A new copper containing MALDI matrix that yields high abundances of [Peptide + Cu]+ ions

The dinuclear copper complex (α-cyano-4-hydroxycinnamic acid (CHCA) copper salt (CHCA)₄Cu₂), synthesized by reacting CHCA with copper oxide (CuO), yields increased abundances of [M + xCu − (x−1)H]⁺ (x = 1–6) ions when used as a matrix for matrix-assisted laser desorption ionization (355 nm Nd:YAG laser). The yield of [M + xCu − (x−1)H]⁺ (x = 1∼6) ion is much greater than that obtained by mixing peptides with copper salts or directly depositing peptides onto oxidized copper surfaces. The increased ion yields for [M + xCu − (x−1)H]⁺ facilitate studies of biologically important copper binding peptides. For example, using this matrix we have investigated site-specific copper binding of several peptides using fragmentation chemistry of [M + Cu]⁺ and [M + 2Cu − H]⁺ ions. The fragmentation studies reveal interesting insight on Cu binding preferences for basic amino acids. Most notable is the fact that the binding of a single Cu⁺ ion and two Cu⁺ ions are quite different, and these differences are explained in terms of intramolecular interactions of the peptide-Cu ionic complex.     

Studies on phosphatidylserine by tandem quadrupole and multiple stage quadrupole ion-trap mass spectrometry with electrospray ionization: Structural characterization and the fragmentation processes

Low-energy CAD product-ion spectra of various molecular species of phosphatidylserine (PS) in the forms of [M−H]⁻ and [M−2H+Alk]⁻ in the negative-ion mode, as well as in the forms of [M+H]⁺, [M+Alk]⁺, [M−H+2Alk]⁺, and [M−2H+3Alk]⁺ (where Alk=Li, Na) in the positive-ion mode contain rich fragment ions that are applicable for structural determination. Following CAD, the [M−H]⁻ ion of PS undergoes dissociation to eliminate the serine moiety (loss of C₃H₅NO₂) to give a [M−H−87]⁻ ion, which equals to the [M−H]⁻ ion of a phoshatidic acid (PA) and give rise to a MS³-spectrum that is identical to the MS²-spectrum of PA. The major fragmentation process for the [M−2H+Alk]⁻ ion of PS arises from primary loss of 87 to give rise to a [M−2H+Alk−87]⁻ ion, followed by loss of fatty acid substituents as acids (R_xCO₂H, x=1,2) or as alkali salts (e. g., R_xCO₂Li, x=1,2). These fragmentations result in a greater abundance of [M−2H+Alk−87−R₂CO₂H]⁻ than [M−2H+Alk−87−R₁CO₂H]⁻ and a greater abundance of [M−2H+Alk−87−R₂CO₂Li]⁻ than [M−2H+Alk−87−R₁CO₂Li]⁻; while further dissociation of the [M−2H+Alk−87−R_{2(or 1)}CO₂Li]⁻ ions gives a preferential formation of the carboxylate anion at sn-1 (R₁CO₂⁻) over that at sn-2 (R₂CO₂⁻). Other major fragmentation process arises from differential loss of the fatty acid substituents as ketenes (loss of R_x′CH=CO, x=1,2). This results in a more prominent [M−2H+Alk−R₂′CH=CO]⁻ ion than [M−2H+Alk−R₁′CH=CO]⁻ ion. Ions informative for structural characterization of PS are of low abundance in the MS²-spectra of both the [M+H]⁺ and the [M+Alk]⁺ ions, but are abundant in the MS³-spectra. The MS²-spectrum of the [M+Alk]⁺ ion contains a unique ion corresponding to internal loss of a phosphate group probably via the fragmentation processes involving rearrangement steps. The [M−H+2Alk]⁺ ion of PS yields a major [M−H+2Alk−87]⁺ ion, which is equivalent to an alkali adduct ion of a monoalkali salt of PA and gives rise to a greater abundance of [M−H+2Alk−87−R₁CO₂H]⁺ than [M−H+2Alk−87−R₂CO₂H]⁺. Similarly, the [M−2H+3Alk]⁺ ion of PS also yields a prominent [M−2H+3Alk−87]⁺ ion, which undergoes consecutive dissociation processes that involve differential losses of the two fatty acyl substituents. Because all of the above tandem mass spectra contain several sets of ion pairs involving differential losses of the fatty acid substituents as ketenes or as free fatty acids, the identities of the fatty acyl substituents and their positions on the glycerol backbone can be easily assigned by the drastic differences in the abundances of the ions in each pair.     

