Ion–molecule reaction in the gas phase 4—Stereospecific nucleophilic substitution under ammonia chemical ionization conditions from diastereomeric methyl and benzyl ethers of 3-hydroxy steroids

The ammonia chemical ionization (CI/[NH₄⁺]) mass spectra of a series of diastereomeric methyl and benzyl ethers derived from 3-hydroxy steroids (unsaturated in position 5 and saturated) have been studied. The adduct ions [M+NH₄]⁺ and [MH]⁺ and the substitution product ions [M+NH₄? ROH]⁺ (thereafter called [M_sH]⁺) are characterized by an inversion in their relative stabilites in relation to their initial configuration. [M+NH₄]_α⁺ and [MH]_α⁺ formed from the α-Δ⁵-steroid isomers are stabilized by the presence of a hydrogen bond which is not possible for the β-isomers. This stereochemical effect has also been observed in the mass analysed ion kinetic energy (MIKE) spectra of [M+NH₄]⁺ and [MH]⁺. The MIKE spectra of [M_sH]⁺ indicate that those issued from the β-isomers are more stable than the one originating from the α-isomers. This behavior is also observed in the first field free region (HV scan spectra) for [MH]⁺, [M_sH]⁺ and [M+NH₄]⁺ which are precursors of the ethylenic carbocations (base peak in the conventional CI/[NH₄]⁺ spectra). Mechanisms, such as S_N1 and S_Ni, have been ruled out for the formation of [M_sH]⁺, but instead the data support an S_N2 mechanism during the ion-molecule reaction between [M+NH₄]⁺ and NH₃.     

Further examples of skeletal rearrangements of the Wagner-Meerwein type in chemical ionization mass spectrometry: The case of [C6H9]+ ions

The [C₆H₉]⁺ ions produced either via unimolecular H₂O loss from 13 [C₆H₁₁O]⁺ precursors or direct protonation of 1,3- and 1,4-cyclohexadiene have identical collisional activation mass spectra. The kinetic energy release data for the process [C₆H₁₁O]⁺→[C₆H₉]⁺+H₂O are also very similar (on average T_0.5=24 meV) irrespective of the constitution of the precursor. From the proton affinities of 1,3-cyclohexadiene (PA=837.2 kJ mol^?1) using ion cyclotron resonance mass spectrometry the heat of formation of the [C₆H₉]⁺ ion is determined to 804.6 kJ mol^?1. This value taken together with the results of molecular orbital calculations (MNDO) and the structure indicative losses of CH₃^. and C₂H₄ upon collisional activation suggest that the [C₆H₉]⁺ ion has the structure of the 1-methylcyclopentenylium ion f and not that of the slightly less stable cyclohexenylium ion g. The generator of an easily interconverting system of isomeric [C₆H₉]⁺ ions is unlikely to be due to the high barrier separating the various isomers.     

The mass spectra of organic compounds. 6th Communication.. The Ring Opening Reaction of Isomeric Cyclanones C8H14O\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{+ \atop \dot{}} $\end{document} in metastable and collisional-activation spectra

The molecular ions from three isomeric cyclanones isomerize to the ethyl-2-cyclohexanone ion prior to C₂H₄ elimination. With D- and ¹⁸O-labelled compounds it is shown by Mass Analyzed Ion Kinetic Energy Spectroscopy (MIKES.) that both isomerization and C₂H₄ loss are specific processes. By high resolution collisional activation spectra it is shown that the resultant fragment ion [C₆H₁₀O]\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{+ \atop \dot{}} $\end{document} (m/z = 98) differs in structure from the cyclohexanone molecular ion.     

Investigation by mass spectrometry of metal complexes of new molecular hosts: cyclic oligomer of sugar amino acid and sugar-aza-crown ethers

The affinity of cyclic oligomers of sugar amino acid and sugar-aza-crown ether compounds towards various transition metal cations (Cu(II), Ni(II), Co(II), Fe(II) and Zn(II)) was investigated with positive-ion electrospray mass spectrometry. The binding between the receptors (M) and the different metals (Met) is evidenced mainly by the presence of the [M + Met(II)Cl](+) ion. The experimental results showed that all studied receptors present specificity to Cu(II). An attempt has been made with CuI but no complexation was obtained. The formation of these complexes can be rationalized by considering the presence of two oxygens and two nitrogens on the receptor rim. The lone electron pair can serve as the electron donor to Cu(II). Theoretical calculations were carried out in order to show the structure of the complex and, in particular, to determine if Cu(2+) is situated either on the outer surface, on the rim of the receptor or inside the cavity. Comparison of complex formation was carried out by mixing the four receptors with various amounts of Cu(II) (one equivalent and five equivalents). It appears that the best complexation was obtained with the sugar-aza-crown ethers (amine linker) for both benzylated and methylated compounds. In addition, the stereochemical effects have been investigated.     

