Steric recognition

In the asymmetric reduction of ketones, stereodifferentiation has been explained in terms of the steric recognition of two substituents on the prochiral carbon by chirally modified reducing agents40. Enantiomeric excesses for the reduction of dialkyl ketones, therefore, are low because of the little differences in the bulkiness of the two alkyl groups40. In the reduction of ketoxime ethers, however, the prochiral carbon atom does not play a central role for the stereoselectivity, and dialkyl ketoxime ethers are reduced in the same enantiomeric excess as are aryl alkyl ketoxime ethers. Reduction of the oxime benzyl ethers of (E)- and (Z)-2-octanone with borane in THF and the chiral auxiliary (1 R,2S) 26 gave (S)- and (R)-2-aminooctane in 80 and 79% ee, respectively39. 

In addition to solvophobic and silanophilic interactions other stationary phase effects, including steric recognition and pi-pi interactions between solutes and stationary phase, have been demonstrated. Thus, the specific nature of the stationary phase and solutes in-... 

Receptor layers can respond selectively to particular substances or to a group of substances. The term molecular recognition is used to describe this behaviour. Typical for biosensors is that molecules are recognized by their size or their dimension, i.e. by steric recognition. 

Membranes with steric recognition Layers with intermolecular channels Calixarenes crown ethers Alkali metal ions... 

Membranes with steric recognition of the analyte have properties similar to that of permselective membranes. As mentioned in connection with potentio-metric sensors, some molecules (ligands) can coordinate with specific analyte molecules due to their size. As a result, the bound analyte molecules may cross a phase boundary and will become mobile in the subsequent non-aqueous phase. With potentiometric sensors, partition equilibria were established in this way with subsequent formation of a potential difference across the inter-... 

Moreover, the chromatographic retention and selectivity is always affected to a certain extent by the interplay of the molecular configuration of analyte and stationary phase. This so-called molecular or steric recognition is one of the main reasons for the excellent selectivities that can be achieved in liquid chromatography. These parameters form the basis of the following equation (Eq. 6) to describe the retention relative to that of a reference solute. Therefore, Eq. (6) does not include an intercept value, which usually reflects, among other things, the phase ratio of the column. 

Recognition and description of new interparticle interaction forces such as those owing to magnetic dipoles, steric and electrosteric repulsion,... 

Intermolecular recognition and self-assembly processes both in the solid, liquid, and gas phases are the result of the balanced action of steric and electronic factors related to shape complementarity, size compatibility, and specific anisotropic interactions. Rather than pursuing specific and definitive rules for recognition and self-assembly processes, we will afford some heuristic principles that can be used as guidelines in XB-based supramolecular chemistry. 

Roughly speaking, we may say that dimethylformamide acts in a way analogously to alcohols, with the difference of having its H-bond acceptor and donor functions in sterically distant sites compared to the —OH moiety. The prevailing recognition pattern can be subject to alterations, giving an individual touch to the associate in question, nevertheless it may be traced back to its characteristic form. 

Another hypothesis was provided by Mikio Shimitso (1982) on the basis of studies of steric effects in molecular models. It had been noted years previously that the fourth nucleotide at the 3 end of the tRNA molecules (referred to as the discrimination base) might have a recognition function. In the case of certain amino acids (i.e., their tRNA-amino acid complexes) this base pair, in combination with the anticodon of the tRNA molecule, can select the amino acid corresponding to the tRNA species in question this is done on the basis of the stereochemical properties of the molecule. Since the anticodon of a tRNA molecule and the fourth nucleotide of the acceptor stem are far apart in space, two tRNA molecules must complex in a head-to-tail manner. The pocket thus formed can then fit specifically to the corresponding amino acid. 

Molecular imprinting can be accomplished in two ways (a), the self assembly approach and (b), the preorganisation approach3. The first involves host guest complexes produced from weak intermolecular interactions (such as ionic or hydrophobic interaction, hydrogen bonding) between the analyte molecule and the functional monomers. The self assembled complexes are spontaneously formed in the liquid phase and are sterically fixed by polymerisation. After extraction of the analyte, vacant recognition sites specific for the imprint are established. Monomers used for self assembly are methacrylic acid, vinylpyridine and dimethylamino methacrylate. 

The substituents on the phenyl group can also sterically influence the chiral recognition ability. The main-chain structures of 23a and 23x look similar, but the conformations of side groups are not the same. Three aromatic groups of... 

Fig. 6. Comparison of the biological activities of monovalent glucose and mannose derivatives, multivalent carbohydrate-substituted polymer with two saccharide epitopes per repeat unit, and the less sterically congested carbohydrate-substituted polymer with a single recognition element per repeat unit. All polymers were generated by ROMP using RuC13...

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