Preservation of Protein Zwitterionic States in the Transition from Solution to Gas Phase Revealed by Sodium Adduction Mass Spectrometry

Abstract

The structural characterization of proteins and their interaction network mapping in the gas phase highlights the need to preserve their most nativelike conformers in the transition from the solution to gas phase. Zwitterionic interactions in a protein are weak bonds between oppositely charged residues, which make an important contribution to protein stability. However, it is still not clear whether the native zwitterionic states of proteins can be retained or not when it is transferred from the solution to gas phase. Using the nonspecific Na+ adduction as a novel signature, here we show that the zwitterionic states of proteins can be preserved when a moderated droplet desolvation condition (temperature <30 °C) is used in native electrospray ionization mass spectrometry. The very low-level nonspecific metal adduction to proteins under such conditions also enables rapid and direct determination of the binding states of metal-binding proteins and sensitive detection of proteins from solutions containing highly concentrated involatile salts (e.g., 50 mM NaCl). We believe that our findings can be instructive for performing mass spectrometric analysis of proteins and useful for protein ions desalting which simply involves altering the temperature and flow rate of drying gas in the desolvation region.