Enantiomer labeling

H Frank, GR Nicholson, E Bayer. Enantiomer labelling, a method for the quantitative analysis of amino acids. J Chromatog 167, 187, 1978. 

There are several methods to enantiodifferentiate chiral ammonium ions by FAB-MS. One is the so-called enantiomer-labeled (EL) guest method. The method is based on the preparation of a mixture containing the enantiopure host (denoted as U) and the racemate of the guest. One of the guest enantiomers is isotopically labeled (e.g., [M5]+) and the other is not (e.g., [M ] ). Consequently, the signals for the two diastereomeric host-guest pairs (i.e., [U M/j] and [U-Ms] of equations (21) and (22)) appear at different miz ratios. 

M. Sawada et al., Chiral recognition in host-guest complexation determined by the enantiomer-labeled guest method using fast atom bombardment mass spectrometry. J. Am. Chem. Soc. 117, 7726-7736 (1995)... 

M. Sawada et al., 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. J. Chem. Soc. Perkin Trans. 2, 701-710 (1998)... 

One of the critical aspects of this approach is that two different experiments have to be performed between which the particular instrument conditions must be carefully kept constant in order not to affect the intensity ratios. This problem can be overcome by the enantiomer-labeled guest method [47]. It is based on the mass spectrometric examination of one enantiomer of the host with a pseudo-racemic mixture of the guest. In order to be able to detect both diastereomers separately, one enantiomer of the guest must be isotopically labeled, usually with deuterium. In the same experiment, both diastereotopic complexes are formed and their intensities can be compared directly. However, the stereochemical effect might additionally be superimposed by an unknown isotope effect. A way to separate stereochemical and isotope effects is to perform the same experiment with the second host enantiomer [4B]. In one experiment both stereochemical and isotope effects disfavor the same complex and thus work in the same direction. In the other experiment, they partly cancel each other. If both experiments have been performed, one can use the two experimental values for the intensity ratios of both diastero-meric complexes to deconvolute both effects [49]. 

The combination of stable isotopes and MS gives a powerful tool with extra features for the study of drug metabolism. For example, luilike natural products, most synthetic drugs are racemic mixtures. Administering a racemate with one enantiomer labeled with stable isotopes can show up any en-antioselective route of metabolism. It is important to administer a racemic mixture because the... 

Two different approaches have been developed to determine diastereomeric excess in host-guest complexes in solution by MS (Box 2) the relative peak intensity mefliod makes use of an achiral host as a reference compound and compares the two diastereomeric host-guest complexes with it. The enantiomer-labeled method employs pseudoracemates, that is, 1 1 mixtures... 

B. Enantiomer-Labeled Method Synthesize one isotopically labeled guest enantiomer mix 1 1 with the second unlabeled enantiomer and add a small amount of enantiopure host record ESI mass spectrum and compare the intensities of both diastereomeric complexes directly to separate isotope and stereochemical effects, the other pseudoracemate needs to be subjected to the same experiment. 

Limited radiochemical purity may result in detection of radioactivity from chemical species different from the intentionally labeled one. This is of prime importance in specifically labeled enantiomers. Labeled compounds should be purified immediately before use. Chromatography and reverse IDA (see Section 8.4.2) are important for checking radiochemical purity. 

Besides the chiral resolution of pollutants, GC determination of enantiomeric ratios (ERs) can be used for the quantification of the enantiomers in complex matrices, with a known quantity of the pure enantiomer added as an internal standard. With a method of enantiomer labelling, a known quantity of an enantiopure standard is added to the mixture (or an aliquot of it) and the amount of the enantiomer originally present is calculated from the change in the ER after addition of the standard. This method provides precise knowledge of the ERs of the sample and the standards [87]. In addition, the absolute configurations of the pollutants may be determined directly, and free of chiroptical evidence, by GC via co-injection of reference pollutants with known stereochemistry [87]. 

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