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Confirming the N-terminal tryptic peptide of the full-length spike protein. (A) HCD fragmentation of the doubly charged peptide. The yellow box corresponds to a pyroGlu modification, the blue C corresponds to carbamidomethylated Cys, and the gray box corresponds to a deamidated Asn. (B) ETD fragmentation of the doubly charged peptide. The 90 Da loss on the observed z7 ion corresponds to the radical loss of the alkylated cysteine side chain (•S–CH2–CO–NH2).32
