Tandem Mass Spectrometry of Large Biomolecule Ions by Blackbody Infrared Radiative Dissociation

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Abstract
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 elevating the vacuum chamber to temperatures up to 215 °C. Rate constants for dissociation are measured and found to be independent of pressure below 10-7 Torr. This indicates that the ions are activated by absorption of blackbody radiation emitted from the chamber walls. Dissociation efficiencies as high as 100% are obtained. There is no apparent 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 kDa), did not dissociate at temperatures up to 200 °C. This method is highly selective for low-energy fragmentation, from which limited sequence information can be obtained. From the temperature dependence of the dissociation rate constants, Arrhenius activation energies in the low-pressure limit are obtained. The lowest energy dissociation processes for the singly and doubly protonated ions of bradykinin are loss of NH3 and formation of the b2/y7 complementary pair, with activation energies of 1.3 and 0.8 eV, respectively. No loss of NH3 is observed for the doubly protonated ion; some loss of H2O occurs. These results show that charge−charge interactions not only lower the activation energy for dissociation but also can dramatically change the fragmentation, most likely through changes in the gas-phase conformation of the ion. Dissociation of ubiquitin ions produces fragmentation similar to that obtained by IRMPD and SORI-CAD. Higher charge state ions dissociate to produce y and b ions; the primary fragmentation process for low charge state ions is loss of H2O.

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