H/D Exchange Kinetics: Experimental Evidence for Formation of Different b Fragment Ion Conformers/Isomers During the Gas-Phase Peptide Sequencing

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• 作者：Alireza Fattahi ; Behrooz Zekavat ; Touradj Solouki
• 刊名：Journal of the American Society for Mass Spectrometry
• 出版年：2010
• 出版时间：March 2010
• 年：2010
• 卷：21
• 期：3
• 页码：358-369
• 全文大小：1065 K

文摘

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.