A comparative study of in- and post-source decays of peptide and preformed ions in matrix-assisted laser desorption ionization time-of-flight mass spectrometry: Effective temperature and matrix effect

In-source decay (ISD) and post-source decay (PSD) of a peptide ion ([Y₆ + H]⁺) and a preformed ion (benzyltriphenylphosphonium, BTPP) generated by matrix-assisted laser desorption ionization (MALDI) were investigated with time-of-flight mass spectrometry. α-Cyano-4-hydroxycinammic acid (CHCA) and 2,5-dihydroxybenzoic acid (DHB) were used as matrices. For both ions, ISD yield was unaffected by delay time, indicating rapid termination of ISD. This was taken as evidence for rapid expansion cooling of hot “early” plume formed in MALDI. CHCA was hotter than DHB for [Y₆ + H]⁺ while the matrix effect was insignificant for BTPP. The “early” plume temperature estimated utilizing previous kinetic results was 800–900 K, versus 400–500 K for “late” plume. The results support our previous finding that the temperature of peptide ions interrogated by tandem mass spectrometry was lower than most rough estimates of MALDI temperature.     

Characterization of synthetic N-acetylcysteine conjugates by positive- and negative-ion 252Cf plasma desorption mass spectrometry

N-Acetylcysteine and nine N-acetylcysteine conjugates of synthetic origin were characterized by positive- and negative-ion plasma desorption mass Spectrometry. For sample preparation the electrospray technique and the nitrocellulose spin deposition technique were applied. The fragmentation of these compounds, which are best seen as S-substituted desaminoglycylcysteine dipeptides, shows a similar behaviour to that of linear peptides. In the positive-ion mass spectra intense protonated molecular ion peaks are observed. In addition, several sequence-specific fragment ions (A⁺, B⁺, [Y + 2H]⁺, Z⁺), immonium ions (I⁺) and a diagnostic fragment ion for mercap-turic acids (R_M⁺) are detected. The negative-ion mass spectra exhibit deprotonated molecular ions and in contrast only one fragment ion corresponding to side-chain specific cleavage ([R_XS]^?) representing the xenobiotic moiety. In the case of a low alkali metal concentration on the target, cluster molecular ions of the [nM + H]⁺ or [nM - H]^? ion type (n = 1-3) are observed. The analysis of an equimolar mixture of eight N-acetylcysteine conjugates shows different quasi-molecular ion yields for the positive- and negative-ion spectra.     

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.     

The stereochemistry of Tutton's salts X2[M(H2O)6](YO4)2

Neutron structure determinations have been made of Tutton's salts, X₂[M(H₂O)₆] (YO₄)₂, where Y = Se, X = K⁺, M = Cu²⁺; Y = S, X = K⁺, M = Ni²⁺, Cu²⁺, Zn²⁺; X = Rb⁺, Cs⁺, M = Cu²⁺. This work has shown that there are extensive hydrogen networks with almost linear hydrogen bonds from [M(H₂O)₆]²⁺ to (YO₄)^2?. The (H … O) distance increases in the Cu²⁺ series for X = K⁺ to Cs⁺ but there is no difference for the potassium copper salts when Y = Se or S. Three different distorted [M(H₂O)₆]²⁺ octahedra were found in the series (orthorhombic, tetragonal with two long and four short, or four long and two short bonds). The interatomic distances from X⁺ to the neighboring O in a distorted XO₈⁺ dodecahedron increases with increased cation size, implying that the X⁺ polyhedron is maintaining its shape.     

Mass spectra of sulfinamide derivatives and identification of their thermal decomposition products

