Metal-Mediated Peptide Ion Conformations in the Gas Phase

The influence of metal cations and source temperature on the conformations of oxidized insulin chain A (ICA) anions has been investigated in the gas phase by high-resolution ion mobility techniques. Cross sections for non-metalated [ICA-nH](n-) (n = 2-6) ions show a distinct Coulomb-driven unfolding transition when four or more protons are removed. Studies of [ICA-6H + M2+](4-) ions (where M = Ca, Mn, Co, Ni, Cu, or Zn) show that metal peptide ions favor specific types of conformations that depend upon the binding properties of the metal. Doubly-charged metal ions appear to bind to multiple sites (presumably through interactions with -SO3-, -COO-, and amide groups on various amino acids) and have a pronounced effect on the number and shapes of stable conformations. Generally, [ICA-6H + M2+](4-) ions are found to be more compact than their [ICA-4H](4-) analogues. Studies of [ICA-5H + Na+](4-) ions show that addition of Na+ to the ICA anion has little effect on the peptide structure. The relative stabilities of different conformations for different metal ions are discussed in terms of simple qualitative potential energy surfaces.