Chiral analysis by electrospray ionization mass spectrometry/mass spectrometry. 1. Chiral recognition of 19 common amino acids.

Chiral recognition of 19 common amino acids was achieved by investigating the collision-induced dissociation spectra of protonated trimers that were formed from the electrospray ionization of amino acids in the presence of one of the following chiral selectors: L- or D-N-tert-butoxycarbonylphenylalanine, L- or D-N-tert-butoxycarbonylproline, and L- or D-N-tert-butoxycarbonyl-O-benzylserine. The protonated trimers were dissociated to protonated dimers, and the intensity ratios of the protonated dimer (product ion) to the protonated trimer (precursor ion), i.e., the observed dissociation efficiency, was found to be strongly dependent on the chirality of the amino acids with respect to that of the chiral selectors. The results showed that the chirality of all 19 common amino acids can be definitely differentiated. The method was demonstrated as rapid, sensitive, precise, robust, and requiring no reference standards and only minimal sample preparation. The chirality of all three amino acids in a mixture was determined without prior separation of the amino acids, consuming only 70 pmol of sample and requiring only approximately 14 min of mass spectrometric measurements. A cyclodipeptide with unknown chirality was determined to be cyclo-(L-Pro-L-Leu) by acid hydrolysis followed by the present method, and the results were consistent with the physiochemical properties and NMR data of the compound. This study suggested that ESI-MS/MS can be a promising approach for the chiral recognition of other compounds.