The scarce source of mobile protons in ([ 192 T-K 200 ]-( 1 Z-K 16 )) 2+ precursor ion makes the newly generated carboxyl group as the most prominent source of a mobile proton, therefore it destabilizes the pseudopeptide bond (Figure ) and in consequence linker fragment ions ([ 192 T-K 200 ]+71) + and (( 1 Z-K 16 )+98) + were detected as the two-most intense in the MS/MS spectrum (Figure ). The sequence coverage of ( 1 Z-K 16 ) was very modest (40 %). When the MS/MS is dominated by intense signals of the crosslinked peptide pair due to the linker fragmentation, the sequence coverage deduced from the backbone fragment ions is compromised. In this MS/MS spectrum other linker fragment ions ([ 192 16 ) + and (( 1 Z-K 16 ) -34) + , generated by the thioether bond fragmentation , as well as the fragmentation of the other linker pseudopeptide bond (highlighted in blue, see Figure ) were not observed.