The mass spectra of 30 sulfinamide derivatives (RSONHR', R' alkyl or p-XC₆H₄) are reported. Most of the spectra had peaks attributable to thermal decomposition products. For some compounds these were identified by pyrolysis under similar conditions to be: RSO₂NHR', RSO₂SR, RSSR and NH₂R' (in all kinds of sulfinyl amides); RSNHR' (in the case of arylsulfinyl arylamides); RSO₂C₆H₄NH₂, RSOC₆H₄NH₂ and RSC₆H₄NH₂ (in the case of arylsulfinyl arylamides of the type of X = H) The mass spectra of the three thermally stable compounds showed that there are several kinds of common fragment ions. The mass spectra of the thermally labile compounds had two groups of ions; (i) characteristic fragment ions of the intact molecules and (ii) the molecular ions of the thermal decomposition products. It was concluded that the sulfinamides give the following ions after electron impact: [M]⁺, [M ? R]⁺, [M ? R + H]⁺, [M ? SO]⁺, [RS]⁺, [NHR']⁺, [NHR' + H]⁺, [RSO]⁺, [RSO + H]⁺, [R]⁺, [R + H]⁺, [R']⁺ and [M ? OH]⁺, and that the thermal decomposition products give the following ions: [RSO₂SR]⁺, [RSSR]⁺, [M ? O]⁺, [M + O]⁺ and [RSOC₆H₄NH₂]⁺.     

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.     

A mechanistic study of the H/D exchange reactions of protonated arginine and arginine-containing Di- and tripeptides

The gas-phase H/D exchange reactions of arginine (R) and arginine-containing di- and tri-peptide (gly-arg (GR), arg-gly (RG), gly-gly-arg (GGR), gly-arg-gly (GRG) and arg-gly-gly (RGG)) [M+H]⁺ ions with deuterated ammonia (ND₃) were investigated by using Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR), ion mobility-mass spectrometry (IM-MS), ab initio and density functional theory-based molecular orbital calculations and molecular modeling. Three exchanges are observed for arginine and arginine-containing tri-peptide [M+H]⁺ ions, whereas the di-peptide [M+H]⁺ ions undergo a single H/D exchange. In addition, C-terminal methylation blocks H/D exchange of arginine and the arginine-containing peptide [M+H]⁺ ions, and a single H/D exchange is observed for N-terminal acetylated arginine [M+H]⁺ ions. A general mechanism for H/D exchange involving a collision complex that is best described as a “solvated salt-bridge” structure is proposed.     

Fast atom bombardment mass spectra of telluronium salts

The fast atom bombardment (FAB) mass spectra of telluronium salts were studied. The spectra exhibit the intact cation (C⁺) and cluster ions ([M + C]⁺). The principal fragment ions in the FAB mass spectra of telluronium salts are [RTe]⁺, [R₂Te]⁺˙, [R₂Te − H]⁺, [RTeR′]⁺˙, and [RTeR′ + H]⁺. When the anion was [BPh₄]⁻, interesting cluster ions such as [M + C − BPh₃]⁺ appeared.     

Ions Can Move a Proton‐Coupled Electron‐Transfer Reaction into Tunneling Regime

The effects of charged species on proton‐coupled electron‐transfer (PCET) reaction should be of significance for understanding/application of important chemical and biological PCET systems. Such species can be found in proximity of activated complex in a PCET reaction, although they are not involved in the charge transfer process. Reported here is the first study of the above‐mentioned effects. Here, the effects of Na⁺, K⁺, Li⁺, Ca²⁺, Mg²⁺, and Me₄N⁺ observed in PCET reaction of ascorbate monoanions with hexacyanoferrate(III) ions in H₂O reveal that, in presence of ions, this over‐the‐barrier reaction entered into tunneling regime. The observations are: a) dependence of the rate constant on the cation concentration, where the rate constant is 71 (at I = 0.0023), and 821 (at 0.5M K⁺), 847 (at 1.0M Na⁺), and 438 M ^?1 s^?1 (at 0.011M Ca²⁺); b) changes of kinetic isotope effect (KIE) in the presence of ions, where k_H/k_D=4.6 (at I = 0.0023), and 3.4 (in the presence of 0.5M K⁺), 3.3 (at 1.0M Na⁺), 3.9 (at 0.001M Ca²⁺), and 3.9 (at 0.001M Mg²⁺), respectively; c) the isotope effects on Arrhenius pre‐factor where A_H/A_D=0.97 (0.15) in absence of ions, and 2.29 (0.60) (at 0.5M Na⁺), 1.77 (0.29) (at 1.0M Na⁺), 1.61 (0.25) (at 0.5M K⁺), 0.42 (0.16) (at 0.001M Ca²⁺) and 0.16 (0.19) (at 0.001M Mg²⁺); d) isotope differences in the enthalpies of activation in H₂O and in D₂O, where ΔΔH^?(D,H)=3.9 (0.4) kJ mol^?1 in the absence of cations, 1.3 (0.6) at 0.5M Na⁺, 1.8 (0.4) at 0.5M K⁺, 1.5 (0.4) at 1.0M Na⁺, 5.5 (0.9) (at 0.001M Ca²⁺), and 7.9 (2.8) (at 0.001M Mg²⁺) kJ mol^?1; e) nonlinear proton inventory in reaction. In the H₂O/dioxane 1 : 1, the observed KIE is 7.8 and 4.4 in the absence and in the presence of 0.1M K⁺, respectively, and A_H/A_D=0.14 (0.03). The changes when cations are present in the reaction are explained in terms of termolecular encounter complex consisting of redox partners, and the cation where the cation can be found in a near proximity of the reaction‐activated complex thus influencing the proton/electron double tunneling event in the PCET process. A molecule of H₂O is involved in the transition state. The resulting ‘configuration’ is more ‘rigid’ and more appropriate for efficient tunneling with Na⁺ or K⁺ (extensive tunneling observed), i.e., there is more precise organized H transfer coordinate than in the case of Ca²⁺ and Mg²⁺ (moderate tunneling observed) in the reaction.     

