Stereoisomeric flavor compounds, part LX: Diluted modified cyclodextrins as chiral stationary phases - the influence of the polysiloxane solvent

High enantiomeric excess of the flavor relevant 4-alkyl-branched Fatty acids in milk fat and subcutaneous adipose tissue of sheep and goat

Volatile 4-alkyl-branched fatty acids are characteristic flavor compounds of sheep and goat. Due to the methyl branch, the carbon C-4 represents a stereogenic center with the possible presence of one or both enantiomers in the respective samples. In this study, we used enantioselective gas chromatography to study the enantiomeric composition of 4-methyloctanoic acid (4-Me-8:0) and 4-ethyloctanoic acid (4-Et-8:0) in milk and dairy products from sheep and goat as well as in goat subcutaneous tissue. Different columns coated with modified cyclodextrins were tested to resolve racemic 4-alkyl-branched fatty acid methyl ester standards. The best enantiomer resolution was obtained on 25% octakis(2,3,6-tri-O-ethyl)-γ-cyclodextrin (γ-TECD) diluted in OV-1701. For analysis of the food samples, the lipids were extracted and fatty acids in the extracts were converted into fatty acid methyl esters. Non-aqueous reversed-phase high-performance liquid chromatography was used to fractionate the samples in order to gain one solution enriched in 4-Me-8:0 methyl ester and one solution enriched with 4-Et-8:0 methyl ester. Subsequent analysis by enantioselective gas chromatography with mass spectrometry allowed only the detection of one enantiomer of 4-Me-8:0 and 4-Et-8:0 in the samples. By means of a non-racemic standard of 4-Me-8:0, it was found that the predominant enantiomer was (R)-4-Me-8:0.

Modified cyclodextrins as chiral selectors: molecular modelling investigations on the enantioselective binding properties of heptakis(2,3-di-O-methyl-6-O-tert.-butyldimethylsilyl)-beta-cyclodextri n

Molecular modelling methods have been used to investigate the enantioselective binding properties of chiral dihydrofuranones on heptakis(2,3-di-O-methyl-6-O-tert.-butyldimethylsilyl)-beta-cyclod extrin in capillary gas chromatography. A conformational analysis of the modified beta-cyclodextrin was performed using annealed molecular dynamics. With the program GRID the molecular interaction potential for each of the received energetically reasonable structures of the beta-cyclodextrin and the dihydrofuranones was evaluated using different probe groups. The results of these computations have been used as starting points for constructing geometrically reasonable host-guest complexes between the beta-cyclodextrin and the dihydrofuranones. The subsequently performed molecular dynamics simulations yielded different complex states reflecting the conformational flexibility of the diastereomeric complexes. Considering the evaluated interaction energy between the beta-cyclodextrin and the dihydrofuranones as a measure of complex stability the results are in close agreement with the experimentally determined elution sequences. The methodology for the construction of the interaction model used in this study is capable of simulating the experimental data. We believe that it may serve as a basis for predictions of hitherto unknown elution sequences at modified cyclodextrins.