Fragmentation of doubly-protonated peptide ion populations labeled by H/D exchange with CD3OD

摘要

Doubly-protonated bradykinin (RPPGFSPFR) and an angiotensin III analogue (RVYIFPF) were subjected to hydrogen/deuterium (H/D) exchange with CD3OD in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. A bimodal distribution of deuterium incorporation was present for bradykinin after H/D exchange for 90 s at a CD3OD pressure of 4 × 10⁻⁷ Torr, indicating the existence of at least two distinct populations. Bradykinin ion populations corresponding to 0–2 and 5–11 deuteriums (i.e., D0, D1, D2, D5, D6, D7, D8, D9, D10, and D11) were each monoisotopically selected and fragmented via sustained off-resonance irradiation (SORI) collision-induced dissociation (CID). The D0–D2 ion populations, which correspond to the slower exchanging population, consistently require lower SORI amplitude to achieve a similar precursor ion survival yield as the faster-reacting (D5–D11) populations. These results demonstrate that conformation/protonation motif has an effect on fragmentation efficiency for bradykinin. Also, the partitioning of the deuterium atoms into fragment ions suggests that the C-terminal arginine residue exchanges more rapidly than the N-terminal arginine. Total deuterium incorporation in the b₁/y₈ and b₂/y₇ ion pairs matches very closely the theoretical values for all ion populations studied, indicating that the ions of a complementary pair are likely formed during the same fragmentation event, or that no scrambling occurs upon SORI. Deuterium incorporation into the y₁/a₈ pseudo-ion pair does not closely match the expected theoretical values. The other peptide, doubly-protonated RVYIFPF, has a trimodal distribution of deuterium incorporation upon H/D exchange with CD₃OD at a pressure of 1 × 10⁻⁷ Torr for 600 s, indicating at least three distinct ion populations. After 90 s of H/D exchange where at least two distinct populations are detected, the D₀–D₇ ion populations were monoisotopically selected and fragmented via SORI-CID over a range of SORI amplitudes. The precursor ion survival yield as a function of SORI amplitude falls into two distinct behaviors corresponding to slower- and faster-reacting ion populations. The slower-reacting population requires larger SORI amplitudes to achieve the same precursor ion survival yield as the faster exchanging population. Total deuterium incorporation into the y₂/b₅ ion pairs matches closely the theoretical values over all ion populations and SORI amplitudes studied. This result indicates the y₂ and b₅ ions are likely formed by the same mechanism over the SORI amplitudes studied.