Electrospray-ionization mass spectrometry for protein conformational studies

The possibility to study large molecules and their non-covalent interactions by mass spectrometry (MS) has opened novel ways to investigate protein folding and binding reactions. MS can be applied to protein conformational studies in two conceptually different ways. One approach uses MS to monitor mass changes produced by conformation-sensitive reactions, such as hydrogen/deuterium (H/D) exchange, alkylation and radiolysis. The second approach directly exploits the conformation dependence of the charge-state distributions (CSDs) of the multiply charged protein ions produced by electrospray-ionization (ESI). This review focuses on the information that has been provided by the latter kind of studies. An attempt is made to summarize and discuss the available evidence about the mechanism underlying this technique and its possible applications. The results of the studies described here include equilibrium and kinetic characterization of protein folding transitions and detection of folding intermediates. The case studies of myoglobin (Mb) and cytochrome c (cyt c) are discussed in particular detail. The unprecedented advantages offered by MS in the analysis of heterogeneous samples can now be applied to the study of dynamic systems involving different conformational states.