Rapid desalting during electrospray ionization mass spectrometry for investigating protein-ligand interactions in the presence of concentrated salts.

Investigation of protein-ligand interactions in physiological conditions is crucial for better understanding of biochemistry because the binding stoichiometry and conformations of complexes in biological processes, such as various types of regulation and transportation, could reveal key pathways in organisms. Nanoelectrospray ionization mass spectrometry is widely used in studies of biological processes and systems biology. However, non-volatile salts in biological fluid may adversely interfere with nanoelectrospray ionization mass spectrometry. In this study, the previously developed method of induced nanoelectrospray ionization was used to facilitate in situ desalting of protein in solutions with high concentrations of non-volatile salts, and direct investigation of protein-ligand interactions for the first time. In situ desalting occurred at the tip of emitters within a short period lasting for a few to tens of milliseconds, enabling the maintenance of nativelike conditions compatible with mass spectrometry measurements. Induced nanoelectrospray ionization was driven by pulsed potential and exhibited microelectrophoresis effect in each spray cycle, which is not observed in conventional nanoelectrospray ionization because the continuous spray procedure is driven by direct current. Microelectrophoresis caused desalting through micron-sized spray emitters (1-20 μm), as confirmed experimentally with proteins in 100 mM NaCl solution. The method developed in this study has been further illustrated as a potential option for fast and direct identification of protein-ligand (small molecules or metal ions) interactions in complex samples. The results of this study demonstrate that the newly developed method may represent a reliable approach for investigations of proteins and protein complexes in biological samples.