Protein folding mechanisms studied by time-resolved electrospray mass spectrometry.

Mass spectrometry (MS) owes its remarkable rise as a bioanalytical tool to the development of two “soft” ionization techniques: matrix-assisted laser desorption/ ionization (MALDI) (1,2) and electrospray ionization (ESI) (3). The salient feature of soft ionization is that a wide range of analytes, from small metabolites to very large biomolecular systems, can be transferred intact into the gas phase, thus making them amenable to mass spectrometric analysis. For MALDI-MS, the sample is embedded into a crystalline matrix prior to analysis. For ESI, in contrast, ionization is initiated directly from the liquid phase by spraying analyte solution from the tip of an electrically charged capillary. The small droplets formed by the ESI emitter undergo rapid evaporation and fi ssion, ultimately releasing the analyte as multiply protonated (or deprotonated) species (4).