Chiral amino acid recognition detected by electrospray ionization (ESI) and fast atom bombardment (FAB) mass spectrometry (MS) coupled with the enantiomer-labelled (EL) guest method

Abstract

Chiral recognition of crown ethers toward amino acids and their esters is detected both by electrospray ionization (ESI) and by fast atom bombardment (FAB) mass spectrometry (MS) and then compared as a series of host (H)–guest (G) pairs. A racemic guest (G_R⁺∶[²H_n]G_S⁺ = 1∶1, of which one enantiomer is deuterium-labelled), is mixed with the target host. The chiral amino acid recognition of the host is determined from the relative peak intensities of the corresponding diastereomeric host–guest complex ions, [eqn. (a)]:

      I[(H·G_R)⁺]/I[(H·[²H_n]G_S)⁺] = IRIS.      (a) 

For the complexation between chiral host 1 and guest MetOMe⁺, FABMS gives IRIS = 5.0 (NBA matrix), which is practically equal to the corresponding equilibrium constant ratio (K_R/K_S) in solution. However, ESIMS gives IRIS = 1.5 for the same complexation (MeOH), which is a remarkable decrease in the IRIS value. Another complex 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^tOMe⁺/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 selectivity, K⁺/Na⁺, of the same host 18-crown-6 gives a good qualitative evaluation of the relative concentrations of the corresponding H–G complex ions in solution. It is demonstrated that the IRIS values from the FABMS coupled with the enantiomer-labelled (EL) amino ester guest method are the most reliable and generally useful of the measures considered for the chiral amino acid recognition.