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 12Call 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