Collision-induced dissociation is carried out for electrosprayed [Fe·GlyGlyHis-H], [Ni·GlyGlyHis-H], [Cu·GlyGlyHis-H], and [Zn·GlyGlyHis-H] complexes. [Fe·GlyGlyHis-H], [Ni·GlyGlyHis-H], and [Zn·GlyGlyHis-H] yield metal-bound peptide sequence ions and dehydrated ions as primary products, whereas [Cu·GlyGlyHis-H] generates a more extensive series of metal-bound sequence ions and a product arising from the unusual loss of a formaldehyde moiety; dehydration is significantly suppressed for this complex. Density functional theory calculations show that the copper ion-deprotonated peptide binding energy is substantially higher than those in other complexes, suggesting that there is a correlation between ion-ligand binding energy and their fragmentation behavior.

Relative proton affinities (PAs) of tyrosine, 3‐chlorotyrosine, and 3‐iodotyrosine were obtained using the kinetic method approach. The measured mean values are 922.5, 912.9, and 917.9 kJ/mol, respectively, with ± 0.1 kJ/mol standard deviation, indicating that halogenation of tyrosine decreases its PA. In general, PA of a molecule increases as its isotropic polarizability increases, but no such correlation has been found for the three molecules investigated in this study. Our findings show that PA decreases with increasing electronegativity of the modifying atom for a halogenated molecule, further supporting computational results of previous work [5].

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