Skeletal rearrangements of protonated molecular ions of some ethers

Skeletal rearrangements are reported of protonated molecular ions in the chemical ionization mass spectra of allyl cyclohexyl ether, benzyl cyclohexyl ether, t-butyl cyclohexyl ether and dibenzyl ether.     

Water loss from molecular protonated ions of benzyl 2-phenylethyl ether and derivatives

The skeletal rearrangement for water loss is the dominant ion reaction in the chemical ionization mass spectrum of benzyl 2-phenylethyl ether. Isotopic distributions obtained for this reaction with specifically labelled derivatives have been interpreted in terms of competing five- and six-centred skeletal rearrangements. Chemical substitution of the alternative aromatic rings strongly influences the balance of the competition.     

Electron loss from multiply protonated lysozyme ions in high energy collisions with molecular oxygen

We report on the electron loss from multiply protonated lysozyme ions Lys-Hn(n)+ (n = 7 - 17) and the concomitant formation of Lys-Hn(n+1)+. in high-energy collisions with molecular oxygen (laboratory kinetic energy = 50 x n keV). The cross section for electron loss increases with the charge state of the precursor from n = 7 to n = 11 and then remains constant when n increases further. The absolute size of the cross section ranges from 100 to 200 A2. The electron loss is modeled as an electron transfer process between lysozyme cations and molecular oxygen.     

脂肪胺分子离子及质子化脂肪胺分子的碎裂反应

应用碰撞诱导解离(CID)技术研究了电子轰击方法产生的脂肪胺分子离子和化学电离方法产生的质子化脂肪胺分子的碎裂反应。质子化脂肪胺碰撞活化后的主要碎裂通道包括丢失C~XH~2~X、C~XH~2~X~+~1、C~XH~2~X~+~2单元及NH~3和生成[C~yH~2~y~+~1]^+及CH~3CH=NH~2^+离子。脂肪胺分子离子碰撞活化后的主要碎裂通道是丢失C~XH~2~X、C~XH~2~X~+~1及NH~3和生成[C~mH~2~m~-~1]^+、CH~2NH~2^+及CH~3CHNH~2^+离子。随着碰撞能的增加，远电荷碎裂反应和电荷诱导碎裂反应之间竞争引起产物离子的分布发生变化，如[C~mH~2~m~-~1]^+和[C~yH~2~y~+~1]^+离子。自由基机理可以解释质子化脂肪胺分子的远电荷反应。分子内氢抽取可以解释脂肪胺分子离子的碎裂反应。     

Balance of xanthophylls molecular and protonated molecular ions in electrospray ionization

This paper reports the chemical evidence of the balance between radical molecular ions and protonatedmolecules of xanthophylls (an oxygen-containing carotenoid) with a conjugated pi-system (polyene) and oxygen as a heteroatom in ESI and HRESI mass spectrometry. The ionization energy of neutral xanthophylls was calculated by semi-empirical methods. The results were compared with those previously published for carotenoids and retinoids, which have also been shown in ESI-MS to form M(+*) and [M + H](+), respectively. This study demonstrates, for the first time, the correlation of an extended conjugation and the presence of oxygen in the formation and balance of M(+*) or [M + H](+) for the carotenoids, neoxanthin, lutein, violaxanthin and zeaxanthin.     

The loss of water from molecular ions of aliphatic ketones in electron-impact mass spectra

In a series of straight chain aliphatic ketones which has been studied, it has been shown that the structural requirement for loss of water from the molecular ions produced by electron-impact is an alkyl group of seven or more carbon atoms attached to the carbonyl function.     

