Interference to linear superposability of collisionally activated decomposition spectra at high resolution

A new factor causing the superposability of collisionally activated decomposition spectra of ions to fail has been established for isobaric ions. If the two isobaric ion beams are not transmitted in their entirety simultaneously to the collision cell, the resulting spectrum is not a linear combination of the two spectra of the individually transmitted complete ion beams. Apparently the energy content of collimated ions varies across the ion beam, so that transmission of different portions of the ion beam produces samples of ions with different energy contents.     

Automated assignment of high‐resolution collisionally activated dissociation mass spectra using a systematic bond disconnection approach

An essential component of the process of characterising chemical unknowns via mass spectrometry is the analysis of collisionally activated dissociation (CAD) mass spectra. Existing tools for the automated assignment of CAD spectra typically use a rule‐based approach which identifies those bonds that are likely to break. While valuable, the failure of explicitly rule‐based approaches to suggest rationalisations for a significant proportion of observed product ions led us to develop an alternative approach (elucidation of product ion connectivity, EPIC) based on high‐resolution mass spectrometry, systematic bond disconnection of the precursor structure, and ranking of the resulting substructures. We exemplify this approach with a reanalysis of published MS/MS data for two compounds taken from the literature. Copyright © 2005 John Wiley & Sons, Ltd.     

The unimolecular and collisionally activated decompositions of fluorine-substituted tetraphenyltetrahedrane ions on the microsecond time-scale

Unimolecular and collision-induced decomposition products of [C₄(C₆H₅)₂(C₆H₄F)₂]⁺˙ generated from four unsymmetrical sources include [C₁₄H₁₀]⁺˙ and [C₁₄H₈F₂]⁺˙ and so provide evidence for a tetrahedral intermediate. Other decompositions show substantial influence of the position of the ρ-fluorophenyl ring on ion energy distributions. This influence may be related to the reported absence of peaks diagnostic for the tetrahedral intermediate from the spectrum of the equivalent ion from the appropriate ¹³C-labeled analog. Alternatively the difference in spectra can be correlated with lifetimes of ions.     

Examination of Ortho effects in the collisionally activated dissociation of closed-shell aromatic ions

The collisionally activated dissociation of a variety of isomeric disubstituted aromatic ions formed by ion–molecule reactions were examined in order to characterize ortho effects in closed-shell systems. Closed-shell ions of methoxyacetophenone, hydroxyacetophenone, methoxyphenol, anisaldehyde and hydroxybenzaldehyde were formed by proton transfer, methyl addition or methyne addition by using dimethyl ether or ethylene oxide as chemical ionization reagents, and then the structures of these adducts were studied by deuterium-labelling methods and by collisionally activated dissociation techniques in a triple quadrupole mass spectrometer or a quadrupole ion trap. Typically, the meta and para isomers have qualitatively similar dissociation spectra which reflect the types of dissociation reactions observed for the corresponding monosubstituted aromatic ions. The predominant dissociation pathways of the [M + H]⁺ and [M + 15]⁺ ions are directed by the electron-withdrawing substituents, whereas the major dissociation pathways of the [M + 13]⁺ ions are related to the electron-releasing substituent. In contrast, the dissociation routes of the corresponding ortho isomers are dramatically different. This is attributed to the opportunity for functional group interactions of the ortho isomers which facilitate alternative pathways.     

Comparison of collisionally activated dissociation mass spectra for the identification of cyclopeptides and cyclodepsipeptides

Collisionally activated dissociation (CAD) spectra of protonated molecules of cyclopeptides and cyclodepsipeptides obtained with two different mass spectrometry systems were compared. Fragmentations were obtained either from collisions induced in the ion source of an electrospray mass spectrometer fitted with a single quadrupole by increasing the extracting cone voltage or from collisions with an inert gas in a free-field area of a fast-atom bombardment (FAB) mass spectrometer. Similar fragmentation pathways were produced with the two configurations even though actual tandem mass spectrometry experiments with magnetic and electric sectors provided more information than cone-induced dissociations. However, only the latter mode allowed us to perform mass spectrometric analyses coupled to liquid chromatography (LC/ESI-MS) at low cost on commercially widespread instruments. Copyright © 1999 John Wiley & Sons, Ltd.     

