Small Emitter Tips for Native Mass Spectrometry of Proteins and Protein Complexes from Nonvolatile Buffers That Mimic the Intracellular Environment

Salts are often necessary to maintain the native structures and functions of many proteins and protein complexes, but many buffers adversely affect protein analysis by native mass spectrometry (MS). Here, protein and protein complex ions are formed directly from a 150 mM KCl and 25 mM Tris-HCl buffer at pH 7 that is widely used in protein chemistry to mimic the intracellular environment. The protein charge-state distributions are not resolved from electrospray ionization MS using 1.6 μm diameter emitter tips, resulting in no mass information. In contrast, the charge-state distributions are well-resolved using 0.5 μm tips, from which the masses of proteins and protein complexes can be obtained. Adduction of salt to protein ions decreases with decreasing tip size below ∼1.6 μm but not above this size. This suggests that the mechanism for reducing salt adduction is the formation of small initial droplets with on average fewer than one protein molecule per droplet, which lowers the salt:protein ratio in droplets that contain a protein molecule. This is the first demonstration of native mass spectrometry of protein and protein complex ions formed from a buffer containing physiological ionic strengths of nonvolatile salts that mimics the intracellular environment, and this method does not require sample preparation or addition of reagents to the protein solution before or during mass analysis.