Electrospray tandem quadrupole fragmentation of quinolone drugs and related ions. On the reversibility of water loss from protonated molecules

Selected reaction monitoring (SRM) of quinolone drugs showed different sensitivities in aqueous solution vs. biological extract. The authors suggested formation of two singly protonated molecules with different behavior, one undergoing loss of H(2)O and the other loss of CO(2), so that SRM transitions might depend on the ratios of these forms generated by the electrospray. These surprising results prompted us to re-examine several quinolone drugs and some simpler compounds to further elucidate the mechanisms. We find that the relative contributions of loss of H(2)O vs. loss of CO(2) in tandem mass spectrometric (MS/MS) experiments depend not only on molecular structure and collision energy, but also, in certain cases, on the cone voltage. We further find that many product ions formed by loss of H(2)O can reattach a water molecule in the collision cell, whereas ions formed by loss of CO(2) do not. Since reattachment of H(2)O can occur after water loss in the cone region and prior to selection of the precursor ion, this effect leads to the dependence of MS/MS spectra on the cone voltage used in creating the precursor ion, which explains the formerly observed effect on SRM ratios. Our results support the earlier conclusion that varying amounts of two ions of the same m/z value are responsible for problems in the analysis of these drugs, but the origin is in dehydration/rehydration reactions. Thus, SRM transitions for certain complex compounds may be comparable only when monitored under equivalent ion-forming conditions, including the voltage used in the production of the protonated molecules in the electrospray ionization (ESI) source.     

Ring H/hydroxyl H scrambling before loss of water from salicylic acid molecular ions

Scrambling before formation of the [M — 18] ion from salicylic acid is considerably reduced in comparison with scrambling before formation of the [M — 17] ion from benzoic acid.     

Oxygen rearrangement of molecular ions of 3-phenylpropionates

The oxygen rearrangement in molecular ions of 3-phenylpropionates has been investigated with the aid of mass analyzed ion kinetic energy spectra. Elimination of an allyl radical followed by expulsion of ketene from the molecular ion of allyl 3-phenylpropionate is shown to result in formation of protonated benzaldehyde. The oxygen rearrangement has been found to be inoperative in ionized methyl 3-methyl-3-phenylbutyrate. [M ? CH₃ ? CH₂CO]⁺ ions in the spectrum of the latter compound are formed by elimination of the 3-methyl substituent and subsequent methoxy migration.     

Cyclization and rearrangement reactions of a(n) fragment ions of protonated peptides

a(n) ions are frequently formed in collision-induced dissociation (CID) of protonated peptides in tandem mass spectrometry (MS/MS) based sequencing experiments. These ions have generally been assumed to exist as immonium derivatives (-HN(+)═CHR). Using a quadrupole ion trap mass spectrometer, MS/MS experiments have been performed and the structure of a(n) ions formed from oligoglycines was probed by infrared spectroscopy. The structure and isomerization reactions of the same ions were studied using density functional theory. Overall, theory and infrared spectroscopy provide compelling evidence that a(n) ions undergo cyclization and/or rearrangement reactions, and the resulting structure(s) observed under our experimental conditions depends on the size (n). The a(2) ion (GG sequence) undergoes cyclization to form a 5-membered ring isomer. The a(3) ion (GGG sequence) undergoes cyclization initiated by nucleophilic attack of the carbonyl oxygen of the N-terminal glycine residue on the carbon center of the C-terminal immonium group forming a 7-membered ring isomer. The barrier to this reaction is comparatively low at 10.5 kcal mol(-1), and the resulting cyclic isomer (-5.4 kcal mol(-1)) is more energetically favorable than the linear form. The a(4) ion with the GGGG sequence undergoes head-to-tail cyclization via nucleophilic attack of the N-terminal amino group on the carbon center of the C-terminal immonium ion, forming an 11-membered macroring which contains a secondary amine and three trans amide bonds. Then an intermolecular proton transfer isomerizes the initially formed secondary amine moiety (-CH(2)-NH(2)(+)-CH(2)-NH-CO-) to form a new -CH(2)-NH-CH(2)-NH(2)(+)-CO- form. This structure is readily cleaved at the -CH(2)-NH(2)(+)- bond, leading to opening of the macrocycle and formation of a rearranged linear isomer with the H(2)C═NH(+)-CH(2)- moiety at the N terminus and the -CO-NH(2) amide bond at the C terminus. This rearranged linear structure is much more energetically favorable (-14.0 kcal mol(-1)) than the initially formed imine-protonated linear a(4) ion structure. Furthermore, the barriers to these cyclization and ring-opening reactions are low (8-11 kcal mol(-1)), allowing facile formation of the rearranged linear species in the mass spectrometer. This finding is not limited to 'simple' glycine-containing systems, as evidenced by the IRMPD spectrum of the a(4) ion generated from protonated AAAAA, which shows a stronger tendency toward formation of the energetically favorable (-12.3 kcal mol(-1)) rearranged linear structure with the MeHC═NH(+)-CHMe- moiety at the N terminus and the -CO-NH(2) amide bond at the C terminus. Our results indicate that one needs to consider a complex variety of cyclization and rearrangement reactions in order to decipher the structure and fragmentation pathways of peptide a(n) ions. The implications this potentially has for peptide sequencing are also discussed.     

