Tandem ionization mass spectrometry of biomolecules

The novel mass spectrometric technique tandem ionization mass spectrometry (TIMS) employs irradiation of gas-phase even-electron molecular ions of bot h polarities with >10 eV electrons. This leads to the production of radical molecular cations and anions of large biomolecules. The parent even-electr on ions are produced by laser desorption, matrix-assisted laser desorption/ ionization CW and IR) or electrospray ionization and trapped in the cell of a Fourier transform mass spectrometer before irradiation with electrons, F or multiply-charged polypeptide cations (up to 16+ for cytochrome c) and di -anions, TIMS produced radical [M + nH]((n+1)+.) cations and previously unr eported [M - 2H](-.) anions, respectively, Subsequent collisional activatio n of these species, in contrast to their even-electron counterparts, gave o nly small neutral losses (mainly CO2) regardless of the ionic charge, polar ity or lability. This process was rationalized through intramolecular hydro gen atom transfer in cations, Measurements of the threshold energies for el ectron ejection has now been extended to the protonated porphyrin C76H94N4 [IE(MH+) = 12.8 +/- 0.3 eV] and to multiply-charged polypeptide cations and anions, Serendipitously, it was found that, in the absence of electrons, [ M + nH]((n+1)+.) polypeptide cations can also be formed in energetic collis ions during the ion isolation process in Fourier transform mass spectrometr y.