Estimation of peptide N–Cα bond cleavage efficiency during MALDI‐ISD using a cyclic peptide

Matrix‐assisted laser desorption/ionization in‐source decay (MALDI‐ISD) induces N–C_α bond cleavage via hydrogen transfer from the matrix to the peptide backbone, which produces a c′/z? fragment pair. Subsequently, the z? generates z′ and [z + matrix] fragments via further radical reactions because of the low stability of the z?. In the present study, we investigated MALDI‐ISD of a cyclic peptide. The N–C_α bond cleavage in the cyclic peptide by MALDI‐ISD produced the hydrogen‐abundant peptide radical [M + 2H]⁺? with a radical site on the α‐carbon atom, which then reacted with the matrix to give [M + 3H]⁺ and [M + H + matrix]⁺. For 1,5‐diaminonaphthalene (1,5‐DAN) adducts with z fragments, post‐source decay of [M + H + 1,5‐DAN]⁺ generated from the cyclic peptide showed predominant loss of an amino acid with 1,5‐DAN. Additionally, MALDI‐ISD with Fourier transform‐ion cyclotron resonance mass spectrometry allowed for the detection of both [M + 3H]⁺ and [M + H]⁺ with two ¹³C atoms. These results strongly suggested that [M + 3H]⁺ and [M + H + 1,5‐DAN]⁺ were formed by N–C_α bond cleavage with further radical reactions. As a consequence, the cleavage efficiency of the N–C_α bond during MALDI‐ISD could be estimated by the ratio of the intensity of [M + H]⁺ and [M + 3H]⁺ in the Fourier transform‐ion cyclotron resonance spectrum. Because the reduction efficiency of a matrix for the cyclic peptide cyclo(Arg‐Gly‐Asp‐D‐Phe‐Val) was correlated to its tendency to cleave the N–C_α bond in linear peptides, the present method could allow the evaluation of the efficiency of N–C_α bond cleavage for MALDI matrix development. Copyright © 2016 John Wiley & Sons, Ltd.     

Positive chemical ionization triple‐quadrupole mass spectrometry and ab initio computational studies of the multi‐pathway fragmentation of phthalates

