Primary to quaternary protein structure determination with electrospray ionization and magnetic sector mass spectrometry

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Abstract
Electrospray ionization with a forward‐geometry magnetic sector mass spectrometer was used for collisionally activated dissociation studies of multiply charged polypeptides and for studying non‐covalently bound protein systems. The high‐resolution capabilities of a high‐performance instrument allow the resolution of isotopic contributions for product ions and molecular ion species. Determination of product ion charge states by this method reduces difficulties in the interpretation of product ion mass spectra from multiply charged precursors, which are generated either in the atmospheric pressure/vacuum electrospray interface or in the collision chamber of the mass spectrometer. Extended tandem mass spectrometric experiments have the potential for sequencing larger polypeptides. However, evidence for isomerization of gas‐phase product ions from substance P and substance P analogues was observed, complicating the interpretation of product ion spectra. Non‐covalent complexes can also be studied by electrospray ionization magnetic sector MS. The higher m/z range of such an instrument is a major advantage for studying weakly bound systems, such as heme–protein systems (myoglobin, hemoglobin) and protein aggregates (concanavalin A), because of their tendency to form complex ions with relatively low charge states.