DISSOCIATION ENERGETICS AND MECHANISMS OF LEUCINE-ENKEPHALIN (M+H)(+)AND (2M+X)(+) IONS (X=H, LI, NA, K, AND RB) MEASURED BY BLACKBODY INFRARED RADIATIVE DISSOCIATION

The dissociation kinetics of protonated leucine enkephalin and its pro ton and alkali metal bound dimers were investigated by blackbody infra red radiative dissociation in a Fourier-transform mass spectrometer. F rom the temperature dependence of the unimolecular dissociation rate c onstants, Arrhenius activation parameters in the zero-pressure limit a re obtained. Protonated leucine enkephalin dissociates to form b(4) an d (M-H2O)(+) ions with an average activation energy (E-a) of 1.1 eV an d an A factor of 10(10.5) s(-1). The value of the A factor indicates s that these dissociation processes are rearrangements. The b(4) ions s ubsequently dissociate to form a(4) ions via a process with a relative ly high activation energy (1.3 eV), but one that is entropically favor ed. For the cationized dimers, the thermal stability decreases with in creasing cation size, consistent with a simple electrostatic interacti on in these noncovalent ion-molecule complexes. The E-a and A factors are indistinguishable within experimental error with values of similar to 1.5 eV and 10(17) s(-1), respectively. Although not conclusive, re sults from master equation modeling indicate that all these BIRD proce sses, except for b(4) --> a(4), are in the rapid energy exchange limit . In this limit, the internal energy of the precursor ion population i s given by a Boltzmann distribution and information about the energeti cs and dynamics of the reaction are obtained directly from the measure d Arrhenius parameters. (C) 1997 American Society for Mass Spectrometr y.