The chemistry of peptides related to metabolites of Trichoderma Spp. 3. An unusual dehydration reaction of 2-methylalanine peptides after electron impact ionization

Abstract

Can. J. Chem. 64, 277 (1986). Peptides of 2-methylalanine, where this residue is not the C-terminus, give molecular ions of low abundance on electron impact ionization, but abundant ions are formed by loss of the elements of water from the molecular ion. The dehydration reaction does not occur in permethylated derivatives, nor under the milder ionization conditions in the chemical ionization source. The reaction does not appear to be time or temperature dependent, but depends on the steric hindrance of the NH group of the 2-methylalanine residue. The rea.:tion occurs at sites where 2-methylalanine is not the C-terminal amino acid. In the cases of two pentapeptides where the dehydration reaction was of high probability, the correct sequence was found by semi-quantitative measurements of reaction rates in the first field-free region of a double focussing mass spectrometer.W. D. JAMIESON et A. TAYLOR. Can. J . Chem. 64, 277 (1986). Lors de la spectromCtrie de masse par ionisation sous impact Clectronique, les peptides de la mtthyl-2 alanine, dans lesquels ce rksidu n'est pas C-terminal, donnent des ions moltculaires dont l'abondance relative est faible; toutefois, il se forme des ions abondants par la perte des ClCments de l'eau 2 partir de l'ion moleculaire. La reaction de deshydratation ne se prodult pas ni avec les dCrivCs permCthylCs ni avec les conditions d'ionisation plus douces prkvalentes lors d'ionisations chimiques. I1 ne semble pas que la reaction depende du temps ou de la temptrature; toutefois, elle depend de llemp&chement stCrique du groupement NH du rCsidu de la mtthyl-2 alanine. La rtaction se produit a des sites oh la methyl-2 alanine n'est pas l'acide amin6 C-terminal. Dans les cas de deux pentapeptides pour lesquels la reaction de dtshydratation Ctait trks probable, on a pu ttablir la sequence correcte en effectuant des mesures semi-quantitatives de vitesses de reactions dans la prernikre region sans champ d'un spectromktre de masse i double focalisation. I [Traduit par le journal] 1Some aspects of the chemistry of the complex fungal I metabolites containing 2-methylalanine moieties have been I reviewed in a recent paper (1). Although these metabolites are resistant to catalytic hydrolysis by peptidases (2), there is a I considerable body of evidence which can best be interpreted by 1 assuming that they are polypeptides (3). Foremost among this evidence are mass spectroscopic studies on degradation products obtained by acid hydrolysis of metabolites (4). More iecentlv. use of fast atom bombardment (FAB) sources has . , provided impressive mass spectroscopic data, which has thrown light on the structure of several of the natural products (5). By contrast, mass spectroscopy of some N-acyl derivatives of synthetic peptides of 2-methylalanine gave anomalous results in the sense that molecular ions were not observed (6). For example, methyl L-prolyl-L-valyl-2-methylalanyl-2-methylalaninate (6), examined directly or as its N-chloroacetyl, N-trifluoroacetyl, N-deuterioacetyl, or phenylmethoxycarbonyl d...

Synthesis and Use of 2H-Azirin-3-amines as Dipeptide Synthons

“…The N-methylation (NaH/MeI in the presence of LiClO 4 ) failed in this case. An analogous case has already been reported in [29]. Therefore, other methods for the introduction of the N-Me group had Azirine 5 5 to be found, as N-Me amino acid esters are commercially not readily available.…”