Charge-surface correlation in electrospray ionization of folded and unfolded proteins

Electrospray-ionization mass spectrometry (ESI-MS) is widely used for protein studies. It has been shown that the extent of protein ionization under nondenaturing conditions correlates well with the solvent-accessible surface area of the tridimensional structure, for either folded monomers or multimeric complexes. The goal of this study was to test whether this relation holds for unfolded proteins as well. In order to overcome the paucity of structural data, the server ProtSA was used to model the conformational ensembles of proteins in the unfolded state and generate estimates of the average solvent accessibility. The results are analyzed along with literature data or original measurements by ESI-MS. It is found that the charge-to-surface relation holds for proteins in the unfolded state, free from solvent effects. A double-log plot is derived, in close agreement with published data for folded proteins. These results suggest that the solvent-accessible surface area is a key factor determining the extent of protein ionization by electrospray, independent of the conformational state. This conclusion helps rationalizing conformational effects in protein ESI-MS. The here reported relation can be used to predict the average solvent accessibility and, hence, the state of folding of unknown proteins from ESI-MS data.