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Conformational Changes with Charge State

Protein ions formed by electrospray ionization usually are observed as a distribution of charge states. As charge state increases for a given protein, the coulombic repulsion energy must also increase. For high charge states, an insufficient number of intramolecular stabilizing interactions are possible to maintain a compact conformation. The result is coulomb-induced unfolding.

As an example of this, the collision cross sections observed for cytochrome c are plotted below as a function of charge state. For low charge states (+3 and +4), the cross section is close to that calculated for coordinates of the protein crystal structure. For charge states +5 to +9, multiple conformations are observed, indicating a range of progressively more unfolded conformers. Higher charge states (>= +10) have cross sections that correspond to relatively open conformations that have little or no tertiary structure.