过渡金属纯簇和混合簇的密度泛函研究:Nb4,Co4和Nb2Co2

用密度泛函方法对过渡金属双原子Nb₂,Co₂和NbCo的电子态进行研究,得到三者的基态分别为³∑_g^-,⁵∑_g⁺和³△.并以此为基础,讨论四核簇Nb₄,Co₄和No₂Co₂的成键情况,发现稳定的单金属簇Nb₄具有高对称性的密堆结构,稳定的Co₄具有低对称性的变形封闭结构,两者都是典型的金属键;而Nb₂Co₂在封闭式结构中是一般的金属键,在线形结构中有强弱交替的定域键,Nb原子易相互靠近成键,Co原子趋于远离不成键.三种团簇的多重度以Nb₄＜Nb₂Co₂＜Co₄顺序依次升高.     

Generation and reactivity of enantiomeric (BINOLato)Ni+ complexes with chiral secondary alcohols in the gas phase

In the presence of secondary alcohols, electrospray ionization of dilute methanolic solutions of nickel(II) salts and 1,1'-bis-2-naphthol (BINOL) leads to complexes of the formal composition [(BINOLato)Ni(CH3CH(OH)R)]+ (BINOLato refers to a singly deprotonated (R)- or (S)-1,1'-bis-2-naphthol ligand; R=CH3, C2H5, n-C3H7, n-C4H9, n-C5H11, n-C6H13, c-C6H11, and C6H5). Upon collision-induced dissociation, each mass-selected nickel complex either loses the entire secondary alcohol ligand or undergoes bond activation followed by elimination of the corresponding ketone, as revealed by deuterium labeling. When enantiomeric BINOLato ligands (R or S) are combined with chiral secondary alcohols (R or S), differences in the branching ratios between these channels for the two stereoisomers of the secondary alcohols provide insight into the chiral discrimination operative in the C--H- and O--H-bond activation processes. For saturated alkan-2-ols, the chiral discrimination is low, and if any preference is observed at all, ketone elimination from the homochiral complexes (R,R and S,S) is slightly favored. In contrast, the diastereomeric (BINOLato)Ni+ complexes of 1-phenylethanol exhibit preferential ketone losses for the heterochiral systems (S,R and R,S).     

Using collision-induced dissociation with corrections for the ion number of degrees of freedom for quick comparisons of relative bonding strength

The number of degrees of freedom-dependent stability of ions and ion-neutral non-covalent complexes under collision-induced dissociation (CID) conditions was studied in a quadrupole ion trap mass spectrometer. It was found that the stability of ions as probed by energy-variable CID has a linear dependence on the total number of degrees of freedom for the ions (or ion-neutral complexes) with the same (or nearly the same) bonding energy. The slope of such a stability vs number of degrees of freedom dependence correlates with the binding energy. Proton-bound amine dimers display the lowest slope as they have weak bonds. Breaking covalent bonds will result in much greater slopes. In addition to the binding energy, the vibrational frequencies of the ion also affect the stability vs number of degrees of freedom behavior. Studying such a dependence of the CID stability in a system paves the way for direct relative binding energy comparisons. The application of this approach is demonstrated by testing the relative heme affinities of anti-malaria drugs and related compounds.     

Surprising Homolytic Gas Phase Co−C Bond Dissociation Energies of Organometallic Aryl-Cobinamides Reveal Notable Non-Bonded Intramolecular Interactions

