Structures and Dissociation Products of Ce/PeptideComplexes: Competition between Coordination and Charge Delocalization

Structures of [Ce(GGG)]3+ and [Ce(GGG – H)]2+ have been investigated by DFT calculations. The two lowest-energy structures of the triply-charged metal complex have the peptide in either the iminol or conventional zwitterionic form and these ions have almost identical energies. In the doubly-charged complex, the iminol and charge-solvated structures are the best structures on the potential energy surface but the latter is favored. In both iminol structures, the metal ion coordinates to the iminol oxygen rather than to the nitrogen, unlike in previously reported iminol-containing complexes. Triply-charged [Ce(peptide)]3+ complexes are fragile and are not easily isolated in a mass spectrometer whereas the doubly-charged [Ce(peptide – H)]2+ complexes are more robust. Here, we studied the fragmentations of 37 [Ce(peptide – H)]2+ and 30 [Ce(peptide)(peptide – H)]2+ complexes and the results are systematically summarized. Loss of CO and/or H2O are the most commonly observed fragmentation channels for the [Ce(pep...