Gas phase activation energy for unimolecular dissociation of biomolecular ions determined by focused RAdiation for gaseous multiphoton ENergy transfer (FRAGMENT)

We present a novel approach for the determination of activation energy for the unimolecular dissociation of a large (>50 atoms) ion, based on measurem ent of the unimolecular dissociation rate constant as a function of continu ous-wave CO2 laser intensity. Following a short (similar to 1s) induction p eriod, CO2 laser irradiation produces an essentially blackbody internal ene rgy distribution, whose 'temperature' varies inversely with laser intensity . The only currently available method for measuring such activation energie s is blackbody infrared radiative dissociation (BIRD), Compared with BIRD, FRAGMENT: (a) eliminates the need to heat the surrounding ion trap and vacu um chamber to each of several temperatures teach requiring hours for temper ature equilibration); (b) offers a three-fold wider range of effective blac kbody temperature; and (c) extends the range of applications to include ini tially cold ions (e.g., gas-phase H/D exchange). Our FRAGMENT-determined ac tivation energy for dissociation of protonated bradykinin, 1.2 +/- 0.1 eV, agrees within experimental error to the value, 1.3 +/- 0.1eV, previously re ported by Williams et al, from BIRD experiments. Copyright (C) 1999 John Wi ley & Sons, Ltd.