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

Abstract

The dissociation kinetics of protonated leucine enkephalin and its proton and alkali metal bound dimers were investigated by blackbody infrared radiative dissociation in a Fourier-transform mass spectrometer. From the temperature dependence of the unimolecular dissociation rate constants, Arrhenius activation parameters in the zero-pressure limit are obtained. Protonated leucine enkephalin dissociates to form b(4) and (M-H(2)O)(+) ions with an average activation energy (E(a)) of 1.1 eV and an A factor of 10(10.5) s(-1). The value of the A factor indicates that these dissociation processes are rearrangements. The b(4) ions subsequently dissociate to form a(4) ions via a process with a relatively high activation energy (1.3 eV), but one that is entropically favored. For the cationized dimers, the thermal stability decreases with increasing cation size, consistent with a simple electrostatic interaction in these noncovalent ion-molecule complexes. The E(a) and A factors are indistinguishable within experimental error with values of approximately 1.5 eV and 10(17) s(-1), respectively. Although not conclusive, results from master equation modeling indicate that all these BIRD processes, except for b(4) --> a(4), are in the rapid energy exchange limit. In this limit, the internal energy of the precursor ion population is given by a Boltzmann distribution and information about the energetics and dynamics of the reaction are obtained directly from the measured Arrhenius parameters.

Figure 1

Schematic diagram of the Berkeley external electrospray ion source Fourier-transform mass spectrometer showing the heated region of the vacuum chamber.

Figure 2

Blackbody infrared radiative dissociation spectrum of protonated leucine enkephalin at 203 °C and a reaction delay of 15 s; * denotes the 2nd harmonic of the (M + H)⁺ ion.

Figure 3

Appearance/depletion curves for protonated leucine enkephalin at (a) 156 °C and (b) 203 °C. The normalized fragment ion abundances are multiplied by 3.

Figure 4

BIRD spectra of leucine enkephalin (a) at 183 °C and a reaction delay of 30 s, (b) under the same conditions but with a single rf excitation to continuously remove the b₄ ion, and (c) continuous removal of the a₄ ion; *denotes the 2nd harmonic of the (M + H)⁺ ion.

Figure 5

Electrospray ionization mass spectrum of a ~10⁻⁴ M leucine enkephalin solution containing 0.5 μg/mL KOAc.

Figure 6

Dissociation data for (a) protonated leucine enkephalin and (b) for the b₄ ion formed by SORI-CAD of protonated leucine enkephalin, fit to unimolecular kinetics at the temperatures indicated.

Figure 7

Arrhenius plots for the dissociation of (a) protonated leucine enkephalin [all processes (filled circles), loss of water (filled diamonds), and formation of the b₄ fragment ion (open diamonds)], and the b₄ fragment ion (open circles) formed by SORI-CAD, and (b) leucine enkephalin (2M + X)⁺ dimers where X is indicated on the plot.

Figure 8

Arrhenius plots of the experimental data (filled circles) and master equation modeled fits (dash-dot line) of the dissociation of (a) protonated leucine enkephalin and (b) the proton bound dimer of leucine enkephalin. Dash line indicates master equation modeling using transition dipole moments at half their estimated value (see text).

Scheme I

Scheme II