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Effects of Cations on Protein and Peptide Charging in Electrospray Ionization from Aqueous Solutions

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Journal of The American Society for Mass Spectrometry

Abstract

The effects of eight different cations with ionic radii between 69 and 337 pm on the charging of peptides and proteins with electrospray ionization from aqueous acetate salt solutions are reported. Significant adduction occurs for all cations except NH4 +, and the average protein charge is lower when formed from solutions containing salts compared with solutions without salts added. Circular dichroism and ion mobility results show the protein conformations are different in pure water compared with salt solutions, which likely affects the extent of charging. The average charge of protein and peptide ions formed from solutions with Li+ and Cs+, which have Gibbs solvation free energies (GSFEs) that differ by 225 kJ/mol, is similar. Lower charge states are typically formed from solutions with tetramethylammonium and tetraethylammonium that have lower GSFE values. Loss of the larger cations that have the lowest GSFEs is facile when adducted protein ions are collisionally activated, resulting in the formation of lower analyte charge states. This reaction pathway provides a route to produce abundant singly protonated protein ions under native mass spectrometry conditions. The average protein and peptide charge with NH4 + is nearly the same as that with Rb+ and K+, cations with similar GSFE and ionic radii. This indicates that proton transfer from NH4 + to proteins plays an insignificant role in the extent of protein charging in native mass spectrometry.

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Acknowledgments

The authors thank the National Institutes of Health (grant no. R01GM097357) for generous financial support. The authors also thank Dr. Haichuan Liu and Professor Ewa Witkowska of the UCSF Sandler-Moore Mass Spectrometry Core Facility for the use of the Synapt G2 instrument, and Professor Bryan A. Krantz for the use of the circular dichroism spectropolarimeter. The authors also thank Catherine A. Cassou for helpful discussions.

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

    Anna C. Susa, Daniel N. Mortensen & Evan R. Williams

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  1. Anna C. Susa
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Susa, A.C., Mortensen, D.N. & Williams, E.R. Effects of Cations on Protein and Peptide Charging in Electrospray Ionization from Aqueous Solutions. J. Am. Soc. Mass Spectrom. 25, 918–927 (2014). https://doi.org/10.1007/s13361-014-0864-5

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