation experiments conducted in the first vacuum region of the mass spectrometer suggest analyte ions being protected by a solvent cage. The charges may be produced by processes similar to those proposed for charge separation under freezing conditions in thunderclouds. By this process, the surface of an ice pellet is charged positive and the interior negative so that removal of surface results in charge separation. A reversal of surface charge is expected for a heated droplet surface, and this is observed by heating rather than cooling the inlet tube. These observations are consistent with charged supercooled droplets or ice particles as intermediates in the production of analyte ions under freezing conditions.

Dimer ion formation and intermolecular fragmentation of 1,2-diacylglycerols revealed by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry for more comprehensive lipid analysis.

The ionization of neutral diacylglycerols (DAGs) by electrospray ionization mass spectrometry (ESI-MS) is challenging compared with other lipid classes which possess ionic head group conjugations. Although ESI-MS is the method of choice in lipidomic analysis, it is questionable whether all lipid classes can be efficiently ionized by this method. Actually, various lipids were not efficiently detected (due to poor ionization) in many studies which claimed to comprehensively describe lipid profiles. Since neutral lipids are precursors for the biosynthesis of most other lipid classes, the necessity for improved or alternative ionization and identification schemes becomes obvious. We identified the 1,2-diacylglycerol (DAG) dimer ion formation in the gas phase by ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) in negative electrospray ionization ((-)ESI) mode. The geometry of the dimer ion was investigated by accurate density functional theory (DFT) calculations at the B3LYP/6-311+G(d)//B3LYP/LANL2DZ level of theory. Fragmentation of the dimer ions of many investigated DAGs has been achieved via collision-induced dissociation (CID) experiments with several elevated collision energies (0-12 eV). We revealed the possibility to ionize neutral DAGs as dimer ions in the negative ESI mode. Quantum mechanical calculations revealed a polar head-to-head intermolecular interaction between one charged DAG and one DAG neutral. This represents an energy minimum structure for the DAG dimer ions. We could furthermore detect CID fragmentation product ions that can only result from intermolecular reactions in this head-to-head conformation (SN2 nucleophilic substitution reactions inside the dimer DAG ion). Here, we present for the first time the opportunity to ionize and identify DAGs as dimer ions. This new finding provides a new alternative for investigations of important diacylglycerol lipids and provides the opportunity to obtain complementary and more comprehensive results in future lipidomic studies.

A mechanistic study of fragmentation of deprotonated N,2-diphenyl-acetamides in electrospray ionization tandem mass spectrometry.

Exploring the fragmentation mechanism of amide ions in mass spectrometry has attracted great interest because of the desire to analyze the amino acid sequences of peptides and proteins. However, the collision-induced dissociation (CID) mechanism of deprotonated small amides has been rarely studied in electrospray ionization mass spectrometry (ESI-MS). The fragmentation of deprotonated N,2-diphenylacetamides exhibited some characteristic fragment ions, which are not derived from the conventional cleavage route. Therefore, clarification of their fragmentation mechanism is very important and useful for structural analysis of related amides and peptides. All CID experiments were carried out using an electrospray ionization ion trap mass spectrometer in negative ion mode. In addition, the accurate masses of fragments were measured on an ESI quadrupole time-of-flight (Q-TOF) mass spectrometer in negative ion mode. Deuterium-labeled 2-phenyl-N-(4-trifluoromethylphenyl)acetamide was synthesized and its ESI fragmentation spectrum had been obtained. Theoretical calculations were carried out by the density functional theory (DFT) method at the B3LYP level of theory with the 6-31G++(d,p) basis set. Deprotonated N,2-diphenylacetamides mainly generate four kinds of ions in CID: benzyl anion, aniline anion, phenyl-ethenone anion and isocyanato-benzene anion bearing respective substituent groups. The benzyl anion and the aniline anion can be generated by direct decomposition. The phenyl-ethenone anion and the isocyanato-benzene anion were proposed to be yielded from proton transfer within an ion-neutral complex, and the intensities of two competitive product ions are well correlated with the substituent constants. The mechanism was also supported by theoretical calculations. The characteristic fragment ions of deprotonated N,2-diphenylacetamides were proposed to be produced via an ion-neutral complex mechanism, which was proved by deuterium-labeling experiments, theoretical calculations and substituent effects.