We report the first positive chemical ionization (PCI) fragmentation mechanisms of phthalates using triple‐quadrupole mass spectrometry and ab initio computational studies using density functional theories (DFT). Methane PCI spectra showed abundant [M + H]⁺, together with [M + C₂H₅]⁺ and [M + C₃H₅]⁺. Fragmentation of [M + H]⁺, [M + C₂H₅]⁺ and [M + C₃H₅]⁺ involved characteristic ions at m/z 149, 177 and 189, assigned as protonated phthalic anhydride and an adduct of phthalic anhydride with C₂H₅⁺ and C₃H₅⁺, respectively. Fragmentation of these ions provided more structural information from the PCI spectra. A multi‐pathway fragmentation was proposed for these ions leading to the protonated phthalic anhydride. DFT methods were used to calculate relative free energies and to determine structures of intermediate ions for these pathways. The first step of the fragmentation of [M + C₂H₅]⁺ and [M + C₃H₅]⁺ is the elimination of [R? H] from an ester group. The second ester group undergoes either a McLafferty rearrangement route or a neutral loss elimination of ROH. DFT calculations (B3LYP, B3PW91 and BPW91) using 6‐311G(d,p) basis sets showed that McLafferty rearrangement of dibutyl, di(‐n‐octyl) and di(2‐ethyl‐n‐hexyl) phthalates is an energetically more favorable pathway than loss of an alcohol moiety. Prominent ions in these pathways were confirmed with deuterium labeled phthalates. Copyright © 2010 John Wiley & Sons, Ltd.     

Functionalized Fluorophosphonium Ions

Efforts to prepare an elusive donor-free phosphenium ion, [R₂P]⁺, led us to synthesize functionalized fluorophosphonium cations of the type [R₂P(F)X]⁺ (X=SiEt₃, H, F), which were obtained from the related neutral fluorophosphines R₂PF and R₂PF₃ upon protonation and reaction with solvated [Et₃Si]⁺ ions (R=2,6-Mes₂C₆H₃). The hypothetical reductive elimination of [R₂P(F)SiEt₃]⁺ and [R₂P(F)H]⁺ affording [R₂P]⁺, Et₃SiF and HF, respectively, was calculated to be endothermic by 40.1 and 190.6 kJ mol⁻¹.     

Ion–molecule reaction in the gas phase 3—nucleophilic substitution of 17ξ-R-5α,14β-androstane-14,17ξ-diols under [NH4]+ chemical ionization conditions1

Under Ammonia chemical Ionization conditions the source decompositions of [M + NH₄]⁺ ions formed from epimeric tertiary steroid alchols 14 OH_β, 17OH_α or 17 OH_β substituted at position 17 have been studied. They give rise to formation of [M + NH₄? H₂O]⁺ dentoed as [MH_sH]⁺, [M_sH? H₂O]⁺, [M_sH? NH₃]⁺ and [M_sH? NH₃? H₂O]⁺ ions. Stereochemical effects are observed in the ratios [M_sH? H₂O]⁺/[M_sH? NH₃]⁺. These effects are significant among metastable ions. In particular, only the [M_sH]⁺ ions produced from trans-diol isomers lose a water molecule. The favoured loss of water can be accounted for by an S_N2 mechanism in which the insertion of NH₃ gives [M_sH]⁺ with Walden inversion occurring during the ion-molecule reaction between [M + NH₄]⁺ + NH₃. The S_N1 and S_Ni pathways have been rejected.     

HPLC‐DAD and HPLC‐ESI‐MS separation,determination and identification of the spin‐labeled diastereoisomers of podophyllotoxin

Spin‐labeled nitroxide derivatives of podophyllotoxin had better antitumor activity and less toxicity than that of the parent compounds. However, the 2‐H configurations of these spin‐labeled derivatives cannot be determined by nuclear magnetic resonance (NMR) methods. In the present paper, a high‐performance liquid chromatography‐diode array detection (HPLC‐DAD) and a high‐performance liquid chromatography‐electrospray ionization tandem mass spectrometry (HPLC‐ESI/MS/MS) method were developed and validated for the separation, identification of four pairs of diastereoisomers of spin‐labeled derivatives of podophyllotoxin at C‐2 position. In the HPLC‐ESI/MS spectra, each pair of diastereoisomers of the spin‐labeled derivatives in the mixture was directly confirmed and identified by [M+H]⁺ ions and ion ratios of relative abundance of [M‐ROH+H]⁺ (ion 397) to [M+H]⁺. When the [M‐ROH+H]⁺ ions (at m/z 397) were selected as the precursor ions to perform the MS/MS product ion scan. The product ions at m/z 313, 282, and 229 were the common diagnostic ions. The ion ratios of relative abundance of the [M‐ROH+H]⁺ (ion 397) to [M+H]⁺, [A+H]⁺ (ion 313) to [M‐ROH+H]⁺, [A+H‐OCH₃]⁺ (ion 282) to [M‐ROH+H]⁺ and [M‐ROH‐ArH+H]⁺ (ion 229) to [M‐ROH+H]⁺ of each pair of diastereoisomers of the derivatives specifically exhibited a stereochemical effect. Thus, by using identical chromatographic conditions, the combination of DAD and MS/MS data permitted the separation and identification of the four pairs of diastereoisomers of spin‐labeled derivatives of podophyllotoxin at C‐2 in the mixture.     

