Determination of the activation energy for unimolecular dissociation of a non-covalent gas-phase peptide:substrate complex by infrared multiphoton dissociation fourier transform ion cyclotron resonance mass spectrometry

Abstract

The activation energy for the unimolecular dissociation of a non-covalent supramolecular complex between an Artificial Cationic Receptor A ([Gua-Val-Val-Val-Amide] +, in which Gua is guanidiniocarbonyl pyrrole) and an Anionic Tetrapeptide B ([N-Acetyl-Val-Val-Ile-Ala] −) has been determined by measurement of the dissociation rate constant as a function of infrared CO 2 laser power density. Singly-charged quasimolecular [A + B + H] + ions are isolated, stored in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer, and irradiated by IR photons. The rate constant for dissociation of the non-covalent complex is determined at five different laser power densities. A plot of the natural logarithm of the first-order rate constant versus the natural logarithm of the laser power density yields a straight line, the slope of which provides an approximate measure of the activation energy (E a laser) for dissociation. E a laser is calculated by a relationship derived earlier by Dunbar and with a newly proposed equation by Paech et al. The results of the two approaches deliver significantly different activation energy values for the unimolecular dissociation of the non-covalent complex. We obtain E aI laser = 0.67 eV (Dunbar approximation) and E aII laser = 1.12 eV (Paech et al. approximation). Differences between the two approaches are discussed with respect to non-covalent complexes.