CHIRAL AMINO-ACID RECOGNITION DETECTED BY ELECTROSPRAY-IONIZATION (ESI) AND FAST-ATOM-BOMBARDMENT (FAB) MASS-SPECTROMETRY (MS) COUPLED WITHTHE ENANTIOMER-LABELED (EL) GUEST METHOD

Chiral recognition of crown ethers toward amino acids and their esters is detected both by electrospray ionization (ESI) and by fast atom bo mbardment (FAB) mass spectrometry (MS) and then compared as a series o f host (H)-guest (G) pairs, A racemic guest (G(R)(+):[H-2(n)]G(S)(+) = 1:1, of which one enantiomer is deuterium-labelled), is mixed with th e target host. The chiral amino acid recognition of the host is determ ined from the relative peak intensities of the corresponding diastereo meric host-guest complex ions, [eqn, (a)]: I[(H . G(R))(+)]/I[(H .[H-2 (n)]G(S))(+)] = IRIS. (a) For the complexation between chiral host 1 a nd guest MetOMe(+), FABMS gives IRIS = 5.0 (NEA matrix), which is prac tically equal to the corresponding equilibrium constant ratio (K-R/K-S ) in solution, However, ESIMS gives IRIS = 1.5 for the same complexati on (MeOH), which is a remarkable decrease in the IRIS value. Another c omplex between chiral host 8 and guest MetOMe(+), gives IRIS = 2.0 by FABMS but IRIS = 1.2 by ESIMS. Moreover, in a much simpler system, the amino ester ion selectivity, Leu(t)OMe(+)/MetOMe(+), of host 18-crown -6 is depressed to such an extent that we must conclude that ammonium ion selectivity cannot be evaluated by ESIMS, but the metal ion select ivity, K+/Na+, of the same host 18-crown-6 gives a good qualitative ev aluation of the relative concentrations of the corresponding H-G compl ex ions in solution, It is demonstrated that the IRIS values from the FABMS coupled with the enantiomer-labelled (EL) amino ester guest meth od are the most reliable and generally useful of the measures consider ed for the chiral amino acid recognition.