Characterization of chiral host-guest complexation in fast atom bombardment mass spectrometry.

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).