Substance P in the Gas Phase: Conformational Changes and Dissociations Induced by Collisional Activation in a Drift Tube.

The work presented below is related to our companion paper in this issue, entitled: Substance P in solution: trans-to-cis configurational changes of penultimate prolines initiate non-enzymatic peptide bond cleavages. Two-dimensional ion mobility spectrometry (IMS-IMS) and mass spectrometry techniques are used to investigate structural transitions for [M+3H]3+ ions of substance P (subP) upon collisional activation (CA) in the gas phase. In this approach, different conformations of ions having a specified mobility are selected after an initial IMS separation, collisionally activated to produce new conformers, and these product structures are separated again using a second IMS region. In this way, it is possible to follow folding and unfolding transitions of different conformations. The analysis shows evidence for five conformations. Unlike other systems, every transition is irreversible. Studies as a function of activation voltage are used to discern pathways of structural changes prior to reaching the energy required for dissociation. Thresholds associated with the onsets of transitions are calibrated to obtain estimates of the energetic barriers between different structures and semi-quantitative potential energy diagrams are presented. Overall, barriers associated with structural transitions of [subP+3H]3+ in the absence of solvent are on the order of ~ 40 kJ mol-1, substantially lower than the ~ 90 kJ mol-1 required for some similar structural transitions in solutions of ethanol. Comparisons of the transition energies in the gas phase with thermochemistry for similar transitions in solution provide clues about why reverse transitions are prohibited. Graphical Abstract.