FIGURE 6.

MS/MS analysis of a cross-linked product derived from a trypsin-digest of PKG, which was cross-linked with W45-[Phe^(Bz)-2]. A, the [M+3H]³⁺ precursor ion at m/z 649–652 that was selected by the quadrupole using a broad selection window (∼5 m/z units). The characteristic doublet is indicative for the presence of the isotopically labeled leucine from the photoaffinity-labeled peptide. B, MS/MS spectrum of the [M+3H]³⁺ at m/z 649–652. C, primary sequence of the cross-linked product and identified fragment ions. The b and y″ ions corresponding the primary sequence of PKG, without covalently attached inhibitor are annotated by b_n or y_n, whereas b or y″ ions of the primary sequence of PKG that carries the covalent attached photolabel are labeled *b_n or *y_n. The cysteine was found to carry a dithiothreitol adduct that was subsequently modified by iodoacetamide.