Electron molecule interactions of amino acids and peptides Public Deposited



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  • Electron-biomolecule interactions are a biologically relevant field of study because there are several ion fragmentation techniques that have come to the forefront in mass spectrometric analysis that involve the interaction of charged peptides and free electrons. By studying the electron energies that cause fragmentation in neutral amino acids, amino acid derivatives, and peptides, new insights into the mechanisms of biomolecule damage and ion fragmentation have been gained. Low energy electron capture experiments performed on a standard mass spectrometer using a specifically designed probe for non-volatile compounds provided proof of principle for low energy electron capture leading to peptide fragments. The largest assigned ion from a peptide was the z7-1 ion from the peptide substance P. For peptides with carboxylic acids on the C-terminus, z-1 ions were observed along with (M-H)- ions for peptides smaller than a hexamer. The assignment of the ions produced by low energy electron capture to c and z-1 ions was supported by high resolution low electron energy negative ion mass spectrometry with the alanine dimer. Using resonance electron capture – mass spectrometry, the amino acid esters, i.e. ethyl, isopropyl, and t-butyl esters of glycine, alanine, and phenylalanine were studied and these showed effective yield peaks at 3.5-3.7 eV and 8.8–9.5 eV for the carboxylate negative ions that were not observed for the underivatized amino acids or their methyl esters. The effective yield peaks in the carboxylate negative ions are attributed to electronically excited Feshbach resonances. N-Acetyl amino acids and peptides show strong ions with effective yield maxima at 1-2 eV electron energy that are due to shape resonances of the π* orbital. The dominant ions are either the (M-H)- or z-1 ions for the compounds with cterminal carboxylic acids. Their methyl esters show c ions as the dominant fragments. The cleavage of the peptide backbone involves π* - σ* orbital mixing because capture occurs by the π orbitals but sigma bonds are ultimately cleaved. The general conclusion of this thesis is that electrons with energy between 1-2 eV are captured by shape resonances of the π* orbital at the amide bond leading to formation of z-1 and c type ions.
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