Charge location on gas phase peptides

Studying biological systems to determine structure has been performed by a number of analytical techniques, including electrospray ionization (ESI)/ion mobility spectrometry (IMS) with mass spectrometry (MS) detection. ESI/IMS/MS enables the determination of gas phase ionic molecular size and can be correlated to computational modeling for structural evaluation. In this study, a molecular modeling program (CHARMm) coupled with a novel method of experimentally determining ion radii with ambient pressure IMS/MS was utilized to determine the charge position on gas phase peptides. Molecular modeling predicted the relative sizes of several isomeric peptides previously separated by IMS in a nitrogen buffer gas, although the modeled radii were smaller than the experimental radii due to the large polarizability of the drift gas. To correct for the polarization of the ambient pressure gas through which the ions migrate in the ion mobility experiment, ionic radii in drift gases of differing polarizability were plotted as a function of drift gas polarizability. To compare with the zero-polarizability ionic radii (y-intercept of the linear regression), the modeling experiment mimicked conditions of the actual experiment and the modeled and measured ionic size matched well. Moreover, when all possible charge locations on the peptides were modeled, only one modeled structure matched the experimental data, indicating that the combination of modeled and mobility data can determine charge location on gas phase peptides. (Int J Mass Spectrom 219 (2002) 23–37) © 2002 Elsevier Science B.V. All rights reserved.