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Effects of Metal Ion Adduction on the Gas-Phase Conformations of Protein Ions

  • Focus: Electron Transfer Dissociation: Research Article
  • Published:
Journal of The American Society for Mass Spectrometry

Abstract

Changes in protein ion conformation as a result of nonspecific adduction of metal ions to the protein during electrospray ionization (ESI) from aqueous solutions were investigated using traveling wave ion mobility spectrometry (TWIMS). For all proteins examined, protein cations (and in most cases anions) with nonspecific metal ion adducts are more compact than the fully protonated (or deprotonated) ions with the same charge state. Compaction of protein cations upon nonspecific metal ion binding is most significant for intermediate charge state ions, and there is a greater reduction in collisional cross section with increasing number of metal ion adducts and increasing ion valency, consistent with an electrostatic interaction between the ions and the protein. Protein cations with the greatest number of adducted metal ions are no more compact than the lowest protonated ions formed from aqueous solutions. These results show that smaller collisional cross sections for metal-attached protein ions are not a good indicator of a specific metal–protein interaction in solution because nonspecific metal ion adduction also results in smaller gaseous protein cation cross sections. In contrast, the collisional cross section of α-lactalbumin, which specifically binds one Ca2+, is larger for the holo-form compared with the apo-form, in agreement with solution-phase measurements. Because compaction of protein cations occurs when metal ion adduction is nonspecific, elongation of a protein cation may be a more reliable indicator that a specific metal ion–protein interaction occurs in solution.

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Acknowledgments

The authors gratefully acknowledge financial support from the National Institutes of Health (grants no. R01GM096097 and no. 5F32GM093593-02 for fellowship support for S.I.M.). The authors also thank Dr. Haichuan Liu and Professor Ewa Witkowska of the UCSF Sandler-Moore Mass Spectrometry Core Facility for the use of their Synapt G2 instrument.

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  1. Tawnya G. Flick

    Present address: Department of Analytical Research and Development, Amgen, Inc., Thousand Oaks, CA, 91320, USA

Authors and Affiliations

  1. Department of Chemistry, University of California, Berkeley, CA, 94720-1460, USA

    Tawnya G. Flick, Samuel I. Merenbloom & Evan R. Williams

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  1. Tawnya G. Flick
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  2. Samuel I. Merenbloom
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Correspondence to Evan R. Williams.

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Flick, T.G., Merenbloom, S.I. & Williams, E.R. Effects of Metal Ion Adduction on the Gas-Phase Conformations of Protein Ions. J. Am. Soc. Mass Spectrom. 24, 1654–1662 (2013). https://doi.org/10.1007/s13361-013-0664-3

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