Atmospheric pressure ionization mass spectrometry techniques for the analysis of alkyl ethoxysulfate mixtures

This paper compares two liquid introduction atmospheric pressure ionization techniques for the analysis of alkyl ethoxysulfate (AES) anionic surfactant mixtures by mass spectrometry, i. e., electrospray ionization (ESI) in both positive and negative ion modes and atmospheric pressure chemical ionization (APCI) in positive ion mode, using a triple quadrupole mass spectrometer. Two ions are observed in ESI(+) for each individual AES component, [M + Na]⁺ and a “desulfated” ion [M − SO₃ + H]⁺, whereas only one ion, [M − Na]⁻ is observed for each AES component in ESI(−). APCI(+) produces a protonated, “desulfated” ion of the form [M − NaSO₃ + 2H]⁺ for each AES species in the mixture under low cone voltage (10 V) conditions. The mass spectral ion intensities of the individual AES components in either the series from ESI(+) or APCI(+) can be used to obtain an estimate of their relative concentrations in the mixture and of the average ethoxylate (EO) number of the sample. The precursor ions produced by either ESI(+) or ESI(−), when subjected to low-energy (50 eV) collision-induced dissociation, do not fragment to give ions that provide much structural information. The protonated, desulfated ions produced by APCI(+) form fragment ions which reveal structural information about the precursor ions, including alkyl chain length and EO number, under similar conditions. APCI(+) is less susceptible to matrix effects for quantitative work than ESI(+). Thus APCI(+) provides an additional tool for the analysis of anionic surfactants such as AES, especially in complex mixtures where tandem mass spectrometry is required for the identification of the individual components.     

Theoretical and experimental study of the structure and stability of multiply Na-substituted glucose and 2,4,6-trihydroxyacetophenone derivatives

The equilibrium geometric parameters and the energetic and spectroscopic characteristics of low lying conformers for series of polyhydroxyl molecules and ions in which sodium atoms are successively substituted for the hydroxyl hydrogen atoms have been calculated by the density functional theory B3LYP method with the 6−31G* and 6−311+G** basis sets. The glucose derivatives [Glu − nH + nNa] and [Glu − nH + (n + 1)Na]⁺ (n = 1−5) and the 2,4,6-trihydroxyacetophenone derivatives [THAP − nH + nNa] and [THAP − nH + (n + 1)Na]⁺ (n = 1−4) have been considered. The affinities of the neutral [Glu − nH + nNa] and [THAP − nH + nNa] molecules for adding Na⁺ cations, as well as the energies of successive substitution of Na atoms for H atoms in the Glu and THAP molecules and the Glu⁺ and THAP⁺ ions in their reaction with sodium acetate molecules, have been estimated. Computations show that the first substitution of Na for H in ions is slightly exothermic and, presumably, can spontaneously occur under common conditions. Further substitutions are endothermic, but the required energy inputs are small. Therefore, successive substitutions for two, three, or more hydroxyl H atoms in the molecules and ions under consideration are possible at relatively low energy inputs. The computation results and conclusions are compared with the MALDI TOF mass spectral data for Na-substituted glucose and 2,4,6-trihydroxyacetophenone derivatives in the [glucose + CH₃COONa + THAP] system where, in addition to common Glu · Na⁺ and THAP · Na⁺ ion-molecular complexes, multiply substituted positive ions of the [Glu − nH + (n + 1)Na]⁺ (n = 1−4) and [THAP − nH + (n + 1)Na]⁺ (n = 1−3) type have been identified.     