Mass spectrometric monitoring of oxidation of aliphatic C6–C8 hydrocarbons and ethanol in low pressure oxygen and air plasmas

Experimental and theoretical studies on the oxidation of saturated hydrocarbons (n‐hexane, cyclohexane, n‐heptane, n‐octane and isooctane) and ethanol in 28 Torr O₂ or air plasma generated by a hollow cathode discharge ion source were made. Ions corresponding to [M + 15]⁺ and [M + 13]⁺ in addition to [M ? H]⁺ and [M ? 3H]⁺ were detected as major ions where M is the sample molecule. The ions [M + 15]⁺ and [M + 13]⁺ were assigned as oxidation products, [M ? H + O]⁺ and [M ? 3H + O]⁺, respectively. By the tandem mass spectrometry analysis of [M ? H + O]⁺ and [M ? 3H + O]⁺, H₂O, olefins (and/or cycloalkanes) and oxygen‐containing compounds were eliminated from these ions. Ozone as one of the terminal products in the O₂ plasma was postulated as the oxidizing reagent. As an example, the reactions of C₆H₁₄^+? with O₂ and of C₆H₁₃⁺ (CH₃CH₂CH⁺CH₂CH₂CH₃) with ozone were examined by density functional theory calculations. Nucleophilic interaction of ozone with C₆H₁₃⁺ leads to the formation of protonated ketone, CH₃CH₂C(=OH⁺)CH₂CH₂CH₃. In air plasma, [M ? H + O]⁺ became predominant over carbocations, [M ? H]⁺ and [M ? 3H]⁺. For ethanol, the protonated acetic acid CH₃C(OH)₂⁺ (m/z 61.03) was formed as the oxidation product. The peaks at m/z 75.04 and 75.08 are assigned as protonated ethyl formate and protonated diethyl ether, respectively, and that at m/z 89.06 as protonated ethyl acetate. Copyright © 2016 John Wiley & Sons, Ltd.     

Atmospheric pressure chemical ionization and fragmentation of aminomonosaccharides in H2O and D2O

Aminomonosaccharides (glucosamine, galactosamine, and mannosamine) in H₂O and D₂O were ionized by atmospheric pressure chemical ionization (APCI) and their fragmentation patterns were investigated to identify them. All the aminomonosaccharides showed the same fragment ions but their relative ion intensities were different. Major product ions generated in H₂O were [M + H]⁺, [M + H – H₂O]⁺, and [2M + H – 3H₂O]⁺, while in D₂O were [M_D6 + D]⁺, [M_D6 + D – D₂O]⁺, and [2M_D6 + D – D₂O – 2HDO]⁺. At a high fragmentor voltage above 120 V, the relative ion intensities of the major product ions showed different trends according to the aminomonosaccharides. For the use of H₂O as solvent and eluent, the order of the ion intensity ratio of [M + H – H₂O]⁺/[2M + H – 3H₂O]⁺ was galactosamine > mannosamine > glucosamine. When using D₂O as solvent and eluent, the order of the ion intensity ratios of [M_D6 + D – D₂O]⁺/[M_D6 + D]⁺ and [2M_D6 + D – D₂O – 2HDO]⁺/[M_D6 + D]⁺ was mannosamine > galactosamine > glucosamine. It was found that glucosamine, galactosamine, and mannosamine could be distinguished by the specific trends of the major product ion ratios in H₂O and D₂O. Copyright © 2009 John Wiley & Sons, Ltd.     

