Multiple solution structures of the disordered peptide indolicidin from IMS-MS analysis

The solution-favored conformations of the 13-residue disordered peptide, indolicidin (Ile¹-Leu²-Pro³-Trp⁴-Lys⁵-Trp⁶-Pro⁷-Trp⁸-Trp⁹-Pro¹⁰-Trp¹¹-Arg¹²-Arg¹³), are evaluated using electrospray ionization (ESI) coupled to ion mobility spectrometry-mass spectrometry (IMS-MS). The ESI-IMS-MS distributions for the dominant [M+4H]⁴⁺ ions indicate that three populations of structures coexist in a range of aqueous to non-aqueous solutions (water:dioxane, water:trifluoroethanol, and water:hexafluoroisopropanol). Conformer types and their relative abundances change in response to different solution environments suggesting that the gas phase conformers reflect on the solution populations present in different solvent environments. Collisional activation of isolated gas phase conformations with IMS-IMS-MS experiments provides additional insight about the relative stabilities of different structural types in the absence of solvent. Simulated annealing studies suggest that proline configuration may be important for the presence of multiple conformations.