CONFORMER SELECTION OF PROTEIN IONS BY ION MOBILITY IN A TRIPLE QUADRUPOLE MASS-SPECTROMETER

Electrospray mass spectra of multiply charged protein molecules show t wo distinct charge state distributions proposed to correspond to a mor e highly charged, open conformational form and a lower charged, folded form. Elastic collisions carried out in the radiofrequency-only colli sion cell of a triple quadrupole mass spectrometer have dramatic effec ts on the appearance of the mass spectra. The different cross sectiona l areas of the conformers allow preferential selection of one charge s tate distribution over the other on the basis of ion mobility. Prefere ntial selection is dependent on the nature and pressure of the target gas as well as the nature of the protein. In the case of positively ch arged horse heart apomyoglobin (MW 16,951 da), a high charge state dis tribution centered around (M + 20H)20+ predominates at low target gas pressures and a second distribution centered around (M + 10H)10+ predo minates at high target gas pressures. Bimodal distributions are observ ed at intermediate pressures and, remarkably, charge states between th e two distributions are not effectively populated under most of the co nditions examined. Hard sphere collision calculations show large diffe rences in collision frequencies and in the corresponding kinetic energ y losses for the two conformational states and they demonstrate that t he observed charge state selectivity can be explained through elastic collisions.