Effects of size of noncovalent complexes on their stability during collision-induced dissociation

The size-dependent stability of noncovalent complexes under collision-induced dissociation (CID) conditions was studied on a quadrupole ion trap mass spectrometer. Complexes of heme, tetraphenylporphyrin iron(III) (TPP-Fe), and tetraphenylporphyrin manganese(III) (TPP-Mn) with several histidine-containing peptides and model compounds were formed by electrospray ionization (ESI) and their stability was probed by variable-energy CID. It was found that the stability of complexes with the same (or nearly the same) binding energy has a linear dependence on the complex size (or total number of degrees of freedom). This approach will allow comparisons of variable-energy CID data for noncovalent complexes with different binding energies and could be used to help in structural elucidation of some complexes formed by multidentate ligands. The linearity of size effects on the stability of the complexes was also tested in three crown ether/protonated primary amine systems.