Electrosonic spray ionization. A gentle technique for generating folded proteins and protein complexes in the gas phase and for studying ion-molecule reactions at atmospheric pressure.

Electrosonic spray ionization (ESSI), a variant on electrospray ionization (ESI), employs a traditional micro ESI source with supersonic nebulizing gas. The high linear velocity of the nebulizing gas provides efficient pneumatic spraying of the charged liquid sample. The variable electrostatic potential can be tuned to allow efficient and gentle ionization. This ionization method is successfully applied to aqueous solutions of various proteins at neutral pH, and its performance is compared to that of the nanospray and micro ESI techniques. Evidence for efficient desolvation during ESSI is provided by the fact that the peak widths for various multiply charged protein ions are an order of magnitude narrower than those for nanospray. Narrow charge-state distributions compared to other ESI techniques are observed also; for most of the proteins studied, more than 90% of the protein ions can be accumulated in one charge state using ESSI when optimizing conditions. The fact that the abundant charge state is normally as low or lower than that recorded by ESI or nanospray indicates that folded protein ions are generated. The sensitivity of the ionization technique to high salt concentrations is comparable to that of nanospray, but ESSI is considerably less sensitive to high concentrations of organic additives such as glycerol or 2-amino-2-(hydroxymethyl)-1,3-propanediol (Tris base). Noncovalent complexes are observed in the case of myoglobin, protein kinase A/ATP complex, and other proteins. The extent of dissociation of protein ions in ESSI is comparable to or even smaller than that in the case of nanospray, emphasizing the gentle nature of the method. The unique features of ESSI are ascribed to very efficient spraying and the low internal energy supplied to the ions. Evidence is provided that the method is capable of generating fully desolvated protein ions at atmospheric pressure. This allows the technique to be used for the study of ion-molecule reactions at atmospheric pressure and examples of this are shown.