Aryl-cobalamins are a new class of organometallic structural mimics of vitamin B₁₂ designed as potential ‘antivitamins B₁₂’. Here, the first cationic aryl-cobinamides are described, which were synthesized using the newly developed diaryl-iodonium method. The aryl-cobinamides were obtained as pairs of organometallic coordination isomers, the stereo-structure of which was unambiguously assigned based on homo- and heteronuclear NMR spectra. The availability of isomers with axial attachment of the aryl group, either at the ‘beta’ or at the ‘alpha’ face of the cobalt-center allowed for an unprecedented comparison of the organometallic reactivity of such pairs. The homolytic gas-phase bond dissociation energies (BDEs) of the coordination-isomeric phenyl- and 4-ethylphenyl-cobinamides were determined by ESI-MS threshold CID experiments, furnishing (Co−C )-BDEs of 38.4 and 40.6 kcal mol⁻¹, respectively, for the two β-isomers, and the larger BDEs of 46.6 and 43.8 kcal mol⁻¹ for the corresponding α-isomers. Surprisingly, the observed (Co−C )-BDEs of the Co_β-aryl-cobinamides were smaller than the (Co−C )-BDE of Co_β-methyl-cobinamide. DFT studies and the magnitudes of the experimental (Co−C )-BDEs revealed relevant contributions of non-bonded interactions in aryl-cobinamides, notably steric strain between the aryl and the cobalt-corrin moieties and non-bonded interactions with and among the peripheral sidechains.     

Charge state dependent collision-induced dissociation of native and reduced porcine elastase

The [M + 20H](20+)-[M + 12H](12+) charge states of native and reduced porcine elastase, a 25.9 kDa serine protease, were subjected to collisional activation in a quadrupole ion trap. For most charge states, ion parking was used to increase the number of parent ions over that yielded directly by electrospray. Ion-ion proton transfer reactions were used to reduce product ion charge states largely to +1 to simplify spectral interpretation. Both forms of the protein show charge state dependent fragmentation behavior. The native protein, which contains four disulfide linkages, shows almost no evidence for fragmentation within the regions of the protein linked by disulfide bonds. However, at the lowest charge states studied, evidence for cleavage of a least one of the disulfide bonds was evident in the appearance of a c-type ion. The highest charge states of native elastase showed several prominent cleavages C-terminal to valine residues. As the charge state decreased, however, preferential cleavages at acidic amino acid residues became important. The reduced form of the protein did not show particularly prominent cleavages at valine residues. However, many of the same preferential cleavages at acidic amino acid residues noted for the native protein were also observed in the same charge states of the reduced protein. The reduced protein also showed additional cleavages from regions of the protein that are ordinarily protected by disulfide linkages in the native form.     

Azidoacetone as a complexing agent of transition metals Ni2+/Co2+ promoted dissociation of the C-C bond in azidoacetone

The relevance of metal interactions with azides has led us to the study of the complexation of some transition metals, nickel and cobalt, by azidoacetone by means of electrospray ionization mass spectrometry (ESI-MS). Complexes were obtained from solutions of NiCl(2) and CoCl(2) , in methanol/water. Nickel was electrosprayed with other counter ion, bromide (Br), as well as other solvent (ethanol/water). For nickel and cobalt, the complexes detected were single positively charged, with various stoichiometries, some resulted from the fragmentation of the ligand, the loss of N(2) being quite common. The most abundant species were [Ni(II)Az(2)X](+) where X = Cl, Br and Az = azidoacetone. Some of the complexes showed solvation with the solvent components. Metal reduction was observed in complexes where a radical was lost, resulting from the homolytic cleavage of a metal coordination bond. Collision-induced dissociation (CID) experiments followed by tandem mass spectrometry (MS-MS) analysis were not absolutely conclusive about the coordination site. However, terminal ions of the fragmentation routes were explained by a gas-phase mechanism proposed where a C-C bond was activated and the metal inserted subsequently. Density functional theory calculations provided structures for some complexes. In [Ni(II)Az(2)X](+) species, one azidoacetone ligand is monodentate and the dominant binding location is the alkylated nitrogen and not the carbonyl group. The other azidoacetone ligand is bidentate showing coordination through alkylated nitrogen and the carbonyl group. These are also the preferential binding sites for the most stable isomer of [Ni(II)AzX](+) species.     