N-glycan analysis of mannose/glucose specific lectin from Dolichos lablab seeds

An affinity purified mannose/glucose specific lectin from the seeds of Dolichos lablab (Indian bean/lablab bean) resolves into five subunits upon SDS-PAGE in the range of Mr 12–20kDa. Partial de novo sequencing of subunits resulted in 88% and 73% sequence coverage for α and β subunits of the cDNA derived FRIL (Flt3 receptor interacting lectin) sequence, respectively and suggested that four bands correspond to the α-subunits while the band of lowest molecular mass is designated as β. It was proposed in an earlier study on FRIL that the difference in molecular mass of α-subunits is due to differences in C-terminal processing and differential N-glycosylation i.e. numbers of N-glycans present (Colucci et al., 1999). Thus, differential N-glycosylation of the purified mannose/glucose specific lectin was unravelled by in-gel trypsin/chymotrypsin digestion of the α-subunits followed by desalting and ZIC-HILIC enrichment of N-glycopeptides. Subsequently, analyses by nano electrospray ionisation quadrupole time of flight mass spectrometry and low-energy collision-induced dissociation experiments revealed the presence of a typical paucimannose type N-glycan (Man2(Xyl)GlcNAc2(Fuc)) in α subunits 2–4.

Experimental Gas-Phase Thermochemistry for Alkane Reductive Elimination from Pt(IV)

The gas-phase reactivity of the [(NN)PtIVMe3]+ (NN = α-diimine) complex 1 and its acetonitrile adduct has been investigated by tandem mass spectrometry. The only observed reaction from the octahedral d6 complex 1·MeCN is the simple dissociation of the coordinated solvent molecule with a binding energy of 24.5(6) kcal mol–1 measured by energy-resolved collision-induced dissociation experiments. Further reactions of 1 are observed. In addition to the expected reductive elimination of ethane from 1, competitive loss of methane occurs. Methane is generated from the initially formed ethane agostic complex via either C–H activation/bond formation or σ-bond metathesis with the third methyl group. Energy-resolved collision-induced dissociation experiments indicate that the initial reductive C–C coupling step is rate limiting for both ethane and methane elimination, and afford a gas-phase barrier of 22.6(7) kcal mol–1 for this process. Density functional theory calculations confirm the reaction mechanisms, and a v...

Fragmentation reactions of N-benzyltetrahydroquinolines in electrospray ionization mass spectrometry: the roles of ion/neutral complex intermediates.

Electrospray ionization mass spectrometry (ESI-MS) combined with the collision-induced dissociation (CID) technique has assumed increasing importance as an invaluable tool for the structural analysis of organic and biological molecules. However, general rules for elucidating the fragmentation behaviors of charged molecules in the gas phase are still lacking. Therefore, explorations on the mechanistic information are desirable at all times. CID experiments of protonated N-benzyltetrahydroquinolines were carried out on ESI ion trap mass spectrometer and accurate mass measurements were performed on a high-resolution ESI quadrupole time-of-flight (Q-TOF) mass spectrometer in positive ion mode. An ion/neutral complex, [RC6H4CH2(+)/tetrahydroquinoline], resulting from cleavage of the C-N bond induced by the positive charge brought in by protonation, was proposed to be the intermediate to elucidate the fragmentation reactions. For all the compounds investigated, benzyl cation transfer, electron transfer and hydride transfer reactions mediated by the complex were observed. Moreover, for the compound substituted by a methyl group at the para-position of the benzylic phenyl ring, proton transfer reaction via the complex also occurs. This study is a case for better understanding the intriguing roles of ion/neutral complexes in gas-phase fragmentation reactions and enriching the knowledge about the gas-phase chemistry of the benzyl cation. In addition, it provides useful information for researchers working on analysis or structural elucidation of complicated compounds which contain the N-benzyltetrahydroquinoline substructure.

Imidate-based cross-linkers for structural proteomics: increased charge of protein and peptide ions and CID and ECD fragmentation studies.

