Chiral recognition and enantiomer assays of N-(3,5-dinitrobenzoyl)amino acid derivatives using electrospray ionization–mass spectrometry

A pair of pseudoenantiomers, anilide derivatives of N-pivaloylproline were prepared and used as chiral selectors for enantiomer discrimination of amides or esters of N-(3,5-dinitrobenzoyl)amino acids in single-stage electrospray ionization/mass spectrometric experiments. Addition of a chiral analyte to a solution of the two pseudoenantiomeric chiral selectors affords selector–analyte complexes in the electrospray ionization mass spectrum where the ratio of these complexes is dependent on the enantiomeric composition of the analyte. The relationship between the ratio of the selector–analyte complexes in the electrospray ionization mass spectrum and the enantiomeric composition of the analyte can be used to relate the extent of the measured enantioselectivity and for quantitative enantiomeric composition determinations. Effects of the added cationic ions (H+, Li+, Na+ and K+) and instrument conditions on the selector–analyte ion intensity and the enantioselectivity (αMS) were investigated. The percent ratio of the sum of the selector–analyte ion counts and the total ion counts decreases accordingly with the increase of the desolvation temperature for H+, Na+ and K+. The ratio for Li+ kept almost constant. The best αMS was observed at a desolvation temperature of 200 °C with the added H+. The cone voltage has little effects on the αMS values though the intensities of selector–analyte complexes are decreased at higher cone voltages. The observed MS enantioselectivities are comparable to the HPLC enantioselectivities and the sense of chiral recognition by MS is consistent with what is observed chromatographically. Quantitative enantiomeric composition determinations for five different samples of N-(3,5-dinitrobenzoyl)leucinyl butylamide at four different concentrations were performed. The average % difference between the HPLC and MS enantiomer determinations is 6.8% and 3.7% for the calibration lines constructed at a concentration of the analyte of 125 μM and 12.5 μM, respectively.

We performed chiral recognition and enantiomer assays of N-(3,5-dinitrobenzoyl)amino acid derivatives in the gas phase using electrospray ionization/mass spectrometry (ESI/MS).Figure optionsDownload as PowerPoint slide