TANDEM MASS-SPECTROMETRY OF LARGE BIOMOLECULE IONS BY BLACKBODY INFRARED RADIATIVE DISSOCIATION

A new method for the dissociation of large ions formed by electrospray ionization is demonstrated, Ions trapped in a Fourier transform mass spectrometer at pressures below 10(-8) Torr are dissociated by elevati ng the vacuum chamber to temperatures up to 215 degrees C. Rate consta nts for dissociation are measured and found to be independent of press ure below 10(-7) Torr, This indicates that the ions are activated by a bsorption of blackbody radiation emitted from the chamber walls, Disso ciation efficiencies as high as 100% are obtained, There is no apparen t mass limit to this method; ions as large as ubiquitin (8.6 kDa) are readily dissociated, Thermally stable ions, such as melittin 3+ (2.8 k Da), did not dissociate at temperatures up to 200 degrees C. This meth od is highly selective for low-energy fragmentation, from which limite d sequence information can be obtained, From the temperature dependenc e of the dissociation rate constants, Arrhenius activation energies in the low-pressure limit are obtained, The lowest energy dissociation p rocesses for the singly and doubly protonated ions of bradykinin are l oss of NH3 and formation of the b(2)/y(7) complementary pair, with act ivation energies of 1.3 and 0.8 eV, respectively, No loss of NH3 is ob served for the doubly protonated ion; some loss of H2O occurs, These r esults show that charge-charge interactions not only lower the activat ion energy for dissociation but also can dramatically change the fragm entation, most likely through changes in the gas-phase conformation of the ion, Dissociation of ubiquitin ions produces fragmentation simila r to that obtained by IRMPD and SORI-CAD, Higher charge state ions dis sociate to produce y and b ions; the primary fragmentation process for low charge state ions is loss of H2O.