Effects of Transition Metal Ion Identity and π-Cation Interactions in Metal—Bis(Peptide) Complexes Containing Phenylalanine

Abstract

Electrospray ionization–tandem mass spectrometry was used to study the effects of the metal ion identity and π-cation interactions on the dissociation pathways of metal–bis(peptide) complexes, where the metal is either Mn2+, Co2+, Ni2+, Cu2+ or Zn2+; and the peptide is either FGGF, GGGG, GF or GG, where G is glycine and F is phenylalanine. The [(FGGF)(FGGF – H) + M2+]+ and [(GGGG)(GGGG – H) + M2+]+ complexes dissociated by losing one FGGF or GGGG, respectively. Relative binding affinities were measured using the cross-over points, where the parent and product ions were equal in ion abundance and a normalized-collision energy scale. The results indicate the relative binding affinities for FGGF and GGGG follow the same order with respect to the transition metal ion identity: Cu2+ < Ni2+ < Mn2+ ≈ Zn2+ < Co2+ and the π-cation interactions in the FGGF complex have a measureable stabilizing effect. In contrast, the main fragmentation channels of [(GF)(GF – H) + M2+]+ and [(GG)(GG – H) + M2+]+ are loss of CO2 and 2CO2 with the [(GF)(GF – H) + M2+]+ complex also exhibiting cinnamic acid, GF, residual glycine, cinnamate and styrene loss.