Host-Guest FAB Mass Spectrometry between Carbohydarates and Organic Cations

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Enantioselective host-guest complexation has been demonstrated in FAB mass spectrometry. Among several carbohydrates, a modified beta-mannofuranoside 1 differentiates a chiral ammonium ion guest such as the 1-(1-naphthyl)ethylammonium ion as well as a chiral crown ether 2. The relative peak intensity (RPI) method has been developed to the evaluation of such diastereomeric complex ions where the peak intensity of the target host (M) - guest (A^+) complex ion [I(M+A^+)] is compared to that of the internal standard host (R) - guest (A^+) one [I(R+A^+)]: the RPI value = [I(M+A)^+/I(R+A)^+]. The present enantioselectivity is characterized by the RPI ratio value [I(M+A_<(R)>)^+/I(R+A_<(S)>)^+] in the range of 1.2 to 1.6 (1.0*0.1; non-enantioselectivity). The observed preference of the (R)-enantiomer guest is compatible with CPK model examinations and related coloration results. With a chiral crown ether 2, a close correspondence exists between the FABMS/RPI enantioselection and that of strictly related solution phase energetics in association equilibria. The usefulness of the RPI method is also shown by comparison to the relevant equilibrium constants (NMR titration) in solution.