Mass spectrometry of natural and synthetic peptide derivatives

INTRODUCTION Preliminary studies on the use of mass spectrometry for the determination of amino acid sequences in peptides have been published by Biemann a al.1' 2 who reduced the peptide bonds and the terminal carboxyl by LiA1H4 or LiA1D4 to obtain polyamino-alcohols or the corresponding polyamines. For the purpose of obtaining sufficiently volatile derivatives, all later authors have used N-acyl derivatives of oligopeptide methyl (or ethyl, or t-butyl) esters. Thus, Stenhagen3 and Weygand et al.4 have obtained mass spectra of N-trifluoroacetyl methyl esters of oligopeptides up to pentapeptides and Manusadzhyan et al.5 have used N-acetyl oligopeptide ethyl esters. With a direct inlet system, Heyns and Grutzmacher6'7 have studied N-acetyl peptides and have obtained a fairly satisfactory sequential splitting, the highest peptide measured being a pentapeptide derivative of mass 539. Detailed studies of the fragmentation of peptides and their derivatives have been performed by Weygand et al.8, and Shemyakin et al.'°, the latter having especially studied depsipeptides and cyclic peptides'1' ha, The usefulness of mass spectrometry for the determination of the sequence of amino acid residues was clearly recognized in 1984, due to structural studies on natural peptidolipids12. Since then mass spectrometry has become a very helpful tool for correcting12' 13, or confirming'4 the structure of oligopeptide derivatives established by classical analytical methods; the most interesting application of mass spectrometry in this field is, however, the determination of the sequence of amino acids in oligopeptides and their derivatives. The minute quantities (10—100 g) needed for obtaining a mass spectrum and the rapidity of the measurements as well as the possibilities of automation compete favourably with other well established methods. We shall not go into details of fragmentation mechanisms, our principal aim being to try to show how the "normal" splitting of the peptide bond can be used for sequence determination. For recent reviews see references 15—19.