A new method has been developed for the characterization of complexion between host and guest molecules. Adduct formation between chiral crown ethers 1 and 2 and enantiomeric ammonium ions 4 and 5 was examined. The reference compound 3 (achiral host) was chosen to be similar in structure to the chiral crown ethers for quantitative measurements. Our approach is based on a formalism assuming an equilibrium: [chiral host + H](+) + [achiral host + chiral guest](+) ⇌ [chiral host + chiral guest](+) + [achiral host + H](+). The equlibrium constant for this process was calculated using the relative peak intensities of the corresponding species in the FAB mass spectra. It was found that these provide significantly better reproducibility and more reliable results than the relative peak intensity method described before (Sawada, M.; et al. J. Am. Chem. Soc. 1992, 114, 4405; 1993, 115, 7381; Org. Mass Spectrom. 1993, 28, 1525).(1)(-)(3) In the examples studied, the equilibrium constants corresponding to the formation of heterochiral adducts (S,S-R or R,R-S) were higher than those for the formation of homochiral aggregates (S,S-S or R,R-R).
An intensity function combining peak intensities in the same manner as that in which equilibrium constants combine ion activities is suggested for characterization of enantioselectivity by fast-atom bombardment mass spectrometry. The intensity function combined with the use of two internal standards, one similar to the host, one to the guest compounds is shown to give excellent results: The enantioselectivity is better by up to factor of 10 than that obtained using one internal standard only, and its reproducibility is ca. a factor of 10 better, than that obtained using relative peak intensities (the RPI method).
Four lantibiotics namely epidermin, gallidermin, lanthiopeptin and mersacidin, have been studied by fast atom bombardment mass spectrometry. The molecular ion clusters of these compounds can be detected with reasonable abundance. The low-mass regions of the spectra show the presence of ions characteristic of the amino acids in the peptides. The mass distribution of the sequence ions provides information about the location of sulphur bridges the occurrence of which is a common feature of these kinds of molecules. The two isomeric compounds epidermin and gallidermin differ only in a leucine/isoleucine exchange at position 6. These two compounds can be distinguished on the basis of the tandem mass spectrum of m/z 86, the immonium ion of leucine and isoleucine.
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