Conformational dependence of polyproline metal binding using ion mobility mass spectrometry

Due to its unique amide-bonded cyclized functional group, proline has a higher propensity to form cis peptide bonds than other amino acids. This results in polyproline peptides forming two major conformations. Polyproline I (PPI) is a tight right-handed helix with only cis peptide bonds, which is favored in non-polar solvents. Polyproline II (PPII) contains only trans peptide bonds, forming a loose left-handed helix which is favored in aqueous solutions. This solvent dependance allows the folding pathways and intermediates between PPI and PPII to be explored by changing the polarity of the solution. Our lab has previously characterized the mechanisms for interconversion between PPI and PPII using ion mobility-mass spectrometry (IMS-MS). The present work examines the effect of metal ions on the PPI to PPII transition using IMS-MS. Multiple metalated and un-metalated conformers of polyproline-13 (Pro13) are identified and characterized as reactant, intermediate, or product states. Tracking these conformers as the transition proceeds shows that the formation of a Pro13-metal complex is conformation dependent as calcium and zinc only bind after several isomerizations have generated a suitable binding site. Parallel dissociation (or IMS-CID-MS) of metalated Pro13 indicates both metals bind in a loop formed by a trans peptide bond in the middle of the peptide. This characterization of transient intermediates and metalated conformations provides insight into the adaptation of peptide folding pathways to perturbations in the system.