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. Author manuscript; available in PMC: 2019 Jun 27.
Published in final edited form as: J Am Soc Mass Spectrom. 2015 Aug 25;26(12):1981–1991. doi: 10.1007/s13361-015-1231-x

Figure 4.

Figure 4.

MS/MS spectrum of a peptide containing S-cyanocysteine from the tryptic database. This peptide is a fragment of thyroglobulin. The peptide was not labeled with TMAB tags, consistent with cyclization of the N-terminal cyano group. The observed ion was 525.73 and was 2+, corresponding to a mass of 1049.46 Da. The theoretical mass of CLETGEFAR is 1024.46, consistent with the addition of cyano on the Cys (25.00 Da). The expected tryptic fragment is DLFIPTCLETGEFAR, indicating cleavage at a non-tryptic site probably due to cyanide-mediated chemical cleavage of the peptide. The source of the cyanide is unknown; none of the reagents are known to contain cyanide.