Enantioselective binding

Rather than compute AG for eqn. 11 and AG for eqn. 12 to obtain AAG, recognize that the left hand sides of both equilibria are identical. This arises from an enantiomeric relationship where A = A in an unbound state (recall from above that enantiomers have identical properties in an achiral environment which, in this case, is the unbound state). Consequently one need only compute the energies of the two diastereomeric complexes to determine which analyte is more tightly bound to the CSP and, accordingly, has the longer retention time on the column. 

There are many assumptions typically made in these calculations. These include assuming the rate of complex formation is the same for R as for S analyte and that only the relative stabilities of the complexes are important complete neglect of mobile phase additives, ions or solvent, although we know that diastereomers have differential solvation free energies and experimentally we can sometimes find reversal in retention orders depending on solvent elimination or truncation of the 

To model the interaction between CSP and analyte one must account for 1) the shapes of the two molecules in the binary complex, 2) the relative position of the two molecules, i.e. the analyte should be at its proper binding site on or around the CSP, and 3) the orientation of the two molecules with respect to each other. This is just a simple way of saying that some sort of ensemble average is needed wherein a molecular dynamics protocol must ensure adequate sampling of phase space, or, if using a Monte Carlo strategy, a sufficient number of important configurations must be sampled. 

An excellent example of motif based search strategies is found in the work of Dappen, Karfunkel and Leusen [18]. These scientists were interested in understanding how chiral separations take place and then to use that knowledge to design enhanced stationary phases. The authors first determined experimentally that the R enantiomer of anal el is bound more tightly to 2, a chiral stationary phase selector that is tethered to silica by the amino group. 

Because these authors had determined experimental AAH binding energies they focused their efforts on computing enthalpies rather than free energies. The following steps were used to do this. 

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Shihukawa, A., Kadohara, M., He, J.Y., Nishimura, M., Naito, S., Nakagawa, T. (1995). Study of the enantioselective binding between BOF-4272 and serum albumins by means of high-performance frontal analysis. J. Chromatogr. A 694, 81-89. 

Nair, U.B. et al.. Elucidation of vancomycin s enantioselective binding site using its copper complex. Chirality, 8, 590, 1996. 

Gagne and coworkers utilized this combination to discover enantioselec-tive receptors for (-)-adenosine [12]. A racemic dipeptide hydrazone [( )-pro-aib] generated a stereochemically diverse DCL of n-mer. The dimers were composed of two chiral (DD/LL) and one achiral isomer (DL), the four trimers (DDD, LLL, DDL, and LLD), the tetramers of four chiral and two achiral isomers, etc. Two techniques were used to measure the enan-tio-imbalance that was caused by the enantioselective binding of the chiral analyte to the enantiomeric receptors (Fig. 5.11). Since the unperturbed library is optically inactive, the optical enrichment of each library component could be measured by a combined HPLC optical rotation detection scheme (laser polarimeter, LP). LP detection differentiated unselective binding (amplification but not optical enrichment) from enantioselective recognition of the analyte (amplification and optical enrichment). In this manner the LL dimer (SS) of the dipeptide was amplified and identified as the enantioselective match for (-)-adenosine. 

B. Capillary Electrophoresis Methods for Determination of Enantioselective Binding Constants Between Chiral Drugs and Cyclodextrins... 

Similar equations can be written for both enantiomers of chiral analyte. Based on Eq. (14), nonlinear regression techniques allow one to determine the enantioselective binding constants (KR and Ks) and the mobilities of related transient diastereomeric complexes (/4°mplex and /x ""plex). 

The frontal analysis technique has been used for the determination of enantioselective binding constants of chiral drugs such as warfarin, verapamil, nilvadipine, and semotidil with proteins such as bovine serum albumin (BSA), human serum albumin (HSA), and plasma lipoproteins (45-51). 

III. APPLICATION OF CAPILLARY ELECTROPHORESIS FOR DETERMINATION OF ENANTIOSELECTIVE BINDING CONSTANTS OF CHIRAL DRUG/CYCLODEXTRIN COMPLEXES... 