Mass spectrometric behavior of anabolic androgenic steroids using gas chromatography coupled to atmospheric pressure chemical ionization source. Part I: Ionization

The detection of anabolic androgenic steroids (AAS) is one of the most important topics in doping control analysis. Gas chromatography coupled to (tandem) mass spectrometry (GC–MS(/MS)) with electron ionization and liquid chromatography coupled to tandem mass spectrometry have been traditionally applied for this purpose. However, both approaches still have important limitations, and, therefore, detection of all AAS is currently afforded by the combination of these strategies. Alternative ionization techniques can minimize these drawbacks and help in the implementation of a single method for the detection of AAS. In the present work, a new atmospheric pressure chemical ionization (APCI) source commercialized for gas chromatography coupled to a quadrupole time‐of‐flight analyzer has been tested to evaluate the ionization of 60 model AAS. Underivatized and trimethylsylil (TMS)‐derivatized compounds have been investigated. The use of GC–APCI–MS allowed for the ionization of all AAS assayed irrespective of their structure. The presence of water in the source as modifier promoted the formation of protonated molecules ([M+H]⁺), becoming the base peak of the spectrum for the majority of studied compounds. Under these conditions, [M+H]⁺, [M+H‐H₂O]⁺ and [M+H‐2·H₂O]⁺ for underivatized AAS and [M+H]⁺, [M+H‐TMSOH]⁺ and [M+H‐2·TMSOH]⁺ for TMS‐derivatized AAS were observed as main ions in the spectra. The formed ions preserve the intact steroid skeleton, and, therefore, they might be used as specific precursors in MS/MS‐based methods. Additionally, a relationship between the relative abundance of these ions and the AAS structure has been established. This relationship might be useful in the structural elucidation of unknown metabolites. Copyright © 2014 John Wiley & Sons, Ltd.     

A study of metal chelation of dinucleotide analogs in the gas phase by fast-atom bombardment mass spectrometry

Fast-atom bombardment (FAB) mass spectrometry was used to investigate the interaction of proton and alkali metal ions with dinucleotide analogs such as T-n-T (T = thymine moiety, n = polyether chain, e.g., triethylene, tetraethylene, pentaethylene, and hexaethylene ether 1–4), A-n-T (A = adenine unit 5–8), and T-n-OMe (9–12) in 3-nitrobenzyl alcohol matrix. The [M + H]⁺ ion is the most abundant ion for the A-n-T series, whereas in 1–4 and 9–12 the (TC₂H₄)⁺ ion is the most abundant. Formation of [M + H -C₂H₄O]⁺ ions, a characteristic fragmentation of crown ethers under electron ionization, is observed for compounds 1–12 and is more pronounced in 6 and 7. An abundant [M ? H]^? ion is observed for all the compounds studied under negative ion FAB due to the presence of the (-CO-NH-CO-) group of thymine, an indication of existence of intramolecular H bonding. The FAB mass spectra of 1–12 with alkali metal ions (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) showed formation of abundant metal-coordinated ions ([M + Met]⁺ and [TC₂H₄ + Met]⁺). Compounds 3, 4, 6, 7, and 10–12 showed ions due to the substitution of the thymine moiety by a hydroxyl group ([M + Met ? 108]⁺, Met = metal ion). For compound 3 alone, substitution of two thymine groups ([M + Met - 216]⁺) was observed. Metastable ion studies were used to elucidate the structures of these potentially significant ions, and the ion formule were confirmed with high resolution measurements. Selectivity toward metal complexation with ligand size was seen in the T-n-T and A-n-T series and was even more pronounced in A-n-T series. These dinucleotide analogs fall in the following order of chelation of alkali metal ions, acyclic glymes < dinucleotide analogs (acyclic glymes substituted with nitrogen bases) < crown ethers, which places them in perspective as receptor models.     