Effects of transition metal ion coordination on the collision-induced dissociation of polyalanines

Transition metal-polyalanine complexes were analyzed in a high-capacity quadrupole ion trap after electrospray ionization. Polyalanines have no polar amino acid side chains to coordinate metal ions, thus allowing the effects metal ion interaction with the peptide backbone to be explored. Positive mode mass spectra produced from peptides mixed with salts of the first row transition metals Cr(III), Fe(II), Fe(III), Co(II), Ni(II), Cu(I), and Cu(II) yield singly and doubly charged metallated ions. These precursor ions undergo collision-induced dissociation (CID) to give almost exclusively metallated N-terminal product ions whose types and relative abundances depend on the identity of the transition metal. For example, Cr(III)-cationized peptides yield CID spectra that are complex and have several neutral losses, whereas Fe(III)-cationized peptides dissociate to give intense non-metallated products. The addition of Cu(II) shows the most promise for sequencing. Spectra obtained from the CID of singly and doubly charged Cu-heptaalanine ions, [M + Cu - H](+) and [M + Cu](2+) , are complimentary and together provide cleavage at every residue and no neutral losses. (This contrasts with [M + H](+) of heptaalanine, where CID does not provide backbone ions to sequence the first three residues.) Transition metal cationization produces abundant metallated a-ions by CID, unlike protonated peptides that produce primarily b- and y-ions. The prominence of metallated a-ions is interesting because they do not always form from b-ions. Tandem mass spectrometry on metallated (Met = metal) a- and b-ions indicate that [b(n) + Met - H](2+) lose CO to form [a(n) + Met - H](2+), mimicking protonated structures. In contrast, [a(n) + Met - H](2+) eliminate an amino acid residue to form [a(n-1) + Met - H](2+), which may be useful in sequencing.     

Density functional theory studies on the dissociation energies of metallic salts: relationship between lattice and dissociation energies

The formation and physicochemical properties of polymer electrolytes strongly depend on the lattice energy of metal salts. An indirect but efficient way to estimate the lattice energy through the relationship between the heterolytic bond dissociation and lattice energies is proposed in this work. The heterolytic bond dissociation energies for alkali metal compounds were calculated theoretically using the Density Functional Theory (DFT) of B3LYP level with 6‐311+G(d,p) and 6‐311+G(2df,p) basis sets. For transition metal compounds, the same method was employed except for using the effective core potential (ECP) of LANL2DZ and SDD on transition metals for 6‐311+G(d,p) and 6‐311+G(2df,p) calculations, respectively. The dissociation energies calculated by 6‐311+G(2df,p) basis set combined with SDD basis set were better correlated with the experimental values with average error of ca. ±1.0% than those by 6‐311+G* combined with the LANL2DZ basis set. The relationship between dissociation and lattice energies was found to be fairly linear (r>0.98). Thus, this method can be used to estimate the lattice energy of an unknown ionic compound with reasonably high accuracy. We also found that the dissociation energies of transition metal salts were relatively larger than those of alkaline metal salts for comparable ionic radii. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 827–834, 2001     

Protein identification via ion-trap collision-induced dissociation and examination of low-mass product ions

A whole-protein tandem mass spectrometry approach for protein identification based on precursor ion charge state concentration via ion/ion reactions, ion-trap collisional activation, ion/ion proton-transfer reactions involving the product ions, and mass analysis over a narrow m/z range (up to m/z 2000) is described and evaluated. The experiments were carried out with a commercially available electrospray ion-trap instrument that has been modified to allow for ion/ion reactions. Reaction conditions and the approach to searching protein databases were developed with the assumption that the resolving power of the mass analyzer is insufficient to distinguish charge states on the basis of the isotope spacings. Ions derived from several charge states of cytochrome c, myoglobin, ribonuclease A, and ubiquitin were used to evaluate the approach for protein identification and to develop a two-step procedure to database searching to optimize specificity. The approach developed with the model proteins was then applied to whole cell lysate fractions of Saccharomyces cerevisiae. The results are illustrated with examples of assignments made for three a priori unknown proteins, each selected randomly from a lysate fraction. Two of the three proteins were assigned to species present in the database, whereas one did not match well any database entry. The combination of the mass measurement and the product ion masses suggested the possibility for the oxidation of two methionine residues of a protein in the database. The examples show that this limited whole-protein characterization approach can provide insights that might otherwise be lacking with approaches based on complete enzymatic digestion.     

