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Eur. J. Mass Spectrom. 7, 447 - 459 (2001) |
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Chiral recognizable hostguest interactions detected by fast-atom bombardment mass spectrometry: application to the enantiomeric excess determination of primary amines | ||
Masami Sawada,* Yoshio Takai, Hiroyuki Imamura, Hitoshi Yamada and Shigetoshi Takahashi |
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ABSTRACT: | ||
The enantiomeric excess (ee) of organic amine compounds has been determined using fast-atom bombardment (FAB) mass spectrometry based on the chiral hostguest complexation systems. The method uses a 1 : 1 mixture of the given chiral crown ether hosts (HRRRR and HSSSS-dn), one of whose enantiomers is isotopically labeled, for the ee-determination of a given amine salt guest (G+). The peak intensity ratio {I[(HRRRR + G)+] / I[(HSSSS-dn + G)+] = IRIS} of the two diastereomeric hostguest complex ions clearly changes with the change in the ee of the guest. The intensity excess (Ie) of the two complex ion peaks is newly defined as Ie = (IRIS 1) / (IRIS + 1). The two sets of mass spectrometrically obtained Ie values vs a set of ee values of phenylalanine methyl ester and leucine methyl ester hydrochlorides show excellent linear relationships through the zero point (R2 = 0.9989 and R2 = 0.9970, respectively). This indicates the potential utility of the present ee-determination method to within ñ 3% ee. Furthermore, based on the solution equilibrium distributions of the complex ions, the linearity is mathematically justified. Therefore, the ee-determination can be simplified as ee (%) = (|Ie| / |Ie(100)|) x 100: here, Ie(100) is the corresponding Ie value obtained using an enantiomerically pure (100% ee) guest as a reference. The present chiral dimethoxyphenyl crown ether host pairs employed are effective (IRIS ³ 1.5) for the ee-determination of most a-amino acid esters, but not effective (IRIS » 1.0) for most simple alkylamines. | ||
Keywords: chiral recognition, enantiomeric excess, hostguest complexation, chiral crown ether, fast-atom bombardment, deuterium labeling |
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