Chemical cross-linking is an attractive low-resolution technique for structural studies of protein complexes. Distance constraints obtained from cross-linked peptides identified by mass spectrometry (MS) are used to construct and validate protein models. Amidinating cross-linkers such as diethyl suberthioimidate (DEST) have been used successfully in chemical cross-linking experiments. In this work, the application of a commercial diimidate cross-linking reagent, dimethyl suberimidate (DMS), was evaluated with model peptides and proteins. The peptides were designed with acetylated N-termini followed by random sequences containing two Lys residues separated by an Arg residue. After cross-linking reactions, intra- and intermolecular cross-linked species were submitted to CID and ECD dissociations to study their fragmentation features in the gas phase. Fragmentation of intramolecular peptides by collision induced dissociation (CID) demonstrates a unique two-step fragmentation pathway involving formation of a ketimine as intermediate. Electron capture and electron transfer dissociation (ECD and ETD) experiments demonstrated that the cyclic moiety is not dissociated. Intermolecular species demonstrated previously described fragmentation behavior in both CID and ECD experiments. The charge state distributions (CSD) obtained after reaction with DMS were compared with those obtained with disuccinimidyl suberate (DSS). CSDs for peptides and proteins were increased after their reaction with DMS, owing to the higher basicity of DMS modified species. These features were also observed in LC-MS experiments with bovine carbonic anhydrase II (BCA) after cross-linking with DMS and tryptic proteolysis. Cross-linked peptides derived from this protein were identified at high confidence and those species were in agreement with the crystal structure of BCA.

Generation of gas-phase zirconium fluoroanions by electrospray of an ionic liquid.

New approaches for forming anions are sought that have strong abundance and no isobaric overlap, attributes that are compatible with the measurement of isotope ratios. Fluoroanions are particularly attractive because fluorine is monoisotopic, and thus will not have overlapping isobars with the isotope of interest. Since many elements do not have positive electron affinity values, they do not form stable negative atomic ions, and hence are not compatible with isotope ratio measurement using high sensitivity isotope ratio mass spectrometers such as accelerator mass spectrometers. Zirconium fluoroanions were prepared using the fluorinating ionic liquid (IL) 1-ethyl-3-methylimidazolium fluorohydrogenate, which was used to generate abundant [ZrF5](-) ions using electrospray ionization. The IL was dissolved in acetonitrile, combined with a dilute solution of either Zr(4+) or ZrO(2+), and then electrosprayed. Mass analysis and collision-induced dissociation experiments were conducted using a time-of-flight mass spectrometer. Cluster structures were predicted using density functional theory calculations. The fluorohydrogenate IL solutions generated abundant [ZrF5](-) ions starting from solutions of both Zr(4+) and ZrO(2+). The mass spectra also contained IL-bearing cluster ions, whose compositions indicated the presence of [ZrF6](2-) in solution, a conclusion supported by the structural calculations. Rinsing out the zirconium-IL solution with acetonitrile decreased the IL clusters, but enhanced [ZrF5](-), which was sorbed by the polymeric electrospray supply capillary, and then released upon rinsing. This reduced the ion background in the mass spectrum. The fluorohydrogenate-IL solutions are a facile way to form zirconium fluoroanions in the gas phase using electrospray. The approach has potential as a source of fluoroanions for isotope ratio measurements, which would enable high-sensitivity measurement of minor zirconium isotopes without overlapping isobars caused by the charge carrier (i.e., the monoisotopic fluorine atoms).

Discovery and Mechanistic Studies of Facile N-Terminal Cα–C Bond Cleavages in the Dissociation of Tyrosine-Containing Peptide Radical Cations

Fascinating N-terminal Cα-C bond cleavages in a series of nonbasic tyrosine-containing peptide radical cations have been observed under low-energy collision-induced dissociation (CID), leading to the generation of rarely observed x-type radical fragments, with significant abundances. CID experiments of the radical cations of the alanyltyrosylglycine tripeptide and its analogues suggested that the N-terminal Cα-C bond cleavage, yielding its [x2 + H](•+) radical cation, does not involve an N-terminal α-carbon-centered radical. Theoretical examination of a prototypical radical cation of the alanyltyrosine dipeptide, using density functional theory calculations, suggested that direct N-terminal Cα-C bond cleavage could produce an ion-molecule complex formed between the incipient a1(+) and x1(•) fragments. Subsequent proton transfer from the iminium nitrogen atom in a1(+) to the acyl carbon atom in x1(•) results in the observable [x1 + H](•+). The barriers against this novel Cα-C bond cleavage and the competitive N-Cα bond cleavage, forming the complementary [c1 + 2H](+)/[z1 - H](•+) ion pair, are similar (ca. 16 kcal mol(-1)). Rice-Ramsperger-Kassel-Marcus modeling revealed that [x1 + H](•+) and [c1 + 2H](+) species are formed with comparable rates, in agreement with energy-resolved CID experiments for [AY](•+).

