Practical Protein Chemistry - A. Darbre 1989

Application of electron impact mass spectrometry for determining the amino acid sequence of peptides and proteins
Interpretation of Mass Spectra
Introduction of a ²H-Label

To enhance the reliability of determining Amino acid sequences, Peptides are modified using ²H₆-acetic anhydride or ²H₃-methyl iodide instead of conventional Reagents. The analysis of mass spectra of these derivatives yields more reliable results. Consider the following example illustrating the above statement. An ion with m/z 126 in the mass spectrum of a permethylated acetylpeptide may belong to an N-terminal pyrrolidonecarboxylic acid residue, an asparagine residue formed As a result of N—C Cleavage, or an N-terminal Serine residue that has lost a methanol molecule. Analysis of the mass spectra of ²H-labeled derivatives allows for the unambiguous Determination of the nature of this ion. If unlabeled methyl iodide and ²H₆-acetic anhydride are used for the chemical modification of this peptide, the mass of the ion in question will remain unchanged in the first two cases, whereas in the third case, an ion with m/z 129 will be formed instead of the ion with m/z 126. If the peptide is modified with ²H₃-methyl iodide and unlabeled acetic anhydride, an ion with m/z 129 should form in the first two cases, and an ion with m/z 132 in the third case.