Determination of IGF-I in horse plasma by LC electrospray ionisation mass spectrometry

The insulin-like-growth factor (IGF-I) peptide is considered to be the main indirect marker for growth hormone administration (GH) in a horse. Further to a previous investigation on measurement of IGF-I in plasma samples by mass spectrometry, this study focuses on quantitative and qualitative analysis of intact IGF-I in horse plasma. First, protein-transposing software has been developed for IGF-I to facilitate its quantification by HPLC–electrospray–ion-trap mass spectrometry. Second, product-ion scan experiments on IGF-I have been conducted on standard samples, non-fortified equine plasma samples, fortified plasma samples, and equine GH post-administration samples. This “top-down” approach method enables characterisation of fragment ions corresponding to the carboxy terminal end, which can be useful for the confirmation of the presence of IGF-I in plasma samples. Figure Structure of IGF-I and amino acid sequences of IGF-I and R3 IGF-I. Deconvolution mass spectra of the IGF-I and R3 IGF-I mixture     

Negative-charge driven fragmentations for evidencing zwitterionic forms from doubly charged coppered peptides

In aqueous solution, amino acids (AA) and peptides are known to exist as zwitterions over a large pH range. However, in the gas phase, i.e. in electrospray (ESI), the zwitterionic form becomes unfavorable owing to the absence of stabilizing effects from intermolecular solvation. Nevertheless, during mass spectrometry experiments, the presence of a metallic cation can reinforce the zwitterionic character of the molecule and thus influence its fragmentation under low energy collision-induced dissociation (CID) conditions. The [M + Cu(II)](2+) complexes of six pentapeptides (YGGFL, YGGFL(NH(2)), YGGFK, YGGFQ, KYGGF and QYGGF) were analyzed by collision to highlight the presence of zwitterions. The experiments were performed on a 3D-ion trap equipped with an orthogonal ESI source. For each peptides studied, negative-charge driven fragmentations on globally positively charged ions were observed. These fragmentation mechanisms, generally observed in the negative mode, suggest the competitive deprotonation of the C-terminal carboxylic acid or of the tyrosine side-chain residue for each peptide studied and thus a zwitterionic form to preserve the charge balance. Moreover, the specific loss of (CH(3)--C(6)H(4)--O)(*) characterizes YGGFK compared to YGGFQ and the specific loss of styrene characterizes KYGGF compared to QYGGF. These results allow the differentiation of the two couples of isobaric pentapeptides. An unusual loss of NH(4) (+), which occurred from the N-terminus, was also observed for YGGFL, YGGFL(NH(2)), YGGFK and YGGFQ. Finally, the reduction of Cu(II) to Cu(I), concomitant with the (CH(3)--C(6)H(4)--O)(*) release, was pointed out for YGGFK.     

Mass spectrometric studies on 17β-estradiol-17-fatty acid esters: Evidence for the formation of anion-dipole intermediates

The behaviour towards low collision energy processes (eV range) of [M  H]⁻ prepared under negative ion chemical ionization (NICI) ammonia conditions from 17β-estradiol-17-fatty acid esters has been investigated. From such bifunctional compounds containing two acidic sites (i.e. phenol and ester groups), two isomeric forms (i.e. phenoxide and enolate forms) characterize the [M  H]⁻ ion structures, whose distribution depends on the ion preparation mode. Here NICI (ammonia) provides both phenoxide and enolate forms as the [M  H]⁻ species. This behaviour contrasts with the regioselectivity observed for proton abstraction from phenol under NICI (N₂O) and fast atom bombardment conditions. Production of both phenoxide and enolate forms in NICI (ammonia) is demonstrated under NICI (ND₃) conditions in which DO-labelled [M_d  H]⁻ enolate ions are produced in a similar yield to unlabelled [M_d  D]⁻ phenoxide ions. Collisionally activated dissociation (CAD) spectra of both isomeric deprotonated molecules differ strongly by the presence of two different pairs of complementary daughter ions, suggesting that these ionic species are unconvertible. This is due to a steric hindrance effect on the long-distance proton transfer. A mechanistic investigation on the formation of fragment ion pairs produced under CAD was performed with various deuterium-labelled molecules. From these experiments, evidence is provided for molecular isomerizations into ion-dipole complexes (prior to dissociation) which are structurally dependent on the initial charge location. Direct dissociation of these intermediates competes with the occurrence of exothermic proton transfer(s) yielding the formation of other isomeric intermediate forms. The orientation of these proton transfers is dictated by the relative acidities of both moieties of the complex.     

Ion-dipole complex formation from deprotonated phenol fatty acid esters evidenced by using gas-phase labeling combined with tandem mass spectrometry

The behavior of para-hydroxy-benzyl and hydroxy-phenylethyl fatty acid esters and methoxy derivatives toward the NH₃/NH₂ ^? system was investigated. Under these negative ion chemical ionization (NICI) conditions, proton abstraction takes place mainly at the more acidic site (i.e., phenol); however, this reaction is not entirely regioselective. Using NICI-ND₃ conditions, both isomeric phenoxide and enolate molecular species are produced in competition from these phenol esters. Their respective low-energy collision-activated dissociation spectra are studied, and they strongly differ, showing that these molecular species are not convertible to a common structure. Analysis of specific fragmentations of the OD-enolate parent species labeled by ND₃ in the gas phase, indicates that by charge-promoted cleavage, isomerization into an ion-dipole intermediate takes place prior to dissociation. This complex, containing a ketene moiety, isomerizes into different isomeric forms via two consecutive proton transfers: the first, which is very exothermic, is irreversible in contrast to the second, less exothermic reaction, which occurs via a reversible process. It is evidenced by the loss of labeling at phenol or enolizable sites in the fragment ions. Such a stepwise process does not take place from the phenoxide parent ion, which preferentially yields a very stable carboxylate ion. A thermochemical approach, using estimated acidity values, yields a rationalization of the observed reactivities of the various substrates studied.