Mass spectrometric characterization of 3'-imino[60]fulleryl-3'-deoxythymidine by collision-induced dissociation

The primary structure of 3'-imino[60]fulleryl-3'-deoxythymidine ions is studied using mass spectrometry both in the positive and negative modes. Interaction between the subunits is discussed using collision-induced dissociation (CID) spectra. Collisional activation with argon of the sodiated cations leads to the cleavage of the glycosidic bond and the transfer of a radical hydrogen from the deoxyribose to the thymine. The sodiated thymine is the only fragment observed for low collision energies in the positive mode. In the negative mode, two different ionization mechanisms take place, reduction and deprotonation in the presence of triethylamine. The 2.7 eV electron affinity of C60 and its huge cross section compared to the small cross section and predicted 0.44 eV electron affinity of the thymidine subunit most likely localize the radical electron on the fullerene. On the other hand, deprotonation of the 3'-azido-3'-deoxythymidine (AZT) is known to occur in N-3, the most acidic site of the nucleobase. Consequently, deprotonation causes the negative charge to be initially localized on the thymine. Both types of parent anions give the radical anion C60*- as fragment. The other fragments detected are the dehydrogenated 3'-imino[60]fulleryl-3'-deoxyribose anion, C60NH2-, C60N- and C60H-. Since in negative ion mass spectrometry all fragments include the [60]fullerene unit, this suggests that the fragmentation is driven by the electron affinity of the [60]fullerene, likely responsible for a charge transfer between the deprotonated thymine and the C60.     

Improving fragmentation of poorly fragmenting peptides and phosphopeptides during collision-induced dissociation by malondialdehyde modification of arginine residues

Despite significant technological and methodological advancements in peptide sequencing by mass spectrometry, analyzing peptides that exhibit only poor fragmentation upon collision-induced dissociation (CID) remains a challenge. A major cause for unfavorable fragmentation is insufficient proton 'mobility' due to charge localization at strongly basic sites, in particular, the guanidine group of arginine. We have recently demonstrated that the conversion of the guanidine group of the arginine side chain by malondialdehyde (MDA) is a convenient tool to reduce the basicity of arginine residues and can have beneficial effects for peptide fragmentation. In the present work, we have focused on peptides that typically yield incomplete sequence information in CID-MS/MS experiments. Energy-resolved tandem MS experiments were carried out on angiotensins and arginine-containing phosphopeptides to study in detail the influence of the modification step on the fragmentation process. MDA modification dramatically improved the fragmentation behavior of peptides that exhibited only one or two dominant cleavages in their unmodified form. Neutral loss of phosphoric acid from phosphopeptides carrying phosphoserine and threonine residues was significantly reduced in favor of a higher abundance of fragment ions. Complementary experiments were carried out on three different instrumental platforms (triple-quadrupole, 3D ion trap, quadrupole-linear ion trap hybrid) to ascertain that the observation is a general effect.     

