Enantiomers, discrimination, purity

In her initial investigation, Lundquist studied the monolayer behavior of racemic and optically active forms of both tetracosan-2-ol and its acetate derivative on 0.0 lA aqueous HCl over a considerable range of temperature (77). In each case, it was possible to demonstrate chiral discrimination between pure enantiomers versus the racemic substance. Furthermore, the extent of enantiomer discrimination was significantly temperature dependent, being enhanced at lower temperatures and frequently disappearing at higher ones. Under favorable conditions of temperature, however, the appearance of the force-area curves could be very sensitive to the optical purity... 

The nitroaldol-lipase DCR process could not only amplify specific (3-nitroalcohol derivatives, but also lead to their asymmetric discrimination. HPLC analysis proved that the enantioselectivity of the process is very high, resulting in products of very high optical purity. The R-enantiomer of the ester 45 was resolved to 99% ee, and the R-enantiomer of the ester 46 to 98% ee. 

Biological Discrimination of Enantiomers 189 5-6 Racemic Mixtures 191 5-7 Enantiomeric Excess and Optical Purity 192 5-8 Chirality of Conformationally Mobile Systems 193 5-9 Chiral Compounds without Asymmetric Atoms 195 5-10 Fischer Projections 197... 

Because synthetic ( )-87 was pheromonally inactive, Tumlinson et al. carefully studied in 1977 the relationship between the enantiomeric purity of 87 and its pheromone activity.60 They synthesized the enantiomers of 87, starting from the enantiomers of glutamic acid. The bioactive enantiomer is (/ )-(—)-87, and (5 )-(+)-87 severely inhibits the action of (R)-87. Accordingly, (R)-87 of 99% ee is only two-thirds as active as pure (R)-S7 that of 90% ee is one-third as active, that of 80% ee is one-fifth as active as pure (R)-87. Both (7v )-87 of 60% ee and ( )-87 were inactive. These results illustrate convincingly the importance of enantiomeric composition in chemical communication. Later in 1996, Leal found that the sex pheromone of the female scarab beetle (Anomala osakana) is (S )-87, while (R)-87 interrupts the attraction caused by (.S )-87,61 Thus, chirality accounts for species discrimination. 

Attempts have been made to determine enantiospecific differences between enantiomers and racemates in solution using microcalorimetry [29]. The microcalorimetric method can be used to understand the magnitude of stereoselective interactions that could result from the mixing of solutions of enantiomers of a chiral excipient. The heat evolved or heat of solution (A/T ° ) is measured for the racemate as well as the enantiomers and could be indicative of enantioselective discrimination. However, Horeau and Guette [30] reported that enantioselective interaction in an aqueous medium measured using microcalorimetry may be inconsistent and flawed because of the insufficient purity of the optically active samples used and the insensitivity of the measurement relative to small differences in magnitude of the observed effects. 

The intrinsic ability of homogeneous solutions of chiral cobalt complex 43 (Fig. 9) to discriminate between the enantiomers of amino acids was established for a baseline for the imprinting effect [28]. The results are shown in Table 8. The optical purity of bound phenylalanine was 50%, while the optical puritiy for bound iV-benzylvaline recovered from 43 was found to be 88%. 

The maximum enantiomeric purity (95% ee) that can be obtained for 8 in the PKR experiment depends on the ability of each reagent (5 and 6) to discriminate between (S)-7 and (R)-7 based on relative reactivity (s= kf gt/ siow)- Th same rate dependence that is exploited for simple KR, but its role is more subtle because there are two chiral reagents. If either reagent fails to discriminate between (S)-7 and (R)-7, then the required 1 1 ratio of substrate enantiomers wiU not be maintained, and the advantage of PKR over simple KR (product enantiomeric... 

However, no cells in the males were keyed to the third compound (- )-cubebene which is produced by the host tree. Both sexes of 5. scolytus possess a majority of receptor cells keyed to either of the two male-produced compounds, (-)-threo-4-methyl-3-heptanol and the (-)-erythro-stereo-isomer, and in this species some cells were keyed also to (- )- -multistriatin (Wadhams et al., 1982). The relative sensitivities and specificities of these cells were determined in the same way as described for Ips species. Each cell was minimally activated by pheromone components other than its own key substance. The functional types of olfactory receptor cells discussed here for Scolytus, as with the cells of Ips, apparently are all keyed to the specific isomer of the pheromone compounds. The cells specialized to (- )-a-multistriatin are 1000 times less sensitive to the geometrical isomer d-multistriatin. However, the discrimination of the optical form of multistriatin seemed to be less pronounced. The substantially lower effect of the (-l-)-stereo-isomer (optical purity 99.5%), cannot be solely ascribed to contamination with the (- )-enantiomer. It implies that these cells in fact may also respond to (-l- )-multistriatin, although to a lesser extent. The cells keyed to methyl-heptanol are, however, highly optically specific in both species. 

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