Cyclization reaction of peptide fragment ions during multistage collisionally activated decomposition: An inducement to lose internal amino-acid residues

During characterization of some peptides (linear precursors of the cyclic peptides showing potential to be anticancer drugs) in an ion trap, it was noted that many internal amino acid residues could be lost from singly charged b ions. The phenomenon was not obvious at the first stage of collisionally activated decomposition (CAD), but was apparent at multiple stages of CAD. The unique fragmentation consisting of multiple steps is induced by a cyclization reaction of b ions, the mechanism of which has been probed by experiments of N-acetylation, MS(n), rearranged-ion design, and activation-time adjustment. The fragmentation of synthetic cyclic peptides demonstrates that a cyclic peptide intermediate (CPI) formed by b ion cyclization exhibits the same fragmentation pattern as a protonated cyclic peptide. Although no rules for the cyclization reaction were discerned in the experiments of peptide modification, the fragmentations of a number of b ions indicate that the "Pro and Asn/Gln effects" can influence ring openings of CPIs. In addition, large-scale losses of internal residues from different positions of a-type ions have been observed when pure helium was used as collision gas. The fragmentation is initiated by a cyclization reaction forming an a-type ion CPI. This CPI with a fixed-charge structure cannot be influenced by the "Pro effect", causing a selective ring opening at the amide bond Pro-Xxx rather than Xxx-Pro. With the knowledge of the unique fragmentations leading to internal residue losses, the misidentification of fragments and sequences of peptides may be avoided.     

Synthesis and characterization of poly(DTC‐b‐PEG‐b‐DTC) triblock and poly(TMC‐b‐DTC‐b‐PEG‐b‐DTC‐b‐TMC) pentablock copolymers and kinetics of the polymerization

Dihydroxyl capped biodegradable poly(DTC‐b‐PEG‐b‐DTC) (BCB) triblock copolymer and poly(TMC‐b‐DTC‐b‐PEG‐b‐DTC‐b‐TMC) (ABCBA) pentablock copolymer have been synthesized by PEG and BCB copolymer as macroinitiator in the presence of yttrium tris(2,6‐di‐tert‐butyl‐4‐methylphenolate). The copolymers without random segments have been thoroughly characterized by ¹H, ¹³C‐NMR, SEC, and DSC. Molecular weights of the obtained copolymers are dependent on the amount of PEGs and coincide with the theoretical values. The exchange reaction of yttrium alkoxide and hydroxyl end group is essential for controlling the products' molecular weight. Their thermal behaviors are relevant to the chain lengths of PEG and PDTC segments. The Monte Carlo method has been developed to estimate the chain propagation constant and exchange reaction constant. In average, one exchange reaction will occur after approximately six monomer molecules insert into the growing chain. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1787–1796, 2005     

Observation of internal monosaccharide losses in the collisionally activated dissociation mass spectra of anthracycline aminodisaccharides

The loss of internal monosaccharide residues from anthracycline aminodisaccharides has been studied using high and low energy collisionally activated dissociations (CAD). The mass spectra and low energy CAD spectra obtained using electrospray ionization on a quadrupole mass spectrometer were consistent with a structure in which positions of the two monosaccharide residues were reversed. Key fragment ions observed in the product ion spectra of the peracetylated derivative and the deuterated analog were used to provide evidence that a rearrangement process had occurred. Mass-analyzed ion kinetic energy spectrometry data were also consistent with a rearrangement.     

Evaluation of noncovalent interactions between peptides and polyether compounds via energy-variable collisionally activated dissociation

Energy-variable collisionally activated dissociation (CAD) was used to analyze noncovalent interactions of protonated peptide/polyether complexes in a quadrupole ion trap complexes were formed with a series of four polyether host molecules and thirteen peptide molecules. Comparison of dissociation thresholds revealed correlations between the gas-phase basicities of the peptides and polyether molecules and the onset of dissociation. The dissociation thresholds of complexes containing the tripeptides or pentapeptides were inversely proportional to the gas-phase basicities of the sites of protonation of the peptides. Intramolecular hydrogen bonding of the pentapeptides affected the observed dissociation thresholds as well. The dissociation thresholds also scaled proportionally to the gas-phase basicities of the polyethers in the complexes, and the importance of the conformational flexibility of the polyether ligand was confirmed for one of the histidine-containing tripeptide complexes.     

Mass-analysed ion kinetic energy and collisionally induced dissociation mass spectra of clusters of acetylated xylo-oligosaecharides with protonated ammonia and methylannine

Per-O-acetylated methyl glycosides of D -xylan-type di- and trisaccharides were studied by mass-analysed ion kinetic energy (MIKE) and collisionally induced dissociation (CID) mass Spectrometry using protonated ammonia and methylamine, respectively, as reaction gases in chemical ionization (CI). The oligosaccharides form abundant cluster ions, [M + NH₄]⁺ or [M + CH₃NH₃]⁺, and the main fragmentation of these ions in the MIKE and CID spectra is the cleavage of interglycosidic linkages. Thus, CI (NH₃) or CI (CH₃NH₂) spectra in combination with the MIKE or CID spectra allow the molecular masses, the masses of monosaccharide units and the branching point in oligosaccharides to be established. In the case of disaccharides, it is possible to distinguish the (1 → 2) linkage from the other types of linkages.     