Identification and sequencing analysis of intact proteins via collision-induced dissociation and quadrupole time-of-flight mass spectrometry

Identifying unknown proteins has become a central focal point for proteomic and biopharmaceutical development laboratories. Our laboratory investigated using quadrupole time-of-flight mass spectrometry (Qq/TOFMS) for the analysis of intact proteins for the purpose of identifying unknowns while limiting the number of sample-handling steps between protein extraction and identification. Eight standard proteins, both unmodified and disulfide-bonded and ranging in mass from 5 to 66 kDa, were analyzed using nanoelectrospray and collision-induced dissociation to generate peptide sequence tags. An MS analysis, followed by MS/MS analyses on two to five individual protein charge states, were obtained to make an identification. Peptide sequence tags were extracted from the MS/MS data and used, in conjunction with molecular mass and source origin, to obtain protein identifications using the web-based search engine ProteinInfo (www.proteometrics.com). All of the proteins were unambiguously identified from the input data, after which, all of the major product ions were identified for structural information. In most cases, N- and/or C-terminal ions, and also stretches of consecutive product ions from the protein interior, were observed. This method was applied to the analysis and identification of an unknown detected via reversed-phase high-performance liquid chromatography.     

Heteronuclear diatomic transition-metal cluster ions in the gas phase: Reactivity and thermochemistry of AgFe+

The gas-phase chemistry of AgFe⁺ was studied by using Fourier transform ion cyclotron resonance mass spectrometry. AgFe⁺ is unreactive with alkanes but reacts with cyclic and linear (C4–C8) alkenes. The primary reactions are dominated by dehydrogenation and condensation. In addition, cluster splitting is observed in the reaction of AgFe⁺ with benzene. Secondary reactions generally involve cluster splitting with the loss of Ag, although AgFeC₅H ₆ ⁺ is observed to dehydrogenate cyclopentene to yield AgFeC₁₀H ₁₂ ⁺ . Ion-molecule reactions, collision-induced dissociation, and photodissociation experiments were used to determine the bond energiesD°(Fe⁺–Ag)=53±7 kcal/mol andD°(Ag⁺–Fe)=46±7 kcal/mol. These values in turn were used to calculateH _f (AgFe⁺)=296±7 kcal/mol andIP(AgFe)=6.5±0.3 eV. Related chemical and physical properties of CuFe⁺ are presented for comparison.     

Application of MALDI TOF/TOF mass spectrometry and collision-induced dissociation for the identification of disulfide-bonded peptides

This paper describes a method for the fast identification and composition of disulfide-bonded peptides. A unique fragmentation signature of inter-disulfide-bonded peptides is detected using matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF)/TOF mass spectrometry and high-energy collision-induced dissociation (CID). This fragmentation pattern identifies peptides with an interconnected disulfide bond and provides information regarding the composition of the peptides involved in the pairing. The distinctive signature produced using CID is a triplet of ions resulting from the cleavage of the disulfide bond to produce dehydroalanine, cysteine or thiocysteine product ions. This method is not applicable to intra-peptide disulfide bonds, as the cleavage mechanism is not the same and a triplet pattern is not observed. This method has been successfully applied to identifying disulfide-bonded peptides in a number of control digestions, as well as study samples where disulfide bond networks were postulated and/or unknown.     

Neutral loss of amino acid residues from protonated peptides in collision-induced dissociation generates N- or C-terminal sequence ladders

The widespread occurrence of the neutral loss of one to six amino acid residues as neutral fragments from doubly protonated tryptic peptides is documented for 23 peptides with individual sequences. Neutral loss of amino acids from the N-terminus of doubly charged tryptic peptides results in doubly charged y-ions, forming a ladder-like series with the ions [M + 2H](2+) = y(max) (2+), y(max - 1) (2+), y(max - 2) (2+), etc. An internal residue such as histidine, proline, lysine or arginine appears to favor this type of fragmentation, although it was sometimes also observed for peptides without this structure. For doubly protonated non-tryptic peptides with one of these residues at or near the N-terminus, we observed neutral loss from the C-terminus, resulting in a doubly charged b-type ion ladder. The analyses were performed by Q-TOF tandem mass spectrometry, facilitating the recognition of neutral loss ladders by their 2+ charge state and the conversion of the observed mass differences into reliable sequence information. It is shown that the neutral loss of amino acid residues requires low collision offset values, a simple mechanistic explanation based on established fragmentation rules is proposed and the utility of this neutral loss fragmentation pathway as an additional source for dependable peptide sequence information is documented.     

