Molecular and Chiral Recognition

MOLECULAR AND CHIRAL RECOGNITION. SELF-ORGANIZATION, SELF-ASSEMBLY AND PREORGANIZATION 

The building of a cavity around the guest is an extension of induced fit concept. This is the case with hexokinase enzyme [ 13b] and foldamers [ 13c] that wrap themselves around the guest. 

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Noncovalent interactions play a key role in biodisciplines. A celebrated example is the secondary structure of proteins. The 20 natural amino acids are each characterized by different structures with more or less acidic or basic, hydrophilic or hydrophobic functionalities and thus capable of different intermolecular interactions. Due to the formation of hydrogen bonds between nearby C=0 and N-H groups, protein polypeptide backbones can be twisted into a-helixes, even in the gas phase in the absence of any solvent." A protein function is determined more directly by its three-dimensional structure and dynamics than by its sequence of amino acids. Three-dimensional structures are strongly influenced by weak non-covalent interactions between side functionalities, but the central importance of these weak interactions is by no means limited to structural effects. Life relies on biological specificity, which arises from the fact that individual biomolecules communicate through non-covalent interactions." " Molecular and chiral recognition rely on... 

While the distinct amino acid residues have mostly only a modulating effect (see Table 1.9) (e.g., FMOC-protected amino acids), the type of protection group or derivative formed decides on the molecular and chiral recognition mechanism and hence on the obtained elution order as well as the level of enantiomer recognition (i.e., magnitudes of a-values) that can be afforded. From a practical point of view, we may distinguish between two groups of IV-derivatives ... 

Predicting molecular and chiral recognition of CDs on the basis of model calculations... 

Chromatography is one of the most important methods for direct studies of molecular and chiral recognition by CyDs. Today it has split into several branches, e.g. gas chromatography, GC, high-performance liquid chromatography, HPLC, and capillary electrophoresis and other electromigration techniques, that enable us not only to detect the recognition but also to estimate the complex stoichiometry and formation constant and, consequently, the enthalpies and entropies of complex for-... 

Liu Y, Yang EC, Yang YW, Zhang HY, Fan Z, Ding F, Cao R, Thermodynamics of the molecular and chiral recognition of cyclo-alkanols and camphor by modified y -cyclodextrins possessing simple aromatic tethers, J. Org. Chem. 2004 69 173-180. 

Liu Y, Li XY, Zhang HY, Wada T, Inoue Y, Synthesis of phosphoryl-tethered-y -cyclodextrins and their molecular and chiral recognition thermodynamics, J. Org. Chem. 2003 68 3646-3657. 

Our own interest in CyD modeling has evolved from the experimental study of molecular and chiral recognition of decalin isomers 1 by j8-CyD 2 [20, 8, 21]. These molecules provide a unique set for such studies since at room temperature the cis-isomer undergoes rapid ring inversions between the lb and Ic forms, which are enantiomers as well. Thus, decalin isomers allow us to study both molecular and chiral recognition. Chromatographic analysis by Sybilska s group [20] has shown that the trans isomer la forms weak complexes with 2 while the complexes with... 

As mentioned before, decalin isomers present an interesting case for analysis of the reliability of modeling of CyD complexation since they enable us to study molecular and chiral recognition for one set of isomeric guests. However, the lack of experimental data on the energy difference between the complexes involving the cis-decalin enantiomers prompted us to use another example to model chiral recognition, namely, the 1 2 complexes of enantiomers of a-pinenes 12 with a-CyD. 

The cleft of Troger s bases provides only limited sueeess in supramoleeular applieations, however, their well-defined structures were used as armatures for aligning further functionality in order to provide receptors for molecular and chiral recognition. That goal was achieved with scaffold-type derivatives 19, 20, 24 (cf. 18) and by embedding Troger s bases in macrocycles that were called trogerophanes (21-23, 25) (Fig. 3). 

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