Critical Examination of Gas-Phase Protein Conformation/Multimer Ion Formation by Electrospray Ion Mobility-Mass Spectrometry

The generally accepted view of protein structure in the gas-phase is that protein ions produced by electrospray ionization (ESI) exist in a number of different states, and the resulting charge state distribution (CSD) and ion mobility spectrum is interpreted as evidence for protein ions retaining some memory of solution-phase conformation. Even with the inclusion of ion mobility information, reports of protein ion structure in the gas-phase are oftentimes in disagreement not only within the discipline but also as interpreted by other gas-phase techniques. The focus of this work will be to correctly distinguish truly different ion conformations formed by ESI versus homomultimeric complexes with the same m/z. The concentration of cytochrome c in solution was varied over a wide range, and the multiply charged multimers (MCMs) present in the ion mobility/mass spectrum were unambiguously assigned by m/z selection and dissociation prior to ion mobility/mass spectrometry analysis. The results revealed false negatives for protein oligomer formation and false positives for protein conformational states and no evidence that gas-phase cytochrome c ions retain memory of solution-phase conformation, characteristics of great importance for structural biology. The results also suggest that the total IM-MS distribution for a protein is the complex result of individual MCMs either surviving until detection (undissociated) or dissociating into lower order multimers or a number of product ions for each m/z.