Combined ion-mobility and mass-spectrometry investigations of metallothionein complexes using a tandem mass spectrometer with a segmented second quadrupole.

Rabbit metallothionein (MT) 2A complexes with Cd(II), Zn(II), Ag(I), Cu(I), Hg(II), arsenite, monomethylarsonous acid (MMA), and dimethylarsinous acid (DMA) have been examined using ion-mobility measurements and mass spectrometry in a triple-quadrupole mass spectrometer equipped with a segmented second quadrupole that doubled as an ion-mobility cell [Guo, Y.; Wang, J.; Javahery, G.; Thomson, B. A.; Siu, K. W. M. An Ion-Mobility Spectrometer with Radial Collisional Focusing. Anal. Chem.2005, 77, 266-275]. The metal ions confer conformational rigidity on the MT complexes, which counteracts Coulombic repulsion among protons added as a result of electrospray. Triply and quadruply protonated Cd(7)MT2A have smaller cross-sections than the Cd(7)MT2A structure deduced from published NMR data. For the 6+ ions, the As(6)MT2A complex has a cross-section of 790 A(2); the MMA(10)MT2A complex, 920 A(2); and the DMA(20)MT2A complex, 1220 A(2). This increase in cross-section of the As(III) species, from As(3+) to MMA to DMA, is interpreted as a consequence of decreasing multiple coordination and increasing number of methyl groups.