Chiral capacity

The accurate determination of the adsorption isotherm parameters of the two enantiomers on a CSP is of fundamental importance to do computer-assisted optimization to scale up the process. Such determinations are usually done with an analytical column and the most traditional method to determine the parameters and saturation capacity is by frontal analysis (see section 3.4.2). The aim of paper III was to investigate the adsorption behavior and the chiral capacity of the newly developed Kromasil CHI-TBB column using a typical model compound. Many of the previous studies from the group have been made on low-capacity protein columns which has revealed interesting information about the separation mechanism [103, 110, 111], For this reason a column really aimed for preparative chiral separations was chosen for investigation [134], As solute the enantiomers of 2-phenylbutyric acid was chosen. 

Figure 17 shows a section of two individual overlaid staircase chromatograms resulting from single component frontal analysis of (S) and (R)-2-phenylbutyric acid, respectively. At the first step up to 30.5 mM the enantiomers are clearly separated from each other, at the second step up to 61.0 mM they are still separated and even at the third step up, to 91.5 mM it is still a very small tendency for separation. This figure indicates that the chiral capacity is somewhat higher than 90 mM. 

The results reported in paper III showed that the chiral Kromasil column has the highest true chiral capacity measured so far, about 130 times larger than columns packed with immobilized proteins. Thus, the column is indeed very suitable for preparative separations. 

Prochiral (Section 7 9) The capacity of an achiral molecule to become chiral by replacement of an existing atom or group by a different one... 

Although the chiral recognition mechanism of these cyclodexttin-based phases is not entirely understood, thermodynamic and column capacity studies indicate that the analytes may interact with the functionalized cyclodextrins by either associating with the outside or mouth of the cyclodextrin, or by forming a more traditional inclusion complex with the cyclodextrin (122). As in the case of the metal-complex chiral stationary phase, configuration assignment is generally not possible in the absence of pure chiral standards. 

Proteins are the indispensable agents of biological function, and amino acids are the building blocks of proteins. The stunning diversity of the thousands of proteins found in nature arises from the intrinsic properties of only 20 commonly occurring amino acids. These features include (1) the capacity to polymerize, (2) novel acid-base properties, (3) varied structure and chemical functionality in the amino acid side chains, and (4) chirality. This chapter describes each of these properties, laying a foundation for discussions of protein structure (Chapters 5 and 6), enzyme function (Chapters 14-16), and many other subjects in later chapters. 

This is because the increased turbulence from higher flow rates decreases the possibility for inclusion complexation, a necessary event for chiral recognition in reversed phase. Some effect has also been observed in the new polar organic mode when (capacity factor) is small (< 1). Flow rate has no effect on selectivity in the typic normal-phase system, even at flow rates up to 3 inL miir (see Fig. 2-11). 

Proteins. A chiral stationary phase with immobilized a -acid glycoprotein on silica beads was introduced by Hermansson in 1983 [18, 19]. Several other proteins such as chicken egg albumin (ovalbumin), human serum albumin, and cellohy-drolase were also used later for the preparation of commercial CSPs. Their selectivity is believed to occur as a result of excess of dispersive forces acting on the more retained enantiomer [17]. These separation media often exhibit only modest loading capacity. 

The big difference between the extent of asymmetric induction on the addition to a prostereogenic carbonyl group of simple carbanions a to a chiral sulfoxide on the one hand and enolates of sulfinyl esters on the other, can be attributed to the capacity of the ester function to chelate magnesium in the transition states and intermediates. The results already described for the addition of chiral thioacetal monosulfoxide to aldehydes (see Section 1.3.6.5.) underscore the importance of other functions, e.g., sulfide, for the extent of asymmetric induction. 

The sulfoximine group provides anion-stabilizing capacity in a chiral environment and a number of synthetic applications have been developed based on these properties. 

Similarly, whole-cell Lactobacillus kefir DSM 20587, which possesses two alcohol dehydrogenases for both asymmetric reduction steps, was applied in the reduction of tert-butyl 6-chloro-3,5-dioxohexanoate for asymmetric synthesis of ft rf-butyl-(31 ,5S)-6-chloro-dihydroxyhexanoate (Figure 7.5), a chiral building block for the HMG-CoA reductase inhibitor [ 17]. A final product concentration of 120 him and a specific product capacity of 2.4 mmol per gram dry cell were achieved in an optimized fed-batch process. Ado 99% was obtained for (3R,5S)- and (3.S, 55)-te/ f-butyl-6-chloro-dihydroxyhexanoate with the space-time yield being 4.7 mmolL-1 h-1. 

The 1,3-dipolar cycloaddition of nitrones to vinyl ethers is accelerated by Ti(IV) species. The efficiency of the catalyst depends on its complexation capacity. The use of Ti( PrO)2Cl2 favors the formation of trans cycloadducts, presumably, via an endo bidentate complex, in which the metal atom is simultaneously coordinated to the vinyl ether and to the cyclic nitrone or to the Z-isomer of the acyclic nitrones (800a). Highly diastereo- and enantioselective 1,3-dipolar cycloaddition reactions of nitrones with alkenes, catalyzed by chiral polybi-naphtyl Lewis acids, have been developed. Isoxazolidines with up to 99% ee were obtained. The chiral polymer ligand influences the stereoselectivity to the same extent as its monomeric version, but has the advantage of easy recovery and reuse (800b). 

As yet, the number of applications is limited but is likely to grow as instrumentation, mostly based on existing CE systems, and columns are improved and the theory of CEC develops. Current examples include mixtures of polyaromatic hydrocarbons, peptides, proteins, DNA fragments, pharmaceuticals and dyes. Chiral separations are possible using chiral stationary phases or by the addition of cyclodextrins to the buffer (p. 179). In theory, the very high efficiencies attainable in CEC mean high peak capacities and therefore the possibility of separating complex mixtures of hundreds of... 

The chiral recognition ability of a CSP is quantitatively evaluated from the results of chromatographic separation of enantiomers. Figure 3.4 shows a chromatogram of the resolution of benzoin (19) on cellulose tris(3,5-dimethylphenylcarbamate). The (+)-isomer elutes first followed by the (—)-isomer complete baseline separation is achieved. The results of the separation can be expressed by three parameters—capacity factors (k1), separation factor (a), and resolution factor (Rs)—defined as follows ... 

ODS columns (250 X 4.6 mm i.d. 250 X 22 mm i.d.). The isocratic mobile phase for the analytical separation consisted of 0.05 M ammonium citrate (pH = 2.5) containing 10 per cent ACN. Preparative separation was performed with 0.4 M formic acid-methanol (6 4, v.v) as the mobile phase. Enantiomers wre measured in a chiral phase column ((R)-3,5-dini-trobenzoylphenylglycinepropylsilyl) (250 X 4.6 mm i.d. particle size 5 //m). Mixtures of 0.5 per cent formic acid and 2-propanol were employed for the separation of the various isomers [336], The good separation capacity of the method is illustrated in Fig. 2.164. 

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