Designing copper(II) ternary complexes to generate radical cations of peptides in the gas phase: Role of the auxiliary ligand

A series of ternary copper(II) complexes of the type [Cu ^II(L)(M)]²⁺, where M represents the hexapeptides GGGFLR, YGGFLR and WGGFLR and L a set of 12 nitrogen donor ligands have been evaluated for their ability to form cationic peptide radicals, M^+̇, in the gas phase. Although the fragmentation chemistry of these ions is complex, two main conclusions emerge: (i) Complexes containing a tri- or tetra-dentate ligand were found to be more effective at producing the peptide radical because in these instances competitive loss of the ligand from the complex is inhibited; (ii) The ligands ought not possess any acidic protons in order to prevent competitive loss of the protonated peptide, [M + H]⁺. There is significant interaction of the N-terminal aromatic residues in YGGFLR and WGGLFR with the copper(II) ion in several of the complexes as revealed by the formation of [Cu^I(L)(p-quinomethide)]⁺ and [Cu ^I(L)(3-methyleneindoline)]⁺ fragment ions. Following its dissociation from the ternary complex, CID of the YGGFLR^+̇ radical cation shows a dependence on the ligand in the complex from which it was formed. This 'memory effect' most likely reflects differences in the coordinated peptide structure induced by the ligand in the precursor complex which are maintained following dissociation.