UV-visible degradation of boscalid - structural characterization of photoproducts and potential toxicity usingin silicotests

Boscalid is a carboximide fungicide mainly used for vineyard protection as well as for tomato, apple, blueberry and various ornamental cultivations. The structural elucidation of by-products arising from the UV-visible photodegradation of boscalid has been investigated by gas chromatography/multi-stage mass spectrometry (GC/MS(n) ) and liquid chromatography/tandem mass spectrometry (LC/MS/MS) couplings. The potential toxicities of transformation products were estimated by in silico calculations. Aqueous solutions of boscalid were irradiated up to 150 min in a self-made reactor equipped with a mercury lamp. Analyses were carried out using a gas chromatograph coupled with an ion trap mass spectrometer operated in both electron ionization (EI) and chemical ionization (CI) modes and a liquid chromatograph coupled with a quadrupole time-of-flight (Q-TOF) mass spectrometer operated in electrospray ionization (ESI) mode. Multiple-stage collision-induced dissociation (CID) experiments were performed to establish dissociation pathways of ions. The QSAR (Quantitative Structure-Activity Relationship) T.E.S.T. program allowed the estimation of the toxicities of the by-products. Eight photoproducts were investigated. Chemical structures were proposed not only on the interpretation of multi-stage CID experiments, but also on kinetics data. These structures led us to suggest photodegradation pathways. Three photoproducts were finally detected in Lebanon in a real sample of grape leaves for which routine analysis had led to the detection of boscalid at 4 mg kg(-1). With one exception, the structures proposed for the photoproducts on the basis of mass spectra interpretation have not been reported in previous studies. In silico toxicity predictions showed that two photoproducts are potentially more toxic than the parent compound considering oral rat LD50 while five photoproducts may induce mutagenic toxicity. With the exception of one compound, all photoproducts may potentially induce developmental toxicity.

Detection of Plasticizers in Soil Using Surface Desorption Atmospheric Pressure Chemical Ionization Imaging Mass Spectrometry

To address the widespread contamination of surface soil by phthalate esters (PAEs), a method based on the surface desorption atmospheric pressure chemical ionization tandem mass spectrometry (SDAPCI-MSn) imaging technology was successfully applied for the direct determination of diethyl phthalate (DEP) in soil without any pretreatment. The precursor ion, protonated diethyl phthalate (m/z 223), was confirmed by collision-induced dissociation (CID) experiments to exclude false-positive results. The protonated DEP (m/z 223) and the characteristic fragment (m/z 177) were used to obtain the distribution of DEP molecules in the soil using imaging mass spectrometry. The two ion images without any pretreatment showed that the distribution of DEP was inhomogeneous in the shape of clusters, but these two distributions were changed after adding deionized water evenly into the surface of soil. The spatial resolution and the analysis speed per pixel were 0.25 mm2 and 4.5 s. This new method may be applicable for the study of content and distribution of PAEs in other complex solid samples.

Characterization of multiple fragmentation pathways initiated by collision‐induced dissociation of multifunctional anions formed by deprotonation of 2‐nitrobenzenesulfonylglycine

The correlation of anion structure with the fragmentation behavior of deprotonated nitrobenzenesulfonylamino acids was investigated using tandem mass spectrometry, isotopic labeling and computational methods. Four distinct fragmentation pathways resulting from the collision-induced dissociation (CID) of deprotonated 2-nitrobenzenesulfonylglycine (NsGly) were characterized. The unusual loss of the aryl nitro substituent as HONO was the lowest energy process. Subsequent successive losses of CO, HCN and SO2 indicated that an ortho cyclization reaction had accompanied loss of HONO. Other pathways involving rearrangement of the ionized sulfonamide group, dual bond cleavage and intramolecular nucleophilic displacement were proposed to account for the formation of phenoxide, arylsulfinate and arylsulfonamide product ions at higher collision energies. The four distinct fragmentation pathways were consistent with precursor-product relationships established by CID experiments, isotopic labeling results and the formation of analogous product ions from 2,4-dinitrobenzenesulfonylglycine and the Ns derivatives of alanine and 2-aminoisobutyric acid. The computations confirmed a low barrier for ortho cyclization with loss of HONO and feasible energetics for each reaction step in the four pathways. Computations also indicated that three of the fragmentation pathways started from NsGly ionized at the carboxyl group. Overall, the pathways identified for the fragmentation of the NsGly anion differed from processes reported for anions containing a single functional group, demonstrating the importance of functional group interactions in the fragmentation pathways of multifunctional anions.