Low-temperature activation of nitrogen oxide on Cu-ZSM-5 catalysts

The adsorption and activation of NO molecules on Cu-ZSM-5 catalysts with different Cu/Al and Si/Al ratios (from 0.05 to 1.4 and from 17 to 45, respectively) subjected to different pretreatment was studied by ultraviolet-visible diffuse reflectance (UV-Vis DR). It was found that the amount of chemisorbed NO and the catalyst activity in NO decomposition increased with an increase in the Cu/Al ratio to 0.35–0.40. The intensity of absorption bands at 18400 and 25600 cm⁻¹ in the UV-Vis DR spectra increased symbatically. It was hypothesized that the adsorption of NO occurs at Cu⁺ ions localized in chain copper oxide structures with the formation of mono- and dinitrosyl Cu(I) complexes, and this process is accompanied by the Cu²⁺...Cu⁺ intervalence transfer band in the region of 18400 cm⁻¹. The low-temperature activation of NO occurs through the conversion of the dinitrosyl Cu(I) complex into the π-radical anion (N₂O₂)⁻ stabilized at the Cu²⁺ ion of the chain structure, [Cu²⁺-cis-(N₂O₂)⁻], by electron transfer from the Cu⁺ ion to the cis dimer (NO)₂. This complex corresponds to the L → M charge transfer band in the region of 25600 cm⁻¹. The subsequent destruction of the complex [Cu²⁺-cis-(N₂O₂)⁻] at temperatures of 150–300°C leads to the release of N₂O and the formation of the complex [Cu²⁺O⁻], which further participates in the formation of the nitrite-nitrate complexes [Cu²⁺(NO₂)⁻], [Cu²⁺(NO)(NO₂)⁻], and [Cu²⁺(NO₃)⁻] and NO decomposition products.     

In-source decay and fragmentation characteristics of peptides using 5-aminosalicylic acid as a matrix in matrix-assisted laser desorption/ionization mass spectrometry

The use of 5-aminosalicylic acid (5-ASA) as a new matrix for in-source decay (ISD) of peptides including mono- and di-phosphorylated peptides in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) is described. The use of 5-ASA in MALDI-ISD has been evaluated from several standpoints: hydrogen-donating ability, the outstanding sharpness of molecular and fragment ion peaks, and the presence of interference peaks such as metastable peaks and multiply charged ions. The hydrogen-donating ability of several matrices such as α-cyano-4-hydroxycinnamic acid (CHCA), 2,5-dihydroxybenzoic acid (2,5-DHB), 1,5-diaminonaphthalene (1,5-DAN), sinapinic acid (SA), and 5-ASA was evaluated by using the peak abundance of a reduction product [M + 2H + H]⁺ to that of non-reduced protonated molecule [M + H]⁺ of the cyclic peptide vasopressin which contains a disulfide bond (S-S). The order of hydrogendonating ability was 1,5-DAN > 5-ASA > 2,5-DHB > SA = CHCA. The chemicals 1,5-DAN and 5-ASA in particular can be classified as reductive matrices. 5-ASA gave peaks with higher sharpness for protonated molecules and fragment ions than other matrices and did not give any interference peaks such as multiply-protonated ions and metastable ions in the ISD mass spectra of the peptides used. Particularly, 1,5-DAN and 5-ASA gave very little metastable peaks. This indicates that 1,5-DAN and 5-ASA are more “cool” than other matrices. The 1,5-DAN and 5-ASA can therefore be termed “reductive cool” matrix. Further, it was confirmed that ISD phenomena such as N-Cα bond cleavage and reduction of S-S bond is a single event in the ion source. The characteristic fragmentations, which form a− and (a + 2)-series ions, [M + H − 15]⁺, [M + H − 28]⁺, and [M + H − 44]⁺ ions in the MALDI-ISD are described.     

H/D exchange kinetics: Experimental evidence for formation of different b fragment ion conformers/isomers during the gas-phase peptide sequencing

Electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) combined with H/D exchange reactions was utilized to explore the existence of different b₅⁺ and b₄⁺ fragment ion conformers/isomers of hexapeptide WHWLQL in the gas phase. Distinct H/D exchange trends for protonated WHWLQL ([M + H]⁺) and its b₅⁺ and b₄⁺ fragment ions (with ND₃) were observed. Isolated ¹²C_all isotopomers of both b₅⁺ and b₄⁺ fragment ions yielded bimodal distributions of H/D exchanged product ions. The H/D exchange reaction kinetics also confirmed that b₅⁺ and b₄⁺ fragment ions exist as combination of slow-exchanging (“s”) and fast-exchanging (“f”) species. The calculated rate constant for the first labile hydrogen exchange of [M + H]⁺ (k_{[M + H]} ⁺ = 3.80 ± 0.7 × 10⁻¹⁰ cm³ mol⁻¹ s⁻¹) was ∼30 and ∼5 times greater than those for the “s” and “f” species of b₅⁺, respectively. Data from H/D exchange of isolated “s” species at longer ND₃ reaction times confirmed the existence of different conformers or isomers for b₅⁺ fragment ions. The sustained off-resonance irradiation collision-activated dissociation (SORI-CAD) of WHWLQL combined with the H/D exchange reactions indicate that “s” and “f” species of b₅⁺ and b₄⁺ fragment ions can be produced in the ICR cell as well as the ESI source. The significance of these observations for detailed understanding of protein sequencing and ion fragmentation pathways is discussed.     

