Electron capture dissociation of extremely supercharged protein ions formed by electrospray ionisation

The fragmentation of six protein ions formed by electrospray ionisation (8.6 to 66.5 kDa) using electron capture dissociation (ECD) in a 7 T Fourier transform ion cyclotron resonance mass spectrometer was investigated as a function of charge state. Addition of 1,2-butylene carbonate to electrospray solutions was used to form protein ions with extremely high charge densities. For all six proteins, cleavage sites at 85 to 99% of all inter-residue sites were identified and fragmentation efficiencies of between 75 and 95% were obtained from tandem mass spectra of the highest charge states that could be readily isolated under these conditions. For ECD of both relatively low and high charge states, many rarely reported complementary a/x and b/y sequence ions were identified, whereas complementary a/y ions were comparatively sparse. These data suggest that a ions that are more often associated with the formation of complementary y ions (via CO loss from b ions) can also be formed via direct cleavage of C_α–C_β peptide backbone bonds to yield complementary a and x ions. Overall, these data indicate that it should be possible to obtain highly efficient ion fragmentation and near complete sequence characterisation for proteins with masses greater than 66.5 kDa.