Substituent effects on the gas‐phase fragmentation reactions of protonated peptides containing benzylamine‐derivatized lysyl residues

Motivated by the need for chemical strategies designed to tune peptide fragmentation to selective cleavage reactions, benzyl ring substituent influence on the relative formation of carbocation elimination (CCE) products from peptides with benzylamine‐derivatized lysyl residues has been examined using collision‐induced dissociation (CID) tandem mass spectrometry. Unsubstituted benzylamine‐derivatized peptides yield a mixture of products derived from amide backbone cleavage and CCE. The latter involves side‐chain cleavage of the derivatized lysyl residue to form a benzylic carbocation [C₇H₇]⁺ and an intact peptide product ion [(MH_n)ⁿ⁺ – (C₇H₇)⁺]^(n‐1)+. The CCE pathway is contingent upon protonation of the secondary ε‐amino group (N_ε) of the derivatized lysyl residue. Using the Hammett methodology to evaluate the electronic contributions of benzyl ring substituents on chemical reactivity, a direct correlation was observed between changes in the CCE product ion intensity ratios (relative to backbone fragmentation) and the Hammett substituent constants, σ, of the corresponding substituents. There was no correlation between the substituent‐influenced gas‐phase proton affinity of N_ε and the relative ratios of CCE product ions. However, a strong correlation was observed between the π orbital interaction energies (ΔE_int) of the eliminated benzylic carbocation and the logarithm of the relative ratios, indicating the predominant factor in the CCE pathway is the substituent effect on the level of hyperconjugation and resonance stability of the eliminated benzylic carbocation. This work effectively demonstrates the applicability of σ (and ΔE_int) as substituent selection parameters for the design of benzyl‐based peptide‐reactive reagents which tune CCE product formation as desired for specific applications. Copyright © 2012 John Wiley & Sons, Ltd.     

Low-energy collisionally activated decomposition and structural characterization of cyclic heptapeptide microcystins by electrospray ionization mass spectrometry

Characteristics of electrospray ionization mass spectrometry/collision-induced dissociation (ESIMS/CID) mass spectra of microcystins, cyanobacterial cyclic heptapeptide hepatoxins, were examined. The collision conditions showed remarkable effects on the quality of the CID mass spectra, which were divided into three patterns according to the number of Arg residues. A characteristic cleavage reaction and neutral losses of MeOH, NH3 and guanidine group(s) from the (2S,3S,8S,9S)-3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4 E,6E-dienoic acid (Adda) and Arg residues were observed in the ESI and ESIMS/CID mass spectra, suggesting the most probable protonation sites in [M + H]+ and [M + 2H]2+ ions of microcystins. Microcystins with no Arg residue showed only [M + H]+ ions with a proton reacting at the methoxyl group in the Adda residue, and the ESIMS/CID/MS data revealed their structures unambiguously. The protonation site in [M + H]+ ions of microcystins with Arg residue(s) was the guanidine group. The [M + 2H]2+ ions of microcystins possessing one Arg residue had one proton on the Arg residue and probably another proton on the Adda residue, while the [M + 2H]2+ ions of microcystins having two Arg residues showed protonation at both Arg residues and the ESIMS/CID/MS data assigned their sequences. Structures of microcystins possessing one Arg residue can be assigned by ESIMS/CID/MS of [M + H]+ ions combined with those of [M + 2H]2+ ions.     

Tailoring the LCST of PNIPAAM‐b‐PLA‐b‐PNIPAAM Triblock Copolymers via Stereocomplexation

Poly(N‐isopropylacrylamide)‐block‐poly(l ‐lactic acid)‐block‐poly(N‐isopropylacrylamide) (PNIPAAM‐b‐PLLA‐b‐PNIPAAM) and PNIPAAM‐b‐PDLA‐b‐PNIPAAM triblock copolymers with varying polylactic acid (PLA) lengths are synthesized using a combination of ring‐opening polymerization and atom‐transfer radical polymerization. Results of ¹H NMR and gel permeation chromatography analyses show that the copolymers have a well‐defined triblock structure and the PLA segment lengths can be readily controlled with monomer feed ratio. Stereocomplexation between the enantiomeric PLA segments is confirmed with differential scanning calorimetry and wide‐angle X‐ray scattering. Dynamic light scattering experiments show that (1) the LCST of PNIPAAM in water could be tailored from 32 °C up to 38.5 °C by increasing the length of PLA segments and mixing copolymers of similar molecular weight with enantiomeric PLA segments to induce stereocomplexation, and (2) the LCST of each mixed copolymer system could be tailored within a 2–3 °C range of body temperature by manipulating the ratio of the enantiomeric copolymers in solution.

     

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.