Water loss from ester molecular ions upon electron impact

To date, in mass spectral studies of esters, a single reference to loss of water from the molecular ions has been noted. A series of aromatic esters which exhibit that sparsely observed phenomenon are reported herein. Through a combination of synthetic and deuterium labeling studies, support for a unique fragmentation route for the loss of water from these ester molecular ions has been obtained.     

Isotope effects upon hydrogen atom loss from molecular ions

The isotope effect (T_H/T_D) upon the kinetic energy release and the isotope effect (k_H/k_D) upon ion abundance for unimolecular H· loss from molecular ions has been determined for several compounds. It is suggested that the isotope effect upon abundance might provide a convenient method of estimating the relative life-time of ions which fragment in the metastable region for different instruments or different experimental conditions. The value of k_H/k_D varies from <2 to >1000 for different molecular ions and this variation is apparently largely due to the rate of increase of the reaction rate with internal energy in the threshold region. The magnitude of the isotope effect is thus related to the entropy of activation. The isotope effect upon energy release was found to be slightly less than unity in almost every case studied; this included both reactions in which the reverse activation energy is very small and those in which it is appreciable.     

Hidden rearrangement processes in short-lived negative molecular ions

The study of resonant electron capture by nitrobenzene molecules showed that some fragmentary negative ions are unstable toward electron autodetachment. The measured appearance energy of the neutral component of an [M — H]⁻ ion beam does not agree with the energetics of direct dissociation in a molecular ion. The estimation calculations show that the low appearance energy of [M — H]⁰ neutral components is caused by isomerization of a molecular ion of nitrobenzene to the 2-nitrobenzene structure followed by the formation of a phenoxide ion in the autodetachment state. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 367–370, February, 2006.     

The dichotomy of methyl loss from 2-methylalkane molecular ions

The kinetic energy release for methyl loss from the molecular ion of 2-methylpentane increases c. twofold over the time-range 1–15 μs. This result from a mixture of secondary and tertiary pentyl cations. The appearance energy showed that at threshold (and long times) only tertiary ions are formed. The molecular ion of 2-methylhexane produces tertiary hexyl ions by loss of methyl at threshold, but in the μs time-frame only secondary hexyl ions appear to be formed, because the kinetic energy release is small and independent of the observation time.     

The unorthodox loss of propyl from the molecular ions of methoxycyclohexane

It is shown by field ionization kinetics, D and ¹³C labelling and metastable ion studies that the loss of a propyl radical from the molecular ion of methoxycyclohexane occurs via two routes. At a molecular ion lifetime of <10^?10 s propyl is eliminated in the ‘classic’ way, i.e. by successive cleavage of the C(1)? C(2) bond, 1,5-H shift from C(6) to C(2) and cleavage of the C(4)? C(5) bond. At 10^?10 s the other pathway for propyl loss starts to take place, which is initiated by a hydrogen shift from position 3 or 5 to the methoxy group. This leads in a series of steps to the formation of the 3-methoxyhexene-1 ion, which eventually eliminates a propyl radical. In some of the steps specific hydrogen-deuterium exchange processes have been observed.     

Photochemical generation of nitrenium ions from protonated 1,1-diarylhydrazines

[reaction: see text] Laser flash photolysis experiments, chemical trapping studies, and time-dependent density functional theory calculations demonstrate that photolysis of protonated 1,1-diarylhydrazines generates N,N-diarylnitrenium ions.