Characterization of acid-induced protein conformational changes and noncovalent complexes in solution by using coldspray ionization mass spectrometry

Coldspray ionization (CSI) mass spectrometry, a variant of electrospray ionization (ESI) operating at low temperature (20 to −80°C), has been used to characterize protein conformation and noncovalent complexes. A comparison of CSI and ESI was presented for the investigation of the equilibrium acid-induced unfolding of cytochrome c, ubiquitin, myoglobin, and cyclophilin A (CypA) over a wide range of pH values in aqueous solutions. CSI and nanoelectrospray ionization (nanoESI) were also compared in their performance to characterize the conformational changes of cytochrome c and myoglobin. Significant differences were observed, with narrower charged-state distribution and a shift to lower charge state in the CSI mass spectra compared with those in ESI and nanoESI mass spectra. The results suggest that CSI is more prone to preserving folded protein conformations in solution than the ESI and nanoESI methods. Moreover, the CSI-MS data are comparable with those obtained by other established biophysical methods, which are generally acknowledged to be the suitable techniques for monitoring protein conformation in solution. Noncovalent complexes of holomyoglobin and the protein-ligand complex between CypA and cyclosporin A (CsA) were also investigated at a neutral pH using the CSI-MS method. The results of this study suggest the ability of CSI-MS in retaining of protein conformation and noncovalent interactions in solution and probing subtle protein conformational changes. Additionally, the CSI-MS method is capable of analyzing quantitatively equilibrium unfolding transitions of proteins. CSI-MS may become one of the promising techniques for investigating protein conformation and noncovalent protein-ligand interactions in solution.