Formation of molecular radical cations of aliphatic tripeptides from their complexes with CuII(12-crown-4)

Molecular radical cations have proven to be difficult to generate from aliphatic peptides under electrospray ionization mass spectrometry (ESI-MS) conditions. For a family of small aliphatic peptides GGX, where X=G, A, P, I, L and V, these cations have been generated by electrospraying a mixture of Cu2+, 12-crown-4 and GGX in methanol/water. GGX(.+) is readily formed from the collision-induced dissociation (CID) of [Cu-II(12-crown-4)(GGX)](.2+). The formation of these aliphatic peptide radical ions from these complexes, in cases where it is not possible from the corresponding complexes involving a series of amine ligands instead of 12-crown-4, is likely due to the second ionization energy of the [Cu-I(12-crown-4)(GGX)](+) complex being higher than that of the corresponding [Cu-I(amine)(GGX)](+) complex. Using these 12-crown-4 complexes, GGI can be differentiated from the isomeric GGL by comparing the CID spectra of their [a(3)+H](.+) ions. Copyright (C) 2004 John Wiley Sons, Ltd.