Figure 4. ESI–MS/MS spectrum of the [M+2H]²⁺ 665.7 ion corresponding to the ANA⁴⁷¹CNSVIR, rat SERCA1 tryptic peptide derivatized by TG1.

(A) Fragment y” type ions are indicated in the peptide sequence. (B) The interval from the y”6 (1074.4) peak to the y”5 (588.4) peak is 486.0. This difference is consistent with the cysteine derivatized as TG1 plus water minus the methyl ester (Δ=+4 a.m.u.).