Figure 2.

Labeling procedure for the detection of crosslinked peptides. (A) Crosslinked proteins are digested with trypsin in buffer containing H₂¹⁸O at 25 atom percent excess. All resulting peptides become ¹⁸O labeled on their C-terminus. (B) Four possible types of peptides are produced: (i) unlinked peptides without crosslinker; (ii) dead-end peptides that contain hydrolyzed or aminolyzed crosslinker; (iii) loop-linked peptides where two lysines on a single peptide are linked to each other; (iv) interlinked peptides. Interlinked peptides are distinguished from the other peptides because they have twice the amount of labeling on their C-termini.