Toward total structural analysis of cardiolipins: multiple-stage linear ion-trap mass spectrometry on the [M − 2H + 3Li]<Superscript>+</Superscript> ions

ESI multiple-stage linear ion-trap (LIT) mass spectrometric approaches for a near-complete structural characterization of cardiolipins (CLs), including identification of the fatty acyl substituents, assignment of the fatty acid substituents on the glycerol backbone, and location of the double-bond(s) or cyclopropyl group along the fatty acid chain are described. Upon collisionally activated dissociation (CAD) on the [M − 2H + 3Li]⁺ ions of CL in an ion-trap (MS²), two sets of fragment ions (designated as (a + 136) and (b + 136) ions) analogous to those previously reported for the [M − 2H + 3Na]⁺ ions were observed, leading to assignment of the phosphatidyl moieties attached to 1′- or 3′-position of the central glycerol. Further dissociation of the (a + 136) (or (b + 136)) ions (MS³) gives rise to the (a + 136 − R_{1(or 2)}CO₂Li) (or b + 136 − R_{1(or 2)}CO₂Li) ion pairs that identify the fatty acid moieties and their position on the glycerol backbone. This is followed by MS⁴ on the (a + 136 − R_{1(or 2)}CO₂Li) (or b + 136 − R_{1(or 2)}CO₂Li) ion to eliminate a tricylic glycerophosphate ester residue (136 Da) to yield the (a − R_{1(or 2)}CO₂Li) ion, which is then subjected to MS⁵. The MS5 spectrum contains the structural information that locates the double-bond(s) or cyclopropyl group of the fatty acid substituents. Finally, the subsequent MS⁶ on the dilithiated fatty acid ions generated from MS⁵ also yields feature ions that confirm the assignment.     

Oxidation of Cu(II) aminopolycarboxylates by carbonate radical: A flash photolysis study

Reactions of carbonate radical (Co₃ ⁻) generated by photolysis or by radiolysis of a carbonate solution, with Cu(II) complexes of aminopolycarboxylic acids viz., Cu(II)ethylenediamine tetraacetate [Cu^IIEDTA]²⁻ and Cu(II)-iminodiacetate [Cu^IIIDA] were studied at pH 10. 5 and ionic strength 0.2 mol·dm⁻³. Time-resolved spectroscopy and kinetics for the transients were studied using flash photolysis and stable products arising from the ligand degradation of the complex were ascertained by steady-state radiolysis experiments. From the kinetic data it is observed that CO₃ ⁻, radical reacts initially with Cu^II-complex to form a transient intermediate having maximum absorption at 335 nm and 430 nm. From the subsequent reactions of this intermediate it was assigned to be Cu^III. species. This Cu(III) species undergoes intermolecular electron transfer with the Cu^II-complex to give a radical intermediate which again slowly reacts with Cu^II-complex to give a long lived species containing Cu−C bond. This long lived species, however, slowly decomposed to give glyoxalic reaction between Cu^III-complex and a suitable donor, the one electron reduction potential for [Cu^IIIEDTA]¹⁻/[Cu^IIEDTA]²⁻ and [Cu^IIIIDA]⁺¹/Cu^IIIDA was determined.     

A comparison between positive and negative ion modes in thermospray liquid chromatography-mass spectrometry for the determination of chlorophenols and herbicides

Summary Positive and negative ion modes (Pl and NI, respectively) have been employed for the characterization of 2,4-dichlorophenol, 2,4,5-trichlorophenol, pentachlorophenol, Linuron and Cyanazine in thermospray (TSP) liquid chromatography-mass spectrometry (LC-MS). The PI mode showed no response when 200 ng of the different chlorophenols were injected, while for Linuron and Cyanazine high signals were obtained with [M+NH₄]⁺ and [M+acetic acid]⁺ ions as base peaks, respectively. With the NI mode, the base peak usually corresponds to the [M−H]⁻ ion, with better sensitivities for the chlorophenols than for the herbicides. The chloride adduct [M+Cl]⁻ ion was also obtained for 2,4,5-trichlorophenol and for Linuron. Although the PI mode is more sensitive than the NI mode for the two herbicides, the combination of both ionization modes offers complementary structural information for characterizing such compounds in TSP LC-MS.     

