EFFECTS OF CHARGE-STATE ON FRAGMENTATION PATHWAYS, DYNAMICS, AND ACTIVATION-ENERGIES OF UBIQUITIN IONS MEASURED BY BLACKBODY INFRARED RADIATIVE DISSOCIATION

Blackbody infrared radiative dissociation spectra of the (M + 5H)(5+) through (M + 11H)(11+) ions of the protein ubiquitin (8.6 kDa) formed by electrospray ionization were measured in a Fourier-transform mass s pectrometer, The 5+ ion dissociates exclusively by loss of water and/o r ammonia, whereas the 11+ charge state dissociates only by formation of complementary y and b ions. These two processes are competitive for intermediate charge state ions, with the formation of y and b ions in creasingly favored for the higher charge states, The y and b ions are formed by cleavage of the backbone amide bond on the C-terminal side o f acidic residues exclusively, with cleavage adjacent to aspartic acid favored. Thermal unimolecular dissociation rate constants for the dis sociation of each of these charge states were measured. From the tempe rature dependence of these rates, Arrhenius activation parameters in t he rapid energy exchange limit are obtained, The activation energies ( E(a)) and preexponential factors (A) for the 5+, 8+, and 9+ ions are 1 .2 eV and 10(12) s(-1), respectively, These values for the 6+ and 7+ i ons are 0.9-1.0 eV and 10(9) s(-1), and those for the 10+ and 11+ ions are 1.6 eV and 10(16)-10(17) s(-1). Thus, with the exception of the 5 + ion, the higher charge states of ubiquitin have larger dissociation activation energies than the lower charge states, The different A fact ors observed for production of y and b ions from different precursor c harge states indicate that they are formed by different mechanisms, ra nging from relatively complex rearrangements to direct bond cleavages, These results clearly demonstrate that the relative dissociation rate s of large biomolecule ions by themselves are not necessarily a reliab le indicator of their relative dissociation energies, even when simila r fragment ions are formed.