Implementation of Dipolar Resonant Excitation for Collision Induced Dissociation with Ion Mobility/Time-of-Flight MS

An ion mobility/time-of-flight mass spectrometer (IMS/TOF MS) platform that allows for resonant excitation collision induced dissociation (CID) is presented. Highly efficient, mass-resolved fragmentation without additional excitation of product ions was accomplished and over-fragmentation common in beam-type CID experiments was alleviated. A quadrupole ion guide was modified to apply a dipolar AC signal across a pair of rods for resonant excitation. The method was characterized with singly protonated methionine enkephalin and triply protonated peptide angiotensin I, yielding maximum CID efficiencies of 44% and 84%, respectively. The Mathieu q(x,y) parameter was set at 0.707 for these experiments to maximize pseudopotential well depths and CID efficiencies. Resonant excitation CID was compared with beam-type CID for the peptide mixture. The ability to apply resonant waveforms in mobility-resolved windows is demonstrated with a peptide mixture yielding fragmentation over a range of mass-to-charge (m/z) ratios within a single IMS-MS analysis.

Enantiomeric differentiation of β-amino alcohols under electrospray ionization mass spectrometric conditions

The enantiomeric differentiation of a series of chiral β-amino alcohols (A) is attempted, for the first time, by applying the kinetic method using L-proline, L-tryptophan, 4-iodo-L-phenylalanine or 3, 5-diiodo-L-tyrosine as the chiral references (Ref) and Cu(2+) or Ni(2+) ion (M) as the central metal ion. The trimeric diastereomeric adduct ions, [M+(Ref)2+A-H](+), formed under electrospray ionization conditions, are subjected for collision-induced dissociation (CID) experiments. The products ions, formed by the loss of either a reference or an analyte, detected in the CID spectra are evaluated for the enantiomeric differentiation. All the references showed enantiomeric differentiation and the R(chiral) values are better for the aromatic alcohols than for aliphatic alcohols. Notably, the R(chiral) values of the aliphatic amino alcohols enhanced when Ni(2+) is used as the central metal ion. The experimental results are well supported by computational studies carried out on the diastereomeric dimeric complexes. The computational data of amino alcohols is correlated with that of amino acids to understand the structural interaction of amino alcohols with reference molecule and central metal ion and their role on the stabilization of the dimeric complexes. Application of flow injection MS/MS method is also demonstrated for the enantiomeric differentiation of the amino alcohols.

Photofragmentation spectroscopy of cold protonated aromatic amines in the gas phase

The electronic spectra of cold protonated aromatic amines: anilineH(+) C6H5-NH3(+), benzylamineH(+) C6H5-CH2-NH3(+) and phenylethylamineH(+) C6H5-(CH2)2-NH3(+) have been investigated experimentally in a large spectral domain and are compared to those of their hydroxyl homologues. In the low energy region, the electronic spectra are similar to their neutral analogues, which reveals the ππ* character of their first excited state. A second transition is observed from 0.4 to 1 eV above the origin band, which is assigned to the excitation of the πσ* state. In these protonated amine molecules, there is a competition between different fragmentation channels, some being specific to UV excitation i.e., not observed in low-energy collision induced dissociation experiments. Besides, for one amine a drastic change in the fragmentation branching ratio is observed within a very short energy range that reveals the complex excited state dynamics and fragmentation processes. The experimental observations can be rationalized using a simple qualitative model, the ππ*-πσ* model [A. L. Sobolewski, W. Domcke, C. Dedonder-Lardeux and C. Jouvet, Phys. Chem. Chem. Phys., 2002, 4, 1093-1100], which predicts that the excited state dynamics is controlled by the crossing between the ππ* excited state and a πσ* state repulsive along the XH (X being O or N) coordinate.