Anion-π interactions—interactions between benzo-crown ether metal cation complexes and counter ions

The loss of X^· radical from [M + Cu + X]⁺ ions (copper reduction) has been studied by the so called in-source fragmentation at higher cone voltage (M = crown ether molecule, X⁻ = counter ion, ClO₄⁻, NO₃⁻, Cl⁻). The loss of X^· has been found to be affected by the presence/lack of aromatic ring poor/rich in electrons. Namely, the loss of X^· occurs with lower efficiency for the [NO₂-B15C5 + Cu + X]⁺ ions than for the [B15C5 + Cu + X]⁺ ions, where NO₂-B15C5 = 3-nitro-benzo-15-crown-5, B15C5 = benzo-15-crown-5. A reasonable explanation is that Anion-π interactions prevent the loss of X^· from the [NO₂-B15C5 + Cu + X]⁺ ions. The presence of the electron-withdrawing NO₂ group causes the aromatic ring to be poor in electrons and thus its enhances its interactions with anions. For the ion containing the aromatic ring enriched in electrons, namely [NH₂-B15C5 + Cu + ClO₄]⁺ where NH₂-B15C5 = 3-amino-benzo-15-crown-5, the opposite situation has been observed. Because of Anion-π repulsion the loss of X^· radical proceeds more readily for [NH₂-B15C5 + Cu + X]⁺ than for [B15C5 + Cu + X]⁺. Iron reduction has also been found to be affected by Anion-π interactions. Namely, the loss of CH₃O^· radical from the ion [B15C5 + Fe + NO₃ + CH₃O]⁺ proceeds more readily than from [NO₂⁻B15C5 + Fe + NO₃ + CH₃O]⁺.     

Studies on phosphatidylglycerol with triple quadrupole tandem mass spectrometry with electrospray ionization: Fragmentation processes and structural characterization

Negative-ion low-energy collisionally activated dissociation (CAD) tandem mass spectrometry of electrospray-produced ions permits structural characterization of phosphatidylglycerol (PG). The major ions that identify the structures arise from neutral loss of free fatty acid substituents ([M − H − R _x CO₂H]⁻) and neutral loss of the fatty acids as ketenes ([M − H − R′ _x CH = C = O]⁻), followed by consecutive loss of the glycerol head group. The abundances of the ions arising from neutral loss of the sn-2 substutient as a free fatty acid ([M − H − R₂CO₂H]⁻) or as a ketene ([M − H − R′₂CH = C = O]⁻) are greater than those of the product ions from the analogous losses at sn-1. Nucleophilic attack of the anionic phosphate site on the C-1 or the C-2 of the glycerol to which the carboxylates attached expels the sn-1 (R₁CO₂⁻) or the sn-2 (R₂CO₂⁻) carboxylate anion, resulting in a greater abundance of R₂COO⁻ than R₁COO⁻. These features permit assignments of fatty acid substituents and their position in the glycerol backbone. The results are also consistent with our earlier findings that pathways leading to those losses at sn-2 are sterically more favorable than those at sn-1. Fragment ions at m/z 227, 209 and 171 reflect the glycerol polar head group and identify the various PG molecules. Both charge-remote fragmentation (CRF) and charge-drive fragmentation (CDF) processes are the major pathways for the formation of [M − H − R _x COOH]⁻ ions. The CRF process involves participation of the hydrogen atoms on the glycerol backbone, whereas the CDF process involves participation of the exchangeable hydrogen atoms of the glycerol head group. The proposed fragmentation pathways are supported by CAD tandem mass spectrometry of the analogous precursor ions arising from the H-D exchange experiment, and further confirmed by source CAD in combination with tandem mass spectrometry.     

