Crown ethers conformational flexibility

ID and 2D 13C NMR were carried out in a series of novel nematogens by Bayle and co-workers to study the effects on the conformation and order due to the addition of lateral and/or terminal substituents.249 251 For example, lateral flexible substituents are found to adopt a mean conformation more or less parallel to the mesogenic core. As a result, lateral chains are less disordered than terminal chains. Nematogens containing polyoxyethylene ether (POE) chain as a lateral crown ether and terminal chain(s) have been... 

When 18-crown-6 was co-lyophilized with a-chymotrypsin, a 470-fold activation was seen over the free enzyme in the transesterification of APEE with 1-propanol in cyclohexane (Scheme 3.2) [96]. There was a low apparent specificity for the size and macrocyclic nature of the crown ether additives, suggesting that, during lyophilization, 18-crown-6 protects the overall native conformation and acts as a lyoprotectant. To examine this global effect, FTIR was used to examine the effect of crown ethers on the secondary structure of enzymes. In one study [98], subtilisin Carlsberg was shown to retain its secondary structure in 1,4-dioxane when lyophi-lized in a 1 1 ratio with 18-crown-6. In addition, examination of FTIR spectra from varying incubation temperatures indicated that an increase in crown ether content in the final enzyme preparation resulted in a decreased denaturation temperature in the solvent, indicating a more flexible protein structure. 

The slightly less negative value found for the entropy term can be explained by the preorganization of the ligand in the 1,3-alternate conformation, where only a small part of the crown ether moiety is rather flexible. This flexibility is lost with the large cesium cation, which fits very well into the cavity created by the polyether ring and the aromatic nuclei.34... 

Bis(crown ethers) connected by a flexible spacer are a source of intramolecular sandwich-type complexes with alkali metal ions <1992MI1>. A conformational analysis (based on a combination of semiempirical and ab initio methods) performed on 12-crown-3 38 and 12-crown-4 39 predicted that, in the case of sandwich-type complexation, the nucleophilic cavity of 12-crown-3 rather than that of 12-crown-4 is more prone to complexation with the Na+ ion. Accordingly, ion-selective electrodes based on bis(12-crown-3) derivatives with dialkylmalonate spacers displayed the highest selectivity for Na+ ions among the alkali and alkaline earths investigated, and superior to the Na+ selectivity reached with the bis( 12-crown-4) analogue <2003ANA(480)291>. 

Isoaromatization of dienone macrocycles afforded Horning-crown macro-cycles - flexible macrocycles bearing structural elements reminiscent of those found in both calixarenes and crown ethers. In some cases the Horning-crown macrocycles exhibited solvent-dependent and switchable conformations. For macrocycles with the same short linker, self-complementarity was observed, and dimers tended to crystallize as solvates or inclusion compounds. This tendency was suppressed with longer linkers and in some Horning-crowns derived from trapezoidal macrocycles. These properties suggest potential applications in analysis, separation and detection (Figure 6.8). ... 

Calix[n]arenes are cyclic condensation products of para-substituted phenol derivatives and formaldehyde [29], They are highly interesting for the development of sensitive coatings due to their conformational flexibility and the ease by which they may be modified chemically. Chemical modification can be done either in the meta position, or by reactions at the hydroxy group. In this way, bulky substituents [30], chelating substituents [31], aromatic residues [32], crown ethers [33,34], peptides [35,36], etc. can be introduced. A first approach to combinatorial synthesis of calix[4]arene receptors has been published by Reinhoudt and co-workers [37,38], who prepared calixarenes with different substituents. In solution, these calixarenes lead to formation of hetero-oligomers with barbiturates, and these hetero-oligomers were detected by MALDI-TOF mass spectrometry and H-NMR spectroscopy. 

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