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

• Published in: Journal of the American Society for Mass Spectrometry 1997-08, Vol.8 (8), p.771-780
• Main Authors: Schnier, Paul D., Price, William D., Strittmatter, Eric F., Williams, Evan R.
• Format: Article
• Language: English
• Subjects: Activation energy; Alkali metals; Aminoacids, peptides. Hormones. Neuropeptides; Analytical, structural and metabolic biochemistry; Biological and medical sciences; Blackbody; Boltzmann distribution; Cations; Dimers; Entropy of activation; Fundamental and applied biological sciences. Psychology; Internal energy; Kinetics; Leucine; Mass spectrometry; Proteins; Rate constants; Reaction kinetics; Temperature dependence; Thermal stability
• ISSN: 1044-0305; 1879-1123
• DOI: 10.1016/S1044-0305(97)84129-3

The dissociation kinetics of protonated leucine enkephalin and its proton and alkali metal bound dimers were investigated by blackbody infrared radiative dissociation in a Fouriertransform 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 2O) + 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 ~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.