Conformation of cytochrome c studied by deuterium exchange-electrospray ionization mass spectrometry

Deuterium exchange of bovine cytochrome c has been monitored by electrospray ionization mass spectrometry. Different charge-state distributions in the mass spectrum appear to represent different protein conformations, but rapid interconversion of the conformations can lead to a coincidence of the deuterium exchange rates. When interconversion is blocked, the conformation corresponding to higher m/z (lower charge) exchanges more slowly, indicating a tightly folded state. Furthermore, the data suggest that at least two conformations can have identical charge-state distributions, but have different exchange rates. Thus, neither charge-state distribution nor deuterium exchange rate alone is a sufficient indicator of protein conformation.