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.
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 CO2 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 (Ea(laser)) for dissociation. Ea(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 EaI(laser) = 0.67 eV (Dunbar approximation) and EaII(laser) = 1.12 eV (Paech et al. approximation). Differences between the two approaches are discussed with respect to non-covalent complexes.
Institute for Organic Chemistry, University Cologne, Köln, Germany. mathias.schaefer@uni-koeln.de
Mathias
M
Schäfer
Carsten
C
Schmuck
Martin
M
Heil
Helen J
HJ
Cooper
Christopher L
CL
Hendrickson
Michael J
MJ
Chalmers
Alan G
AG
Marshall
D001920:Bradykinin / Q000737:chemistry; D017357:Cyclotrons; D004745:Enkephalins / Q000737:chemistry; D005583:Fourier Analysis; D007202:Indicators and Reagents; D007259:Infrared Rays; D007930:Leucine / Q000737:chemistry; D013058:Mass Spectrometry / Q000379:methods*; D009842:Oligopeptides / Q000138:chemical synthesis / Q000737:chemistry*; D018000:Receptors, Peptide / Q000737:chemistry
D004745:Enkephalins; D007202:Indicators and Reagents; D009842:Oligopeptides; D018000:Receptors, Peptide; D007930:Leucine; D001920:Bradykinin