The fragmentation of the other pseudopeptide bond present in the linker between the epsilon amino group of Lys and the β-Ala residue in the hydrolyzed linker (highlighted in blue, see Figure ) that would yield the ([ 70 A-K 87 ]+nH) n+ and (( 1 Z-K 16 )+169+nH) n+ fragment ions were not observed. It confirms that despite they are also a pseudopeptide bond itd fragmentation by CID is not favored like the cases of ([ 70 A-K 87 ]+71) 2+ and (( 1 Z-K 16 )+98) 1+ .