Preferential inclusion

Enantioseparation is typically achieved as a result of the differences in interaction energies A(AG) between each enantiomer and a selector. This difference does not need to be very large, a modest A(AG) = 0.24 kcal/mol is sufficient to achieve a separation factor a of 1.5. Another mechanism of discrimination of enantiomers involves the preferential inclusion of one into a cavity or within the helical structure of a polymer. The selectivity of a selector is most often expressed in terms of retention of both enantiomers using the separation factor a that is defined as ... 

Recently, an example of cycloamylose-induced catalysis has been presented which may be attributed, in part, to a favorable conformational effect. The rates of decarboxylation of several unionized /3-keto acids are accelerated approximately six-fold by cycloheptaamylose (Table XV) (Straub and Bender, 1972). Unlike anionic decarboxylations, the rates of acidic decarboxylations are not highly solvent dependent. Relative to water, for example, the rate of decarboxylation of benzoylacetic acid is accelerated by a maximum of 2.5-fold in mixed 2-propanol-water solutions.6 Thus, if it is assumed that 2-propanol-water solutions accurately simulate the properties of the cycloamylose cavity, the observed rate accelerations cannot be attributed solely to a microsolvent effect. Since decarboxylations of unionized /3-keto acids proceed through a cyclic transition state (Scheme X), Straub and Bender suggested that an additional rate acceleration may be derived from preferential inclusion of the cyclic ground state conformer. This process effectively freezes the substrate in a reactive conformation and, in this case, complements the microsolvent effect. 

Table 8 shows the results for the separation of monosubstituted benzenes using DCA as the host. In the binary systems containing benzene, benzene was excluded from the DCA crystal, with the resulting separation factors less than unity. This makes a complete contrast to the previous results with CA. Two guests, toluene and ethylbenzene, were preferentially included in the DCA crystal. The order of preferential inclusion in DCA can be described as follows ... 

To obtain further information on the preferential inclusion of the aromatic residue of one of the two enantiomers electronic and c.d. spectra were recorded in the UV-vis region. The inclusion of several chromophores in the cyclodextrin cavity has been shown to produce induced circular dichroism (i.c.d.), due to the presence of many chiral centres. In several cases, it has been possible to correlate theoretically both the sign and the intensity of the observed i.c.d. and the orientation of the transition moment related to the cyclodextrin axis [S0-S2]. It is expected both that the differential inclusion of the aromatic residue of the amino acid in the CD cavity restricts the conformational mobility of the participating groups and that the different orientation of the transition vectors in the two diastereomeric complexes causes the opposite sign in c.d. activity. 

The results were consistent with the interpretation that residual fluorescence is due to the ternary complexes and that quantum yields are affected by the preferential inclusion of the D-enantiomer side chain within the cyclodextrin cavity. 

Each of the behaviors unique to coalesced PET can be attributed to the g tg + kink conformations assumed by its ethylene glycol fragments when included in the narrow channels of its y-CD-IC, which are compared schematically in Fig. 11.3 (bottom) to the dll-trans crystalline conformation (top) of PET. The kink conformations are nearly as extended as the crystalline, ll-trans conformation, but they have a smaller cross-section, which explains their preferential inclusion in the narrow channels of its y-CD-lC (see Fig. 11.1). When coalesced from its y-CD-IC the PET chains can readily and rapidly convert to the all-trans conformation through simple counter rotations about the —CH2—O— bonds and therefore crystallize. Normal melts of PET consist of chains with predominantly g —CH2—CH2— bonds, which must be rotated to the trans conformation during crystallization. This conformational transition is difficult to accomplish without long-range movements of chain segments, consistent with the normally slow crystallization rate of PET. Furthermore, both solid-state FTIR and C-NMR observations... 

Preferential inclusion of some guests as dimers will have a large effect on the observed fluorescence properties, in cases where the free guests exist as monomers. A striking example of the effect of dimer inclusion will be discussed in Section 4.3 for the case of Nile Red. This is also observed in studies of the... 

Following the realization that the presence of phenylacetic acid in the fermentation led to a simplification of the mixture of penicillins produced by the fungus due to preferential uptake of this acid and its incorporation into benzylpenicillin (4), a wide variety of other acids were added to the growing culture. Inclusion of the appropriate acids in the culture medium thus afforded, respectively, phenoxymethylpenicillin (5, penicillin V), phenethicillin propicillin (7), and phehbencillin... 

To provide a sense of the value of some of these deviations from neutral taxation. Table 2 reports cumulative estimates over five years of selected energy-related tax expenditures. The Congressional Budget and Impoundment Control Act defines a tax expenditure as a revenue loss attributable to a special inclusion, exemption, or deduction from gross income or a special credit, preferential tax rate, or deferral of tax. 

Steels and stainless steels show preferential nucleation of pits at inclusions, most notably sulphide inclusions ". Other sulphur-rich regions in ferrous and nickel-based alloys may also lead to premature failure. It has been shown that accumulation of sulphur on the surface of these materials retards passivity and enhances dissolution of the metal. These effects occur in any solution in which the metal shows an active region and they are also preferential pitting sites in the presence of chloride. A recent notion for... 

This review cannot claim to be a comprehensive account of all the coordination chemistry of zinc since the early 1980s. The approach has been to attempt selection of references by key workers or important results in areas where much work has been carried out. It is hoped that it will be possible by following the key references and articles to gain an overview of achievements and advances in the important areas. In many cases, examples where X-ray structural data is included have been selected preferentially for inclusion. 

---