Gas-phase nazarov cyclization of protonated 2-methoxy and 2-hydroxychalcone: An example of intramolecular proton-transport catalysis

Upon CA, ESI generated [M + H]⁺ ions of chalcone (benzalacetophenone) and 3-phenyl-indanone both undergo losses of H₂O, CO, and the elements of benzene. CA of the [M + H]⁺ ions of 2-methoxy and 2-hydroxychalcone, however, prompts instead a dominant loss of ketene. In addition, CA of the [M + H]⁺ ions of 2-methoxy-β-methylchalcone produces an analogous loss of methylketene instead. Furthermore, the [M + D]⁺ ion of 2-methoxychalcone upon CA eliminates only unlabeled ketene, and the resultant product, the [M + D − ketene]⁺ ion, yields only the benzyl-d ₁ cation upon CA. We propose that the 2-methoxy and 2-hydroxy (ortho) substituents facilitate a Nazarov cyclization to the corresponding protonated 3-aryl-indanones by mediating a critical proton transfer. The resultant protonated indanones then undergo a second proton transport catalysis facilitated by the same ortho substituents producing intermediates that eliminate ketene to yield 2-methoxy- or 2-hydroxyphenyl-phenyl-methylcarbocations, respectively. The basicity of the ortho substituent is important; for example, replacement of the ortho function with a chloro substituent does not provide an efficient catalyst for the proton transports. The Nazarov cyclization must compete with an alternate cyclization, driven by the protonated carbonyl group of the chalcone that results in losses of H₂O and CO. The assisted proton transfer mediated by the ortho substituent shifts the competition in favor of the Nazarov cyclization. The proposed mechanisms for cyclization and fragmentation are supported by high-mass resolving power data, tandem mass spectra, deuterium labeling, and molecular orbital calculations.     

2-(5-Benzoacridine)ethyl-p-toluenesulfonate as sensitive reagent for the determination of bile acids by HPLC with fluorescence detection and online atmospheric chemical ionization-mass spectrometric identification

2-(5-Benzoacridine)ethyl-p-toluenesulfonate (BAETS), a dual-sensitive probe, was reacted with bile acids in the presence of K₂CO₃ catalyst in dimethyl sulfoxide (DMSO) solvent to give BAETS–bile acid derivatives. Derivatives exhibited intense fluorescence (FL) with an excitation maximum at λ _ex 270 nm and an emission maximum at λ _em 510 nm. MS analysis using APCI-MS indicated that derivatives had excellent APCI-MS ionizability with percentage ionization δ values changing from 0 to 88.83% in aqueous acetonitrile and from 0 to 89.15% in aqueous methanol. The collision induced dissociation spectra of m/z [M + H]⁺ contained specific fragment ions at m/z [M + H−H₂O]⁺, [M + H−2H₂O]⁺, [M + H−3H₂O]⁺, 347.3, and 290.1. Repeatability was good for LC separation of BAETS–bile acid derivatives with aqueous acetonitrile as mobile phase. The relative standard deviations (RSDs) of retention time and peak area at 6.6 nmol mL⁻¹ levels with fluorescence detection (FL) were from 0.045 to 0.072% and from 2.16 to 2.73%, respectively. Excellent linear responses were observed, with regression coefficients >0.9995. The FL detection limits (S/N = 3) were in the range of 18.0–36.1 fmol. The online APCI-MS detection limits are in the range of 500–790 fmol (at a signal-to-noise ratio of 3).     

Isobutane chemical ionization mass spectra of unsaturated alcohols

Analysis of the isobutane chemical ionization mass spectra of hexenols, cyclohexenols and various syn/anti pairs of bicyclic and tricyclic homoallylic alcohols shows that: (i) the spectra of the allylic alcohols are dominated by [M + H – H₂O]⁺ and [M + C₄H₉–H₂O]⁺ ions and contain traces of [M + H]⁺ ions; (ii) [M + H]⁺ ions are prominent in the spectra of acyclic and certain cyclic homoallylic alcohols; and (iii) [M + H]⁺ ions dominate the spectra of other acyclic unsaturated alcohols. The [M + H]⁺ ions may result from either: (a) protonation of the hydroxyl group, followed by a very rapid intramolecular proton transfer from the protonated hydroxyl group to the carbon–carbon double bond or internal solvation of the protonated hydroxyl group by the carbon–carbon double bond; and/or (b) direct protonation of the carbon–carbon double bond with significant internal solvation of the resulting carbocation by the hydroxyl group, which may lead to carbon–oxygen bond formation to give a protonated cyclic ether. The consequences of placing various geometric constraints on the possible intramolecular interactions between the hydroxyl group and the carbon–carbon double bond in unsaturated alcohols are explored.     

