FIGURE 8.

Mass spectrometric confirmation of Cys-298 as the site of MSNB modification. A, fragmentation spectrum collected for a peptide spanning Lys-289 to Lys-306 containing a covalent adduct with compound 13 at Cys-298 is shown. The peptide ([M+H]+:2355) was observed as doubly and triply charged species (m/z = 1178.83, z + 2; m/z = 786.00, z + 3). The peptide showed an increase of +57 D relative to the unmodified material following carbamoylation with iodoacetamide. Cys-298 was the only residue modified by 13. B, structure of compound 13 alone following adduct formation with Cys-298 and after subsequent fragmentation.