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 protonate d peptide molecules is studied via laser desorption-chemical ionizatio n 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 pr otonated 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 energ y than can be formed by activation methods such as collision activated dissociation. Furthermore, the upper limit of the activation energy i s well defined and is approximately given by the enthalpy of the chemi cal ionization reaction. Control over the fragmentation of peptide ion s is demonstrated through reactions between desorbed peptide molecules with different reagent ion species. The fragmentation behavior of pep tide ions with different internal energies is established by generatio n 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-NH,, and Arg-Lys-Asp-Val-Tyr. The experimentally derived brea kdown curve of Val-Pro has been fitted by using quasi-equilibrium Rice -Ramsperger-Kassel-Martus theory to model the unimolecular dissociatio n of the protonated peptide to provide a better understanding of the m echanisms for the formation of fragment ions that originate from proto nated peptides.