Previous studies into the dissociation of [CuII(dien)peptide].2+ ions (dien = diethylenetriamine) have shown that NH-containing auxiliary ligands do not favor the formation of [peptide].+ species; instead, they promote proton-transfer reactions, especially for peptides containing basic amino residues. Formation of radical cationic tripeptides of the form GGX.+ [GGX = glycylglycyl(residue X)] becomes feasible upon substituting the open-chain tridentate ligand dien with its analogous cyclic ligand, 1,4,7-triazacyclononane (9-aneN3); i.e., from [CuII(9-aneN3)GGX].2+ ions. Similar enhancements occur when using 1,4,7,10-tetraoxacyclododecane (12-crown-4) in place of its open-chain analog, 2,5,8,11-tetraoxadecane (triglyme). We have demonstrated that a sterically encumbered auxiliary macrocyclic ligand within [CuII(L)GGX].2+ complex ions [where L = 9-aneN3 or 12-crown-4] facilitates the formation of radical cationic peptides through gas-phase fragmentation. We verified our experimental observations by examining the reactivities of a series of 19 tripeptides of the type GGX that differ only in the identity of their C-terminal residue. The energy of the electron-transfer reaction correlates well with the bond-dissociation energy of the peptide–Cu(II) interaction; the presence of a constrained macrocyclic ligand weakens metal–peptide chelation through steric repulsion between the ligand and the peptide, and this situation may lead to more favorable radical cationic peptide formation. Copyright © 2006 John Wiley & Sons, Ltd.