Radical-induced dissociation leading to the loss of CO2 from the oxazolone ring of [b5 − H]&z.rad;+ ionsElectronic supplementary information (ESI) available. See DOI: 10.1039/c6cp01974a

• Main Authors: Lau, Justin Kai-Chi, Zhao, Junfang, Williams, Declan, Wu, Bai-Han Backen, Wang, Yating, Mädler, Stefanie, Saminathan, Irine S, Siu, K. W. Michael, Hopkinson, Alan C
• Format: Journal Article
• Published: 06.07.2016
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c6cp01974a

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