Selector structural

Gilar, M., Uhrova, M.,Tesarova, E. Enantiomer separation of dihydropyridine calcium antagonists with cyclodextrins as chiral selectors structural correlation, J. Chromatogr. B., 1996, 681, 133-141. 

Fig. 1-6. Chemical structures of the chiral selector (8) used in the resolution of 9 by distillation.

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 ... 

To find the most efficient selectors in the library, blue and red dye-labeled enantiomeric probe molecules 6 and 7 were prepared by linking pentafluorophenyl esters of L- and D-proline with Disperse Blue 3 and Disperse Red 1, respectively, through an isophthaloyl (shown in structures 6 and 7) or a succinyl moiety. Eor detection, a... 

It should be stressed that only those surfaces that actually come in contact with the sample need to be bio-compatible and the major parts of the valve can still be manufactured from stainless steel. The actual structure of the valve varies a little from one manufacturer to another but all are modifications of the basic sample valve shown in figure 13. The valve usually consists of five parts. Firstly there is the control knob or handle that allows the valve selector to be rotated and thus determines the load and sample positions. Secondly, a connecting device that communicates the rotary movement to the rotor. Thirdly the valve body that contains the different ports necessary to provide connections to the mobile phase supply, the column, the sample loop if one is available, the sample injection port and finally a port to waste. Then there is the rotor that actually selects the mode of operation of the valve and contains slots that can connect the alternate ports in the valve body to provide loading and sampling functions. Finally there is a pre-load assembly that furnishes an adequate pressure between the faces of the rotor and the valve body to ensure a leak tight seal. 

Affinity liquid chromatography and chiral separations (enantiomer separations) require similar analyte properties. The solutes may have interactions through hydrogen-bonding, ligand formation, or Coulombic forces with the surface of stationary phase materials or the sites of additives however, the selectivity is controlled by the steric effects of the structures of the analyte molecules and the recognition molecules (chiral selectors). 

Along this line, several new linear hosts and flexible cyclic hosts having a C2-symmetry axis (bisDPGP, bisTAGP, and bisTMGP Scheme 8) have been designed and synthesized based on the structural feature of the highly selective MeFruNys." However, these tailor-made flexible hosts prove less effective than MeFruNys as chiral selector (Table 15). 

To verify such a steric effect a quantitative structure-property relationship study (QSPR) on a series of distinct solute-selector pairs, namely various DNB-amino acid/quinine carbamate CSPpairs with different carbamate residues (Rso) and distinct amino acid residues (Rsa), has been set up [59], To provide a quantitative measure of the effect of the steric bulkiness on the separation factors within this solute-selector series, a-values were correlated by multiple linear and nonlinear regression analysis with the Taft s steric parameter Es that represents a quantitative estimation of the steric bulkiness of a substituent (Note s,sa indicates the independent variable describing the bulkiness of the amino acid residue and i s.so that of the carbamate residue). For example, the steric bulkiness increases in the order methyl < ethyl < n-propyl < n-butyl < i-propyl < cyclohexyl < -butyl < iec.-butyl < t-butyl < 1-adamantyl < phenyl < trityl and simultaneously, the s drops from -1.24 to -6.03. In other words, the smaller the Es, the more bulky is the substituent. The obtained QSPR equation reads as follows ... 

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