Investigation of gas phase ion structure for proline-containing b{sub 2} ion
Unusual fragmentation was observed for doubly charged VPDPR in which cleavage C-terminal to proline and N-terminal to aspartic acid yielded b{sub 2} (+ a{sub 2})/y{sub 3} complementary ions. This unique fragmentation is contradictory to trends previously established by statistical analysis of peptide tandem mass (MS/MS) spectra [1–4]. Substitution of alanine for aspartic acid (i.e., VPAPR) did not change the fragmentation, indicating the cleavage was not directed by aspartic acid. Fragmentation patterns for VPAPR and V(NmA)APR (NmA = N-methyl alanine) were compared to determine whether conformational constraints from proline’s cyclic side-chain contribute to b{sub 2} ion formation. While both peptide sequences fragmented to yield b{sub 2}/y{sub 3} ions, only VPAPR produced a{sub 2} ions, suggesting the VP b{sub 2} ion is structurally different from the V(NmA) b{sub 2} ion. Instead, the V(NmA) b{sub 2} ion was accompanied by an ion corresponding to formal loss of 71. The suspected structural differences were confirmed by isolation and fragmentation of the respective b{sub 2} ions (i.e., MS{sup 3} spectra). Evidence supporting a diketopiperazine structure for the VP b{sub 2} ion is reported. Fragmentation patterns for the VP b{sub 2} ion and a synthetic VP diketopiperazine showed great similarity. N-terminal acetylation of VPAPR prevented the formation of the VP b{sub 2} ion, presumably by blocking nucleophilic attack by the N-terminal amine on the carbonyl oxygen of the protonation site. Acetylation of the N-terminus for V(NmA)APR did not prevent the formation of the V(NmA) b{sub 2} ion, indicating the V(NmA) b{sub 2} ion has a structure, presumably that of an oxazolone, which requires no attack by the N-terminus for formation. Finally, high-resolution, accurate mass measurements determined that the V(NmA) (b{sub 2} − 71) ion results from losing a portion of valine from oxazolone V(NmA) b{sub 2} ion, rather than cross-ring cleavage of the alternate diketopiperazine.
- OSTI ID:
- 22774097
- Journal Information:
- Journal of the American Society for Mass Spectrometry, Journal Name: Journal of the American Society for Mass Spectrometry Journal Issue: 1 Vol. 17; ISSN 1044-0305; ISSN JAMSEF
- Country of Publication:
- United States
- Language:
- English
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