FIGURE 9.
LC/MS analyses performed on peptide 5 treated with β-propiolactone. Two products (I and II) were observed by LC/MS containing one or two β-propiolactone molecules attached to the histidine residue, respectively. MS2 analyses revealed the peptide sequence and the attachment of β-propiolactone (+72 Da) (I). IIa, two dominant fragment ions (483.8 and 497.3) were observed during fragmentation of product II, which contains double attachment (+144 Da). IIb, multistage activation performed on fragment 497.3 provided full peptide sequence and the attribution of the modified histidine residue.