Influence of the Coordination of Donor Solvent Molecules to 1,4,7,10-Tetramethyl-1,4,7,10- tetraazacyclododecanenickel(II) and Analogous Nickel(II) Complexes on Their Steric Factors

Coordination equilibrium constants (K _NiS) of some donor solvent molecules to 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecanenickel(II) ([Ni(Me₄[12]aneN₄)]²⁺) were determined in nitrobenzene (a noncoordinating bulk solvent). The first (K _NiS1) and second stepwise coordination equilibrium constants (K _NiS2) for 1,4,7,10-tetraazacyclododecanenickel(II) ([Ni([12]aneN₄)]²⁺), 1,4,8,11-tetraazac yclotetradecane- nickel(II) ([Ni([14] aneN₄)]²⁺), 1,4,8,11-tetrathiacyclotetra-decanenickel(II) ([Ni([14]aneS₄)]²⁺) were also reinvestigated. The K _NiS values for [Ni(Me₄[12]aneN₄)]²⁺ were compared to those of [Ni([12]aneN₄)]²⁺, (1R,4S, 8R,11S)-1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecanenickel(II) (R,S,R,S-[Ni(Me₄[14]aneN₄)]²⁺), R,R,S,S-[Ni(Me₄[14]aneN₄)]²⁺, [Ni([14]aneN₄)]²⁺, and [Ni([14]aneS₄)]²⁺. Coordination of pyridine (Py), N,N,N′,N′-tetramethylurea (TMU), and N,N-dimethylacetamide (DMA) to [Ni(Me₄[12]aneN₄)]²⁺ was observed, although these donor solvent molecules did not coordinate to R,S,R,S-[Ni(Me₄[14]aneN₄)]²⁺. The K _NiS values for Py, TMU, and DMA are 7.9, 2.8, and 9.0 dm³⋅mol⁻¹, respectively. Some hydrogen-bonding waters were coordinated to R,S,R,S-[Ni(Me₄[14]aneN₄)]²⁺, but such waters did not coordinate to [Ni(Me₄[12] aneN₄)]²⁺. Also, the K _NiS2 values were larger than the corresponding K _NiS1 values for [Ni([14]aneS₄)]²⁺. Furthermore, the K _NiS1 values for [Ni([12]aneN₄)]²⁺ were the largest among these nickel(II) complex cations. The K _NiS, K _NiS1, and K _NiS2 values are discussed in terms of properties of the donor solvents and steric strains of these nickel(II) complex cations.     

Mobile Proton Triggered Radical Fragmentation of Nitroarginine Containing Peptides

Protonated nitroarginine, [R^NO2 + H]⁺, which contains the nitroguanidine ‘explosophore,’ undergoes homolytic N – N nitro-imine bond cleavage to expel NO₂ ^? and form a radical cation of arginine in high yield (100 % relative abundance) upon low-energy collision-induced dissociation (CID). Other ionization states of nitroarginine, including [R^NO2 - H]^–, and a fixed-charge derivative of nitroarginine do not expel NO₂ ^? (<1 %), but instead dissociate via heterolytic bond cleavage with abundant losses of small molecules (N₂O and H₂N₂O₂) from the nitroguanidine group. The effects of proton mobility on the CID reactions of nitroarginine containing peptides was investigated for peptide derivatives of leucine enkephalin, including XYGGFLR^NO2, X = D, G, K, and R, by examining the different protonation states: [M – H]^–; [M + H]⁺; and [M + 2H]²⁺. For [M + H]⁺ containing the less basic N-terminal residues (X = D, G) and all [M + 2H]²⁺, mobile proton fragmentation reactions that result in peptide sequence ions dominate. In contrast, for peptides containing the basic N-terminal residues (R and K), the CID spectra of both the [M – H]^– and [M + H]⁺ are dominated by the losses of small even-electron neutrals from the nitroarginine side-chain. The fraction of nitroguanidine directed fragmentation of the nitroarginine side chain that results in bond homolysis to form [XYGGFLR]^+? by expulsion of NO₂ ^? increases by more than 10 times as the protonation state changes from [M – H]^– (<10 %) to [M + 2H]²⁺ (ca. 90 %) and by about four times as the acidity of the [M + H]⁺ N-terminal residue increases from R (19.0 %) to D (76.5 %). These results indicate that protonated peptides containing nitroarginine can undergo non-canonical mobile proton triggered radical fragmentation.