Electrospray ionization multiple-stage linear ion-trap mass spectrometry for structural elucidation of triacylglycerols: Assignment of fatty acyl groups on the glycerol backbone and location of double bonds

Linear ion-trap multiple-stage mass spectrometric approach (MS ⁿ ) towards nearly complete structural elucidation of triacylglycerol (TAG) including (1) assignment the fatty acid substituents on the glycerol backbone and (2) location of the double bond(s) on the unsaturated fatty acyl groups is reported. The characterization is established by the findings that MS² on the [M+Li]⁺ ions of TAG yields more abundant ions reflecting losses of the outer fatty acid substituents either as free acids (i.e., [M+Li-R₁CO₂H]⁺ and [M+Li-R₃CO₂H]⁺ ions) or as lithium salts (i.e., [M+Li-R₁CO₂Li]⁺ and [M+Li-R₃CO₂Li]⁺ ions) than the ions reflecting the similar losses of the inner fatty acid substituent (i.e., [M+Li-R₂CO₂Li]⁺ and [M+Li-R₂CO₂Li]⁺ ions). Further dissociation (MS³) of [M+Li-R _n CO₂H]⁺ (n=1, 2, or 3) gives rise to the ion series locating the double bonds along the fatty acid chain. These ions arise from charge-remote fragmentations involving β-cleavage with γ-H shift, analogous to those seen for the unsaturated long-chain fatty acids characterized as initiated ions. Significant differences in abundances in the ion pairs reflecting the additional losses of the fatty acid moieties, respectively, were also seen in the MS³ spectra of the [M+Li-R _n CO₂H]⁺ and [M+Li-R _n CO₂Li]⁺ ions, leading to confirmation of the fatty acid substituents on the glycerol backbone. MS ⁿ on the [M+Na]⁺ and [M+NH₄]⁺ adduct ions also affords location of fatty acid substituents on the glycerol backbone, but not the position of the double bond(s) along the fatty acid chain. Unique ions from internal losses of the glycerol residues were seen in the MS³ spectra of [M+Alk-R _n CO₂H]⁺ (n=1, 2, 3) and of [M+Alk-R _n CO₂Alk]⁺ (Alk=Li, Na, NH₄; n=1, 3). They are signature ions for glycerides and the pathways leading to their formation may involve rearrangements.     

Intriguing Differences in the Gas-Phase Dissociation Behavior of Protonated and Deprotonated Gonyautoxin Epimers

The aim of this study was to investigate the unusual gas-phase dissociation behavior of two epimer pairs of protonated gonyautoxins (GTX) following electrospray ionization in comparison to their deprotonated counterparts. The chemical structures of the investigated GTX1-4 variants vary in their substitution pattern at N-1 and the stereochemical orientation of the hydroxysulfate group at C-11 (11α for GTX1/2 versus 11β for GTX3/4). The direct comparison of mass spectra in positive and negative ion modes illustrated two distinct features: first, an intriguing difference between protonated 11α and 11β species, where 11α conformations exhibited almost complete dissociation of [M + H]⁺ ions via facile SO₃ elimination, while 11β species remained mostly intact as [M + H]⁺; and second, the lack of such differences for the deprotonated counterparts. In this study, we propose an acid-catalyzed elimination mechanism from density functional theory calculations, initiated by a proton transfer of a guanidinium proton to the hydroxysulfate group with simultaneous SO₃ release, which is only possible for the 11α conformation based on intramolecular distances. The same mechanism explains the lack of a comparable SO₃ loss in the negative ion mode. CID experiments supported this proposed mechanism for GTX1 and GTX2. Computational modeling of product ions seen in the CID spectra of GTX3 and GTX4 established that the lowest energy dissociation pathway for the 11β epimers is elimination of water with the possibility for further SO₃ release from the intermediate product. Experimental data for structurally analogous decarbamoyl gonyautoxins confirmed the evidence for the GTX compounds as well as the proposed elimination mechanisms.     

