of the protein backbone which show reduced fragmentation in the presence of ligands, for example the N-terminal residues 13-46, residues 106-114, and some C-terminal residues from 130-146. Mapping these regions onto the canonical calmodulin-peptide structure (PDB 1C1M), Figure lower panel, shows that they all have close proximity or Hbonding to the peptide binding pocket, suggesting that solvent-accessibility reductions and non-covalent interactions drive the reduced fragmentation. A small difference in the dissociation pattern for the two ligand bound species is observed between residue 135-138, which could be attributed to the reduced length of CaMKII peptide (19 aa) compared to melittin (26 aa) altering the solvent accessibility in this region, however being a short stretch, it is difficult to be completely confident in this conclusion without supporting data.