Theoretical sstudy on the Co2+OH2+/Co3+OH2+ electron transfer reactivity

This paper presents a contact distance dependence analysis scheme and an abinitio calculation application for the electron transfer (ET) reactivity of Co²⁺OH₂/Co³⁺OH² reacting pair. The applicability of these schemes and the corresponding models has been discussed. The contact distance (R _CoCo) dependence of the relevant quantities has been analyzed. The results indicate that the activation energy from the accurate PES method agrees well with that from the anharmonic potential method, and they are obviously better than that from the harmonic potential method. The pair distribution function varies from 10^?2 to 10^?5 along withR _CoCo changing from 1.20 to 0.35 nm. The coupling matrix element exponentially decays along with the increase ofR _CoCo, and the effective electronic coupling requiresR _CoCo smaller than 0.75 nm. In the range from 0.50 to 0.75 nm forR _CoCo, the corresponding electronic transmission coefficient falls within 1.0—10^?6. The local ET rate also exponentially decays along with the increase ofR _CoCo due to the electronic factor. Since the contribution from the pair distribution function to the total ET rate is an inverse measure of that from the electronic factor, the variation of the spherically averaged local ET rate along withR _CoCo exhibits a parabola with a maximum at 0.50 nm ofR _CoCo. This maximum is close to the overall observed ET rate value. For this mono-hydrated transition metal ionic system, the ET rate generally is about 10⁶ L·mol^?1·s^?1 in gaseous process. Further, since it is impossible to experimentally determine the structures and their PESs of these hydrated systems, especially for the unstable intermediate species,ab initio calculations can play an effective auxiliary role in discussing the ET reactivities of these kinds of reacting systems.     

Experimental and computational studies of intracomplex reactions in Mg+(primary, secondary alkylamine) complexes induced by photoexcitation of Mg+

We report herein a comprehensive study of photoinduced reactions in complexes of Mg+ with primary (n-propyl- and isopropylamine) and secondary amines (dipropyl- and diisopropylamine) in the spectral range of 230-440 nm. Similar to the methyl- and ethylamine complexes studied previously, N-H bond activation of these complexes is very unfavorable. Instead, the C(alpha)-C, C-N, and C(alpha)-H bond-cleavage photoproducts are observed after photoexcitation of the Mg+ complexes (3(2)P<--3(2)S). For Mg+(primary amine) complexes, for example, Mg+-NH2CH2CH2CH3, and Mg+-NH2CH(CH3)2, the photoproducts resulting from C(alpha)--C rupture prevail after P(z) and charge-transfer excitations, whereas the Mg+ photofragment is predominant upon P(x,y) excitation. However, with further N-alkyl substitution, as in Mg+(secondary amine) complexes, for example, Mg+-NH(CH2CH2CH3)2 and Mg+-NH[CH(CH3)2]2, a novel intracomplex C-C coupling photoreaction dominates on P(x,y) excitation of Mg+, which is believed to arise from Mg+* insertion into the C-N bond. With P(z) and charge-transfer excitation, the Mg-R elimination photoproducts, arising from C(alpha)-C bond cleavage, predominate. The energetics and possible mechanisms of the intracomplex photoreactions are analyzed in detail with the help of extensive quantum mechanics calculations.     

Molecular recognition: comparative study of a tunable host-guest system by using a fluorescent model system and collision-induced dissociation mass spectrometry on dendrimers

