Chemical ionization mass spectrometry of complex molecules. IV. Amino acids

Abstract

The chemical ionization mass spectra of 21 of the 23 free "natural" amino acids have been measured. All give quasimolecular ions and simple fragmentation patterns which can be rationalized as arising from decomposition of the molecule protonated at several sites. We have recently reported3 the chemical ionization (Cl) mass spectra of a series of natural products, in particular alkaloids4 whose high proton affinity makes them especially susceptible to protonation by the strong acids CHS + and C2H5 + formed by electron bombbardment of methane at pressures of the order of 1 Torr.5This paper deals with the results obtained when this technique is applied to free amino acids. Earlier work in the area of electron impact (El) mass spectrometry of free amino acids,6-8 revealed that satisfactory spectra could be obtained with no contribution from diketopiperazines if either a direct insertion probe6,8 or a crucible ion source7 is used. In general however, the high ionizing energies led to mass spectra in which the molecular ion (M+) was often of very low abundance (< 1 % except for methionine, phenylalanine, and tryptophan) and in which many rearrangements and much homolysis were occurring. Even when recourse is made to ester derivatives,9-11 the molecular ion intensities are only 0.05-0.2% of the base peak except in the cases of methionine and the aromatic amino acids.As an adjunct to our work on the problem of peptide sequencing by Cl mass spectrometry12 we have studied the Cl mass spectra, with methane as the reactant gas, of all the common naturally occurring amino acids.These spectra have been measured on an MS-902 mass spectrometer equipped with the source described pre-viously8 91011using a direct insertion probe. Thus the high temperatures (~200-250°) required to vaporize the free amino acids have not been avoided but since ion formation is now carried out by protonation rather than the (1) Part III:

Fragmentations of Protonated Arginine, Lysine and Their Methylated Derivatives: Concomitant Losses of Carbon Monoxide or Carbon Dioxide and an Amine

“…Concomitant loss of CO plus H 2 O results in the pyrrolinium ion. 23 All other fragment ions are <10% in abundance; two of these are due to losses of 17 and 45 Da, which have been seen and interpreted previously in the fragmentation of protonated arginine and its derivatives as loss of NH 3 and the concomitant loss of CO plus NH 3 . The fragmentation chemistries and proposed structures are summarized in Scheme 4.…”

Fragmentation Reactions of Protonated α-Amino Acids

“…Specific lost of the &-nitrogen was noted previously in CI studies. 16 The observation of ammonia loss and water loss from protonated lysine is surprising since, in the absence of rearrangement, neither fragment ion has any particular claim to stability. This is especially true for [MH+ -H20]+, which is nominally an acylium ion.…”

Fast atom bombardment (FAB) and chemical ionization (CI) mass spectrometry of γ-carboxyglutamic acid, 5-substituted-4,4-dicarboxyprolines, and their monodecarboxy derivatives glutamic acid and 5-substituted-4-carboxyprolines

“…These results confirm the failure of this method in analyzing such very polar, nonvolatile, and thermolabile compounds; the CI technique provides limited information only on a few samples, and therefore it does not seem the ideal technique for dealing with such compounds, as it has been in studies on naturally occurring amino acids (14).…”

“…A significant and FAB behaviour of the compounds mentioned has been improvement in the FAB spectra of some compounds was obtained by investigated. The CI technique, which was successfully applied to the analysis of naturally occumng amino acids (14), failed in 'Present address: Istituto di Chimica Agraria, Facolti di Agraria, Universita degli Studi di Napoli, Via Universiti, 100, 80055 Portici, Napoli, Italy.…”