Collisionally activated dissociation of supercharged proteins formed by electrospray ionization

The dissociation of protein ions formed by ESI ranging in size from 12 to 29 kDa using sustained off-resonance irradiation collisional activation was investigated as a function of charge state in a 9.4-T Fourier transform mass spectrometer. Addition of m-nitrobenzyl alcohol to denaturing solutions of proteins was used to form very high charge states of protein ions for these experiments. For all proteins in this study, activation of the highest charge state results in a single dominant backbone cleavage, often with less abundant cleavages at the neighboring 3-5 residues. This surprising phenomenon may be useful for the "top-down" identification of proteins by producing sequence tags with optimum sensitivity. There is a slight preference for cleavage adjacent to acidic residues and proline. Solution-phase secondary structure does not appear to play a significant role. The very limited dissociation channels observed for the supercharged ions may be due, in part, to the locations of the charges on the protein.