Host-guest interactions between the periphery of adamantylurea-functionalized dendrimers (host) and ureido acetic acid derivatives (guest) were shown to be specific, strong and spatially well-defined. The binding becomes stronger when using phosphonic or sulfonic acid derivatives. In the present work we have quantified the binding constants for the host-guest interactions between two different host motifs and six different guest molecules. The host molecules, which resemble the periphery of a poly(propylene imine) dendrimer, have been fitted with an anthracene-based fluorescent probe. The two host motifs differ in terms of the length of the spacer between a tertiary amine and two ureido functionalities. The guest molecules all contain an acidic moiety (either a carboxylic acid, a phosphonic acid, or a sulfonic acid) and three of them also contain an ureido moiety capable of forming multiple hydrogen bonds to the hosts. The binding constants for all 12 host-guest complexes have been determined by using fluorescence titrations by monitoring the increase in fluorescence of the host upon protonation by the addition of the guest. The binding constants could be tuned by changing the design of the acidic part of the guest. The formation of hydrogen bonds gives, in all cases, higher association constants, demonstrating that the host is more than a proton sensor. The host with the longer spacer (propyl) shows higher association constants than the host with the shorter spacer (ethyl). The gain in association constants are higher when the urea function is added to the guests for the host with the longer spacer, indicating a better fit. Collision-induced dissociation mass spectrometry (CID-MS) is used to study the stability of the six motifs using the corresponding third generation dendrimer. A similar trend is found when the six different guests are compared.     

Internal residue loss produced by rearrangement of a novel cationic glycosphingolipid,glyceroplasmalopsychosine, in collision-induced dissociation

A novel plasmal conjugate of galactosylsphingosine (psychosine), Gro1(3)-O-plasmal-O-6Galbeta-sphingosine (glyceroplasmalopsychosine), was analyzed by electrospray ionization and liquid secondary ion mass spectrometry with low- or high-energy collision-induced dissociation (CID). In the product ion spectra of the [M + H](+) ions, [M + H - glycerol](+) ions arising from the loss of a glycerol were predominant. Unexpectedly, CID of the [M + H - glycerol](+) ion produced an outstanding ion, [(M + H - glycerol) - Hex](+), which required the loss of the galactose from inside the molecule. This ion was greatly reduced in the spectra of N,N-dimethyl derivatives, indicating that the [(M + H - glycerol) - Hex](+) ion is formed from an intramolecular rearrangement with migration of the plasmal residue to the free amino group of sphingosine. It would be expected that the rearrangement occurs simultaneously with the elimination of glycerol or a rearranged [M + H](+) ion leads to the elimination of glycerol, to form a Schiff base-type [M + H - glycerol](+) ion, from which the terminal galactose could be removed by the normal mechanism of glycosidic cleavage. On the other hand, the [M + Na - glycerol](+) ion derived from the sodiated molecule did not produce an ion corresponding to the rearrangement reaction, possibly owing to a higher stability of the sodiated ions against conformational changes.     

Structural characterization of fatty acids cationized with copper by electrospray ionization mass spectrometry under low-energy collision-induced dissociation

Fatty acids have for many years been characterized by mass spectrometry using electron ionization after chemical derivatization. When fatty acids are ionized using desorption/ionization methods such as electrospray ionization or fast atom bombardment, structural information is usually obtained through high-energy collision-induced dissociation (CID) using sector instruments. It has been shown that copper displays very interesting properties in the gas phase during CID. In this study, the reactivity of saturated and unsaturated fatty acid-copper [M-H+Cu(II)]+ complex and the role of the copper ion in promoting fragmentations were investigated under low-energy collisional activation conditions. The decomposition of these species in an ion trap instrument led to diagnostic ion series that reflect C--C bond cleavage, which involves Cu(II) reduction followed by the release of an alkyl radical. It was demonstrated that in this way the localization of one or two homoconjugated double bonds is possible using low-energy CID. Moreover, the distinction of cis and trans isomers is possible through characteristic product ions related to a specific loss of CO2. When these experiments are repeated using a triple-quadrupole instrument with argon as collision gas, a different behavior is observed as in this case, in addition to the product ion distributions observed in the ion trap, other distributions are observed that reflect the influence of the different kinetic shifts and the occurrence of consecutive decompositions. Different examples are presented with various saturated and unsaturated fatty acid chains. Mechanisms are proposed in order to rationalize the experimental observations.