An investigation of the energetics of peptide ion dissociation by laser desorption chemical ionization fourier transform mass spectrometry

The energy dependence of competing fragmentation pathways of protonated peptide molecules is studied via laser desorption—chemical ionization in a Fourier transform ion cyclotron resonance spectrometer. Neutral peptide molecules are desorbed by the technique of substrate-assisted laser desorption, followed by post-ionization with a proton transfer reagent ion species. The chemical ionization reaction activates the protonated peptide molecules, which then fragment in accordance with the amount of excess energy that is deposited. Chemical ionization forms a protonated molecule with a narrower distribution of activation energy than can be formed by activation methods such as collision activated dissociation. Furthermore, the upper limit of the activation energy is well defined and is approximately given by the enthalpy of the chemical ionization reaction. Control over the fragmentation of peptide ions is demonstrated through reactions between desorbed peptide molecules with different reagent ion species. The fragmentation behavior of peptide ions with different internal energies is established by generation of a breakdown curve for the peptide under investigation. Breakdown curves are reported for the peptides Val-Pro, Val-Pro-Leu, Phe-Phe-Gly-Leu-Met NH₂, and Arg-Lys-Asp-Val-Tyr. The derived breakdown curve of Val-Pro has been fitted by using quasi-equilibrium Rice-Ramsperger-Kassel-Marcus theory to model the unimolecular dissociation of the protonated peptide to provide a better understanding of the mechanisms for the formation of fragment ions that originate from protonated peptides.