A Direct Measurement of the Absolute Energies of Protonated Gly-Pro-Gly-Gly Conformations Using Ion Mobility Spectrometry and Guided Ion Beam Tandem Mass Spectrometry.

Threshold collision-induced dissociation of conformations selected by ion mobility spectrometry is described as a method to determine their absolute relative energies. Here, the method is demonstrated with the cis and trans conformers of the protonated tetrapeptide, Gly-Pro-Gly-Gly, which differ in the orientation of the peptide bond at the proline residue. The trans conformation was found to be more stable than the cis conformation by 4.5 ± 2.5 kJ mol^-1 at 0 K. Heating the molecule anneals it to the cis conformer, indicating it has a lower Gibbs energy at higher temperatures. These results are compared to theoretical values calculated here and from the literature. This experimental analysis is the first quantitative measurement of the relative stability of the conformers of a protonated peptide in the gas phase, which has far-reaching impacts on the selection of theoretical methods to describe the energetics of these systems.