Radical-induced dissociation leading to the loss of CO2 from the oxazolone ring of [b5- H]˙(+) ions.
Macrocyclization is commonly observed in large bn(+) (n≥ 4) ions and as a consequence can lead to incorrect protein identification due to sequence scrambling. In this work, the analogous [b5- H]˙(+) radical cations derived from aliphatic hexapeptides (GA5˙(+)) also showed evidence of macrocyclization under CID conditions. However, the major fragmentation for [b5- H]˙(+) ions is the loss of CO2 and not CO loss, which is commonly observed in closed-shell bn(+) ions. Isotopic labeling using CD3 and (18)O revealed that more than one common structure underwent dissociations. Theoretical studies found that the loss of CO2 is radical-driven and is facilitated by the radical being located at the Cα atom immediately adjacent to the oxazolone ring. Comparable energy barriers against macrocyclization, hydrogen-atom transfer, and fragmentations are found by DFT calculations and the results are consistent with the experimental observations that a variety of dissociation products are observed in the CID spectra.
Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario M3J 1P3, Canada. ach@yorku.ca.
Justin Kai-Chi
JK
Lau
Junfang
J
Zhao
Declan
D
Williams
Bai-Han Backen
BH
Wu
Yating
Y
Wang
Stefanie
S
Mädler
Irine S
IS
Saminathan
K W Michael
KW
Siu
Alan C
AC
Hopkinson