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

Abstract

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 Cu .2+, 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 © 2004 John Wiley & Sons, Ltd.