Rate and extent of gas-phase hydrogen/deuterium exchange of bradykinins: evidence for peptide zwitterions in the gas phase

The gas-phase H/D exchange of bradykinin [M + H]⁺, [M + Na]⁺, [M + 2H]²⁺, and [M + H + Na]²⁺ ions; des-Arg¹-bradykinin, des-Arg⁹-bradykinin, and bradykinin fragment 2-7 [M + H]⁺ ions; and O-methylbradykinin [M + H]⁺ and [M + 2H]²⁺ ions with D₂O have been examined by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry at 9.4 T. The different peptides vary widely in exchange rate and extent of deuterium incorporation. H/D exchange was slowest and deuterium incorporation was least for bradykinin [M + H]⁺, [M + H + Na]²⁺ and bradykinin methyl ester [M + 2H]²⁺ ions. In contrast, H/D exchange and extent of deuteration are higher for des-Arg¹-bradykinin, des-Arg⁹-bradykinin, and bradykinin fragment 2-7 [M + H]⁺ ions; and highest for bradykinin [M + Na]⁺ and [M + 2H]²⁺, and O-methylbradykinin [M + H]⁺. Because the most likely site of protonation is the guanidino group of arginine, the above reactivity pattern strongly supports a zwitterion form for protonated gas-phase bradykinin.     

A mass spectrometry study of alkanes in air plasma at atmospheric pressure

The positive APCI-mass spectra in air of linear (n-pentane, n-hexane, n-heptane, n-octane), branched [2,4-dimethylpentane, 2,2-dimethylpentane and 2,2,4-trimethylpentane (i-octane)], and cyclic (cyclohexane) alkanes were analyzed at different mixing ratios and temperatures. The effect of air humidity was also investigated. Complex ion chemistry is observed as a result of the interplay of several different reagent ions, including atmospheric ions O₂^+•, NO⁺, H₃O⁺, and their hydrates, but also alkyl fragment ions derived from the alkanes. Some of these reactions are known from previous selected ion/molecule reaction studies; others are so far unreported. The major ion formed from most alkanes (M) is the species [M − H]⁺, which is accompanied by M^+• only in the case of n-octane. Ionic fragments of C _n H_2n ^+1/+ composition are also observed, particularly with branched alkanes: the relative abundance of such fragments with respect to that of [M − H]⁺ decreases with increasing concentration of M, thus suggesting that they react with M via hydride abstraction. The branched C₇ and C₈ alkanes react with NO⁺ to form a C₄H₁₀NO⁺ ion product, which upon collisional activation dissociates via HNO elimination. The structure of t-Bu⁺(HNO) is proposed for such species, which is reasonably formed from the original NO⁺(M) ion/molecule complex via hydride transfer and olefin elimination. Finally, linear alkanes C₅–C₈ give a product ion corresponding to C₄H₇⁺(M), which we suggest is attributed to addition of [M − H]⁺ to C₄H₈ olefin formed in the charge-transfer-induced fragmentation of M. The results are relevant to applications of nonthermal plasma processes in the fields of air depuration and combustion enhancement.     

Fragmentation of protonated O,O-diethyl O-aryl phosphorothionates in tandem mass spectral analysis

The gas-phase ion chemistry of protonated O,O-diethyl O-aryl phosphorothionates was studied with tandem mass spectrometric and ab initio theoretical methods. Collision-activated dissociation (CAD) experiments were performed for the [M+H]⁺ ions on a triple quadrupole mass spectrometer. Various amounts of internal energy were deposited into the ions upon CAD by variation of the collision energy and collision gas pressure. In addition to isobutane, deuterated isobutane C₄D₁₀ also was used as reagent gas in chemical ionization. The daughter ions [M+H?C₂H₄]⁺ and [M+H?2C₂H₄]⁺ dominate the CAD spectra. These fragments arise via various pathways, each of which involves γ-proton migration. Formation of the terminal ions [M+H?2C₂H₄?H₂O]⁺, [M+H?2C₂H₄?H₂S]⁺, [ZPhOH₂]⁺, [ZPhSH₂]⁺, and [ZPhS]⁺ [Z = substituent(s) on the benzene ring] suggests that (1) the fragmenting [M+H]⁺ ions of O,O-diethyl O-aryl phosphorothionates have protons attached on the oxygen of an ethoxy group and on the oxygen of the phenoxy group; (2) thiono-thiolo rearrangement by aryl migration to sulfur occurs; (3) the fragmenting rear-ranged [M+H]⁺ ions have protons attached on the oxygen of an ethoxy group and on the sulfur of the thiophenoxy group. To get additional support for our interpretation of the mass spectrometric results, some characteristics of three protomers of O,O-diethyl O-phenyl phosphorothionate were investigated by carrying out ab initio molecular orbital calculations at the RHF/3–21G* level of theory.