From Solution to the Gas Phase: Stepwise Dehydration and Kinetic Trapping of Substance P Reveals the Origin of Peptide Conformations

Past experimental results and molecular dynamics simulations provide evidence that, under some conditions, electrospray ionization (ESI) of biomolecules produces ions that retain elements of solution phase structures. However, there is a dearth of information regarding the question raised by Breuker and McLafferty, "for how long, under what conditions, and to what extent, can solution structure be retained without solvent?" (Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 18145). Here, we use cryogenic ion mobility-mass spectrometry to experimentally probe the structural evolution of the undecapeptide substance P (SP) during the final stages of ESI. The results reveal that anhydrous SP conformers originate from evaporation of cluster ions, specifically, SP + 2H(2+) (H2O)n (n = 0 to ∼50) and SP + 3H(3+) (H2O)n (n = 0 to ∼30), and that major structural changes do not occur during the evaporative process. In the case of SP + 3H(3+), the results demonstrate that a compact dehydrated conformer population can be kinetically trapped on the time scale of several milliseconds, even when an extended gas phase conformation is energetically favorable.