Homolytic cleavages in pyridinium ions, an excited state process

Abstract. Elastin is a vital protein of the extracellular matrix of jawed vertebrates and provides elasticity to numerous tissues. It is secreted in the form of its soluble precursor tropoelastin, which is subsequently cross-linked in the course of the elastic fiber assembly. The process involves the formation of the two tetrafunctional amino acids desmosine (DES) and isodesmosine (IDES), which are unique to elastin. The resulting high degree of cross-linking confers remarkable properties, including mechanical integrity, insolubility, and long-term stability to the protein. These characteristics hinder the structural elucidation of mature elastin. However, MS 2 data of linear and cross-linked peptides released by proteolysis can provide indirect insights into the structure of elastin. In this study, we performed energy-resolved collision-induced dissociation experiments of DES, IDES, their derivatives, and DES-/IDES-containing peptides to determine characteristic product ions. It was found that all investigated compounds yielded the same product ion clusters at elevated collision energies. Elemental composition determination using the exact masses of these ions revealed molecular formulas of the type C x H y N, suggesting that the pyridinium core of DES/IDES remains intact even at relatively high collision energies. The finding of these specific product ions enabled the development of a similarity-based scoring algorithm that was successfully applied on LC-MS/MS data of bovine elastin digests for the identification of DES-/IDES-cross-linked peptides. This approach facilitates the straightforward investigation of native cross-links in elastin.

Section: Collision-induced Dissociation Of Des and Idessupporting

confidence: 92%

Fingerprinting Desmosine-Containing Elastin Peptides

Schräder

Heinz

Majovsky

et al. 2015

“…Additional support for the proposed mechanism in Scheme 2 is shown in the product ion spectra of Pellitorine (18), in Figure 3, and 19 which are not doubly allylic. These compounds, in addition to being stable to homolytic cleavage, cannot form the bicyclic intermediate that leads to formation of the cyclopentadiene cationic species at m/z 152-the relevant cationic species for 2E,4E-unsaturated amides.…”

Section: Charge-induced Mechanismmentioning

confidence: 70%

“…Pericyclic 1,4-conjugate elimination, homolytic cleavage, and other gasphase thermolytic processes have been used to model charge-remote fragmentations [11][12][13][14][15][16][17]. Denekamp et al studied fragmentation mechanisms for 1-phenyl-4-pyridyl-3-butene and substituted homologs and analogs [18]. These are doubly allylic quaternary ammonium ions where the fragmentation site is also associated by conjugation with the positive charge.…”

Section: Charge-remote Mechanismmentioning

confidence: 99%

See 1 more Smart Citation

Proposed mechanisms for the fragmentation of doubly allylic alkenamides (tingle compounds) by low energy collisional activation in a triple quadrupole mass spectrometer

Hiserodt

Pope

Cossette

et al. 2004

Tingle compounds are a class of alkenamides with organoleptic properties that include a numbing or a pins and needles effect that is generally perceived on the lips and in the mouth when consumed. They occur in nature in a number of botanical species. Spilanthol and Pellitorine are important examples of tingle compounds. A number of homologs and analogs were synthesized to study the effect of chain length, double bond location, and amide moiety on the tingle effect. This also provided the opportunity to study the behavior of these compounds in the collision cell of a triple quadrupole mass spectrometer. The doubly allylic 2E,6Z-alkenamides, which made up the largest class studied, fragmented in a characteristic way to produce a distonic radical cation and a cyclopropene cation. Mechanisms for the formation of these ions are proposed. The mechanisms are supported by energy-resolved mass spectrometric data, the analysis of deuterated analogs and homologs that are not doubly allylic, and exact mass measurements. Exceptions to the proposed mechanisms are also presented. These data represent the first attempt to apply mechanistic principles to the product ions observed in the MS/MS spectra of these compounds. The authors believe the results of this study will facilitate the identification of these and similar compounds and contribute to the fundamental understanding of the behavior of alkenamides in the collision cell of a triple quadrupole mass

“…The same is observed in the CID spectrum of 15 (not shown) that contains two methyl substituents (Scheme 5). The formation of radicals from stable closed-shell ions, even at low energies, is unique, but known for several systems [16]. The driving force for hydrogen loss, in this case, is the relative stability of the resulting conjugated, ionized system, namely the low ionization potential of the corresponding neutral.…”

Section: -D 10mentioning

confidence: 99%

Benzene loss from trityl cations—A mechanistic study

Denekamp

Yaniv

2006

Self Cite

Triarylmethyl cations eliminate substituted benzene in the gas phase, upon activation. The mechanism of this process has been studied using deuterium labeling, substituent effects, and density functional theory calculations. It is shown that this apparently simple dissociation is in fact a complicated stepwise process that involves several consecutive hydride shifts. . Trityl cations are generated from alcohols in acidic conditions, but they can also be generated from trityl halides in the presence of alkali metal, alkaline earth metal ions [6], or Lewis acids. A useful tool for the study of ionic species like trityl cations is gas-phase ion chemistry using mass spectrometry. Mass spectrometry has been used to understand dissociation kinetics and mechanisms of gas-phase molecular ions. Fragmentation pathways of molecular ions are determined using tandem mass spectrometry (MS/MS) isotopic labeling and molecular orbital calculations that provide valuable information. Owing to its general importance to synthetic organic chemistry, a large body of knowledge on the chemistry of cationic aromatic compounds, like alkylbenzenium ions and benzyl cations has been accumulated in the past four decades. For example, the production of C 7 H 7 ϩ from toluene molecular ion is one of the most extensively studied reactions in the field of gas-phase ion chemistry [7]. Other studies concentrate on protonation, ion-neutral complex formation, and H/D exchange processes in aromatic systems. See for example [8] and references cited therein.Electron ionization spectra of several trityl compounds were recorded by Shupe and Berlin [9]. The authors show several typical fragmentation pathways, the most abundant being the formation of a trityl cation. Another characteristic of these EI spectra is the presence of an ion at m/z165 that corresponds to the loss of 78 Da from the trityl cation of m/z 243. The authors suggest that the resulting ion of m/z 165 is a 9-fluorenyl cation. It is also known that benzhydryl cations can afford an analogous ion by the loss of H 2 [10]. The loss of molecular hydrogen is supported by the presence of a metastable ion.Triphenylmethane has also been studied with nega-Ϫ ion. Upon high-energy collisional activation, two major dissociation processes occur, involving the losses of C 2 H 4 and C 6 H 6 neutral fragments. The authors suggest that the loss of C 6 H 6 , a benzene ring, results in the formation of a 9-fluorenyl anion. Isotopic labeling experiments indicate extensive hydrogen-deuterium scrambling within this process [11].Modern desorption techniques enable the generation of stable trityl cations under relatively mild conditions. In the present work, we studied the characteristic loss of benzene or substituted benzene molecules from trityl cations. A detailed mechanism for the formation of 9-fluorenyl cation is proposed and supported by experimental work and theoretical calculations. Results and DiscussionProtonated trityl alcohols eliminate water readily affording trityl cations, under chemical ionization...