Atmospheric pressure ion mobility spectrometry of protonated and sodiated peptides.

A number of peptides were studied with electrospray ionization--ion mobility spectrometry/mass spectrometry (ESI-IMS/MS). The ion mobility data were used to calculate the average collision cross sections of the different detected peptide ions in the nitrogen drift gas. By comparing the cross sections of related ions, structural information about the most probable location of the charge and the gas-phase ion conformations was deduced. For bradykinin and kemptide, a significant mobility difference between protonated and sodiated species (where sodium replaced a proton in singly and doubly charged peptides) was demonstrated. Surprisingly, the doubly charged sodiated peptides had a smaller collision cross section than the doubly charged protonated ones leading to the conclusion that the gas-phase conformations of these ions are different with respect to intramolecular interactions.