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

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.