Radical-induced dissociation leading to the loss of CO sub(2) from the oxazolone ring of [b sub(5) - H]&z.rad; super(+) ions

• Published in: Physical chemistry chemical physics : PCCP Vol. 18; no. 27; pp. 18119 - 18127
• Main Authors: Lau, Justin Kai-Chi, Zhao, Junfang, Williams, Declan, Wu, Bai-Han Backen, Wang, Yating, Maedler, Stefanie, Saminathan, Irine S, Siu, KWMichael, Hopkinson, Alan C
• Format: Journal Article
• Language: English
• Published: 01.07.2016
• Subjects: Aliphatic compounds; Carbon dioxide; Carbon monoxide; Energy of dissociation; Fragmentation; Mathematical analysis; Radicals; Spectra
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c6cp01974a

Macrocyclization is commonly observed in large b sub(n) super(+) (n greater than or equal to 4) ions and as a consequence can lead to incorrect protein identification due to sequence scrambling. In this work, the analogous [b sub(5) - H]&z.rad; super(+) radical cations derived from aliphatic hexapeptides (GA sub(5)&z.rad; super(+)) also showed evidence of macrocyclization under CID conditions. However, the major fragmentation for [b sub(5) - H]&z.rad; super(+) ions is the loss of CO sub(2) and not CO loss, which is commonly observed in closed-shell b sub(n) super(+) ions. Isotopic labeling using CD sub(3) and super(18)O revealed that more than one common structure underwent dissociations. Theoretical studies found that the loss of CO sub(2) is radical-driven and is facilitated by the radical being located at the C sub( alpha ) 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.