Thus, as shown earlier, CE represents a suitable technique for the determination of enantioselective binding constants between chiral drugs and cyclodextrins. The results obtained under appropriate conditions are reasonable and can be applied for optimization purposes as well as for a better understanding of the fine nuances of chiral CE separations. On the other hand, some care must be taken for the proper application of CE methods for the determination of the binding constants as well as when applying these data. A critical review of the calculation of stability constants for the chiral selector-enantiomer interactions from electrophoretic mobilities has been published by Vespalec and Bocek (40). 

Y Kuroda, A Shibukawa, T Nakagawa. The role of branching glycan of human eq-acid glycoproten in enantioselective binding to basic drugs as studied by capillary electrophoresis. Anal Biochem 268 9-14, 1999. 

NAL Mohamed, Y Kuroda, A Shibukawa, T Nakagawa, ST Elo Gizawy, HF Askal, ME El Kommos. Enantioselective binding analysis of verapamil to plasma lipoproteins by capillary electrophoresis-frontal analysis. J Chromatogr A 875 447-453, 2000. 

KB Lipkowitz. Atomistic modeling of enantioselective binding. Acc Chem Res 33 555-562, 2000. 

The enantioselective binding properties of certain chiral crown ethers have been employed in the resolution of amino add racemates. The racemic amino ester is adsorbed onto silica gel as its ammonium salt and eluted by a chloroform solution of the chiral crown ether. An excellent separation of the two enantiomers is achieved by this method (74JA7100). 

It has also been reported from circular dichroism (CD) studies [36] that polysaccharide-based CSPs can induce chirality in enantiomeric guests such as (4Z,15Z)-bilirubin-Ixoc (BR) (Fig. 5). Although not optically active, BR has two enantiomeric helical conformations maintained by six intramolecular hydrogen bonds between two carboxylic acid moieties and two pyrromethenone — NH— protons. These (R)- and (5)-helical conformers are in dynamic equilibrium in an achiral solution [37], but some optically active compounds can enantioselectively bind to BR to induce CD spectra in solution [38-40]. A significant induced CD... 

The main principle for enantioselective analysis using EPME is to find the lock for the key. The key is the enantiomer that must be analysed and the lock is a substance with a special architecture that can bind enantioselectively with the enantiomer. Accordingly, the principle can be defined as enantioselective binding. 

A chiral molecular cavity, admitting the possibility of enantioselective binding and catalysis. 

Figure 4 Molecular recognition pattern found in a 1 1 1 complex of (R,R,R,R)-(—)-28 with (—)-30 and toluene. One enantiomer of cyanohydrine is bound to the enantioselective binding site of the host. The disordered toluene molecule fits well into the hydrophobic cavity. Reprinted with permission from ref. 48. 2000, Wiley-VCH Verlag GmbH.

In 1992, de Mendoza and coworkers reported the synthesis of the bifunction-alized chiral bicyclic guanidinium 34 for the purpose of enantioselectively binding zwitterionic aromatic amino acids (i.e., tryptophan and phenylalanine) [55]. By including binding elements for the carboxylate and ammonium of the amino acids which were noncomplementary, they hoped to avoid the potential... 

S)-amino acicis, mexiule C, two stereoisomers, and tethered to polystyrene via mcxlule B, two stereoisomers) was prepared by encoded split synthesis on 100 pm polystyrene synthesis beads so that different library members were segregated on different beads (i.e. one bead, one chiral SO). This library was then screened by a two-colour differential binding method amino acid SAs were labelled via a linker with red ((/ )-amino acids) and blue ((S)-amino acids) dyes and the chiral beads treated with an equimolar mixture of the labelled SA enantiomers. Enantioselective binding beads are either red or blue, whereas unselective beads are brown. 

The thermal annealing of the MIP was accompanied by a significant reduction in the swelling and an increase in the apparent dry density of the material. This results in a higher density of the enantioselective binding sites, although they may be less accessible than prior to the thermal treatment. The nitrogen sorption data showed... 

B. Early Reports of Enantioselective Binding to Plasma Proteins... 

The enantioselective binding of tryptophan (16) is a result of differential binding at site II, which is held to be the more enantioselective of the major drug binding sites (29). However, enantioselective binding at site I has been proposed for chiral drugs binding there, such as warfarin (19,30-33). 

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