Probing Folded Proteins and Intact Protein Complexes by Desorption Electrospray Ionization Mass Spectrometry

Native mass spectrometry (Native MS) enables the study of intact proteins as well as non-covalent protein-protein and protein-ligand complexes in their biological state. In this work we present the application of a prototype Waters DESI source for rapid surface measurements of folded and native protein structures. Ions with narrow charge state distribution (CSD), i.e. folded structures are observed in the spectra of protein samples with the molecular weight ranging from 8.6 kDa up to 66.4 kDa. Intact protein complexes of holo-myoglobin and tetrameric hemoglobin are also successfully detected from a surface. These results reveal that DESI could be gentle enough to detect compact structures and noncovalent bond interactions. We also examine whether unfolded proteins and protein complexes can refold during transient spray solvent-sample interactions during DESI. Our results from ion mobility experiments of standards of ubiquitin, cytochrome c and protein complex myoglobin indicate that such phenomenon may occur, presenting artificial native-like spectra. Nevertheless, the observation of hemoglobin tetramer is promising as it demonstrates the capability of DESI to maintain truly native structures.