Analyse und Optimierung von Gasphasen-Reaktionen,XVII. Selenoketen

Analysis and Optimization of Gasphase Reactions, XVII. — Selenoketene The thermal decomposition of 1,2,3-selenadiazole in the gaseous phase has been investigated by photoelectron and mass spectroscopy. At temperatures above 720 K selenoketene is formed, the PE spectrum of which can be assigned based on ab initio SCF calculations as well as on radical cation state comparison with the iso(valence) electronic heterocumulenes H₂C  C  O and H₂C  C  S. The 4-phenyl derivative decomposes above 820 K forming phenylacetylene.     

Evidence for electron‐based ion generation in radio‐frequency ionization

Radio‐frequency ionization (RFI) is a novel ionization method coupled to mass spectrometry (MS) for analysis of semi‐volatile and volatile organic compounds (VOCs). Despite the demonstrated capabilities of RFI MS for VOC analysis in both positive‐ and negative‐ion modes, mechanism of RFI is not completely understood. Improved understanding of the ion generation process in RFI should expand its utility in MS. Here, we studied the possibility of electron emission in RFI using both direct charged particle current measurements and indirect electron detection in a 9.4‐T Fourier transform‐ion cyclotron resonance (FT‐ICR) mass spectrometer. We show that RF‐generated electrons can be trapped in the ICR cell and, subsequently, reacted with neutral hexafluorobenzene (C₆F₆) molecules to generate C₆F₆^●?. Intensity of observed C₆F₆^●? species correlated with the number of trapped electrons and decreased as a function of electron quenching period. We also measured the electron attachment rate constant of hexafluorobenzene using a post‐RF electron trapping experiment. Measured electron attachment rate constant of hexafluorobenzene (1.19 (±0.53) × 10^?9 cm³ molecule^?1 s^?1) for post‐RF FT‐ICR MS agreed with the previously reported value (1.60 (±0.30) × 10^?9 cm³ molecule^?1 s^?1) from low‐pressure ICR MS measurements. Experimental results from direct and indirect electron measurements suggest that RFI process involves RF‐generated electrons under ultrahigh vacuum conditions. Copyright © 2015 John Wiley & Sons, Ltd.     

H/D exchange kinetics: Experimental evidence for formation of different b fragment ion conformers/isomers during the gas-phase peptide sequencing

Electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) combined with H/D exchange reactions was utilized to explore the existence of different b₅⁺ and b₄⁺ fragment ion conformers/isomers of hexapeptide WHWLQL in the gas phase. Distinct H/D exchange trends for protonated WHWLQL ([M + H]⁺) and its b₅⁺ and b₄⁺ fragment ions (with ND₃) were observed. Isolated ¹²C_all isotopomers of both b₅⁺ and b₄⁺ fragment ions yielded bimodal distributions of H/D exchanged product ions. The H/D exchange reaction kinetics also confirmed that b₅⁺ and b₄⁺ fragment ions exist as combination of slow-exchanging (“s”) and fast-exchanging (“f”) species. The calculated rate constant for the first labile hydrogen exchange of [M + H]⁺ (k_{[M + H]} ⁺ = 3.80 ± 0.7 × 10⁻¹⁰ cm³ mol⁻¹ s⁻¹) was ∼30 and ∼5 times greater than those for the “s” and “f” species of b₅⁺, respectively. Data from H/D exchange of isolated “s” species at longer ND₃ reaction times confirmed the existence of different conformers or isomers for b₅⁺ fragment ions. The sustained off-resonance irradiation collision-activated dissociation (SORI-CAD) of WHWLQL combined with the H/D exchange reactions indicate that “s” and “f” species of b₅⁺ and b₄⁺ fragment ions can be produced in the ICR cell as well as the ESI source. The significance of these observations for detailed understanding of protein sequencing and ion fragmentation pathways is discussed.     

Evidence for Sequence Scrambling in Collision-Induced Dissociation of y-Type Fragment Ions

Sequence scrambling from y-type fragment ions has not been previously reported. In a study designed to probe structural variations among b-type fragment ions, it was noted that y fragment ions might also yield sequence-scrambled ions. In this study, we examined the possibility and extent of sequence-scrambled fragment ion generation from collision-induced dissociation (CID) of y-type ions from four peptides (all containing basic residues near the C-terminus) including: AAAAHAA-NH₂ (where “A” denotes carbon thirteen (¹³C₁) isotope on the alanine carbonyl group), des-acetylated-α-melanocyte (SYSMEHFRWGKPV-NH₂), angiotensin II antipeptide (EGVYVHPV), and glu-fibrinopeptide b (EGVNDNEEGFFSAR). We investigated fragmentation patterns of 32 y-type fragment ions, including y fragment ions with different charge states (+1 to +3) and sizes (3 to 12 amino acids). Sequence-scrambled fragment ions were observed from ~50 % (16 out of 32) of the studied y-type ions. However, observed sequence-scrambled ions had low relative intensities from ~0.1 % to a maximum of ~12 %. We present and discuss potential mechanisms for generation of sequence-scrambled fragment ions. To the best of our knowledge, results on y fragment dissociation presented here provide the first experimental evidence for generation of sequence-scrambled fragments from CID of y ions through intermediate cyclic “b-type” ions.