Native Mass Spectrometry from Common Buffers with Salts That Mimic the Extracellular Environment

Abstract

Nonvolatile salts are essential for the structures and functions of many proteins and protein complexes but can severely degrade performance of native mass spectrometry by adducting to protein and protein complex ions, thereby reducing sensitivity and mass measuring accuracy. Small nanoelectrospray emitters are used to form protein and protein complex ions directly from high-ionic-strength (>150 mm) nonvolatile buffers with salts that mimic the extracellular environment. Charge-state distributions are not obtained for proteins and protein complexes from six commonly used nonvolatile buffers and ≥150 mm Na⁺ with conventionally sized nanoelectrospray emitter tips but are resolved with 0.5 μm tips. This method enables mass measurements of proteins and protein complexes directly from a variety of commonly used buffers with high concentrations of nonvolatile salts and eliminates the need to buffer exchange into volatile ammonium buffers traditionally used in native mass spectrometry.

Keywords: electrospray; ion desalting; mass spectrometry; proteins.

Figure 1

Electrospray ionization mass spectra of 5 µM avidin tetramer (a,e,i), concanavalin A (b,f,j), carbonic anhydrase (c,g,k) and myoglobin (d,h,l) ions formed from 150 mM aqueous ammonium acetate with 1.6 µm emitter tips (a–d) or phosphate buffered saline with 1.6 µm emitter tips (e–h) and 0.5 µm emitter tips (i–l)

Figure 2

Electrospray ionization mass spectra of 5 µM avidin tetramer ions formed from aqueous solutions containing 150 mM NaCl and 20 mM of the following buffers (a,f) Tris-HCl, (b,g) MOPS, (c,h) PIPES, (d,i) HEPES, (e,j) MES with 1.6 µm emitter tips (a–e) or 0.5 µm emitter tips (g–j)