Jack bean -mannosidase: Amino acid sequencing and N-glycosylation analysis of a valuable glycomics tool

Jack bean (Canavalia ensiformis) seeds contain several biologically important proteins among which α-mannosidase (EC 3.2.1.24) has been purified, its biochemical properties studied and widely used in glycan analysis. In the present study, we have used the purified enzyme and derived its amino acid sequence covering both the known subunits (molecular mass of ∼66,000 and ∼44,000 Da) hitherto not known in its entirety. Peptide de novo sequencing and structural elucidation of N-glycopeptides obtained either directly from proteolytic digestion or after zwitterionic hydrophilic interaction liquid chromatography solid phase extraction-based separation were performed by use of nanoelectrospray ionization quadrupole time-of-flight mass spectrometry and low-energy collision-induced dissociation experiments. De novo sequencing provided new insights into the disulfide linkage organization, intersection of subunits and complete N-glycan structures along with site specificities. The primary sequence suggests that the enzyme belongs to glycosyl hydrolase family 38 and the N-glycan sequence analysis revealed high-mannose oligosaccharides, which were found to be heterogeneous with varying number of hexoses viz, Man8-9GlcNAc2 and Glc1Man9GlcNAc2 in an evolutionarily conserved N-glycosylation site. This site with two proximal cysteines is present in all the acidic α-mannosidases reported so far in eukaryotes. Further, a truncated paucimannose type was identified to be lacking terminal two mannose, Man1(Xyl)GlcNAc2 (Fuc).

Mass Fragmentation Studies of α-Tomatine and Validation of a Liquid Chromatography LTQ Orbitrap Mass Spectrometry Method for Its Quantification in Tomatoes

Tomatoes, members of the Solanaceae plant family, produce biologically active secondary metabolites, including glycoalkaloids, which may have both adverse and beneficial biological effects. By using the linear ion trap (LIT) mass spectrometry, multistage collision-induced dissociation (CID) experiments (MS n ) were performed to elucidate characteristic fragmentation pathways of the glycoalkaloid α-tomatine. High-resolution with high-accuracy mass analysis using an Orbitrap Fourier transform MS (FTMS) with higher-energy CID (HCD) was used to produce mass spectra data across a wide spectral range for confirmation of proposed ion structures and formulae. In addition, a new liquid chromatography method that utilized LTQ Orbitrap MS was developed for the analysis of α-tomatine in tomatoes. Recoveries of α-tomatine were >96.0 % with relative standard deviations (RSD) below 7.98 %. The limit of detection (LOD) was 0.002 mg/kg. The limit of quantitation (LOQ) was 0.005 mg/kg. The linear range was between 0.010 and 10 mg/kg with an excellent correlation coefficient (R 2) ≥ 0.9991. Various tomato samples were analyzed for method application, and the level of α-tomatine in the 11 samples analyzed ranged from 0.0011to 0.3077 mg/kg.

Interaction of Gold Acetylides with Gold(I) or Silver(I) Cations

Structures and properties of complexes between (trimethylphosphino)gold acetylides and another (trimethylphosphino)gold cation have been studied in the light of the possible “dual activation” reaction mode in gold catalysis. Silver salts are often used as cocatalysts in gold catalysis; therefore, also mixed complexes of acetylides with (trimethylphosphino)gold(I) and silver(I) cations have been investigated. Energy-resolved collision-induced dissociation experiments, ion spectroscopy, and density functional theory calculations show that the gold and silver cations preferentially coordinate to the gold acetylides rather than to neutral CC triple bonds in diyne-type substrates. The effect of the binding of two metal cations to acetylides with respect to nucleophilic additions is discussed.

Selective and Nonselective Cleavages in Positive and Negative CID of the Fragments Generated from In-Source Decay of Intact Proteins in MALDI-MS

Selective and nonselective cleavages in ion trap low-energy collision-induced dissociation (CID) experiments of the fragments generated from in-source decay (ISD) with matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) of intact proteins are described in both positive and negative ion modes. The MALDI-ISD spectra of the proteins demonstrate common, discontinuous, abundant c- and z'-ions originating from cleavage at the N-Cα bond of Xxx-Asp/Asn and Gly-Xxx residues in both positive- and negative-ion modes. The positive ion CID of the c- and z'-ions resulted in product ions originating from selective cleavage at Asp-Xxx, Glu-Xxx and Cys-Xxx residues. Nonselective cleavage product ions rationalized by the mechanism of a "mobile proton" are also observed in positive ion CID spectra. Negative ion CID of the ISD fragments results in complex product ions accompanied by the loss of neutrals from b-, c-, and y-ions. The most characteristic feature of negative ion CID is selective cleavage of the peptide bonds of acidic residues, Xxx-Asp/Glu/Cys. A definite influence of α-helix on the CID product ions was not obtained. However, the results from positive ion and negative ion CID of the MALDI-ISD fragments that may have long α-helical domains suggest that acidic residues in helix-free regions tend to degrade more than those in helical regions.