An IMS-IMS threshold method for semi-quantitative determination of activation barriers: Interconversion of proline cis↔trans forms in triply protonated bradykinin.

Collisional activation of selected conformations by multidimensional ion mobility spectrometry (IMS-IMS), combined with mass spectrometry (MS), is described as a method to determine semi-quantitative activation energies for interconversion of different structures of the nonapeptide bradykinin (BK, Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg). This analysis is based on a calibration involving collision-induced dissociation measurements of ions with known dissociation energies (i.e., "thermometer" ions) such as leucine enkephalin, BK, and amino acid-metal cation systems. The energetic barriers between six conformations of [BK+3H]3+ range from 0.23 ±0.01 to 0.55 ±0.03 eV. Prior results indicate that the major peaks in the IMS distributions correspond to specific combinations of cis and trans configurations of the three proline residues in the peptide sequence. The analysis allows us to directly assess pathways for specific transitions. The combination of structural assignments, experimentally determined barrier heights, onset of the quasi-equilibrium region, and dissociation threshold are used to derive a semi-quantitative potential energy surface for main features of [BK+3H]3+.