Figure 5. In-source fragmentation of a juxtamembrane peptide under APPI.

The peptide (aa 104–119) corresponding to the C-terminal extremity of the third transmembrane domain and the N-terminal portion of the second extracellular loop of the tetraspanin CD9, was investigated by APPI under dopant assisted conditions using toluene and 9 eV photons. The portion of the peptide corresponding to the transmembrane domain is represented in bold. Abundant and intense fragment ions were detected in the mass spectrum corresponding to in-source fragmentation of the peptide. The intensities of each fragment species a-, b-, c- and y-ions were plotted in relation to the peptide sequence.