Calix arene comparison

As neutral carriers for the chemical modification, 16-crown-5 and calix[4]arene derivatives possessing a triethoxysilyl group (7) and (8) were designed for Na sensors. Triethoxysilylethyl-16-crown-5(7) was then mixed with a silicone-rubber precursor for the membrane fabrication accompanying covalent bonding of the neutral carrier. Comparison of IR spectra before and after extraction of the nonbonded neutral carrier... 

The X-ray structure of the L-Sr(Picrate)2 (L = p-tert-butyl-calix[4]arene-tetra(diethylamide)) is reported, as well as MD simulations on the L M2+ complexes in vacuo, in water, and in acetonitrile solutions for alkaline earth cations with a comparison of converging and diverging conformers.130 In the simulated and solid-state structures of the L M2+ complex, the ligand wraps around the complexed cations M2+ (more than it does with alkaline cations), which are completely encapsulated within the polar pseudo-cavity of L, without coordination to its counterion in the crystal or to solvent molecules in solution. In contrast to alkali cation complexes, which display conformational flexibility in solution, computations show that the alkaline earth cation complexes are of the converging type in water and in acetonitrile. Subtle structural changes from Mg2+ to Ba2+ are observed in the gas phase and in solution. Based on FBP calculations, a binding sequence of alkaline earth cations was determined Mg2+ displays the weakest affinity for L, while Ca2+ and Sr2+ are the most stable complexes, which is in agreement with the experiment. 

The comparison of lower rim tetrasubstituted calix[4]arene 13a with the corresponding pentasubstituted calix[5]arene 13b (both compounds immobilised in the cone conformation) revealed that the higher number of ferrocene units in 13b does not positively influence the complexation ability of the receptor. The stability constants of 13a and 13b with chloride anion were found to be 55 and 15 M"1, respectively. As indicated by cyclic voltammetry, both receptors are selective towards the dihydrogen phosphate anion (cathodic shift -160 mV). 

Fig. 2.2.2. Comparison of the structures of a cyclic hexapep-tide (left), a calix[6]arene (right), and a cyclic hexapeptide composed of alternating natural amino acids and 3-aminobenzoic acid (center).

For comparison, let us compare the methanol data with those obtained by Horsewill et al. [80] who have reported an intramolecular quadruple proton transfer in the solid state between the four OH groups of solid calix[4]arene. Almost temperature independent rate constants were observed, which are again indicative of tunneling. An Arrhenius curve can be calculated using reasonable parameters (Table 6.4) which can reproduce both the pure methanol and the calix[4]arene data. It would be interesting to know more about the kinetic isotope effects in both systems. 

These studies have been extended to the calix[6]arenes, which also have been observed to show changes in the HNMR spectra as incremental amounts of strong base, up to six equivalents, are added to a DMSO solution. With the calix[6]arene systems the spectra are considerably more complex than those of the calix[4]arenes, and conformational assignments are much more difficult to make. However, a comparison of the temperature-dependent behavior of the lithium and sodium oxyanions of the calix[6]arenes indicates that the sodium system is less conformationally mobile and suggests that here, also, a crown ether-like complexa-tion of the metal ion may be occurring. The annulus of the calix[6]arene system is complementary in size to the sodium cation, while that of the calix[4]arene is complementary in size to the lithium cation. 

A in smectite [23] and 15.0 A in hydrated sodium vermiculite [24]. Second, the hydrated layer in sodium vermiculite is 9.4 A, while the inorganic layer in Na5[calix[4]arene sulfonate] 12 H2O is 8.3 A. A more complete comparison has been presented [19]. 

Figure 1 Comparison of a substrate inside the binding pocket of an enzyme and a host inside a supramolecular capsule, (a) Schematic representation of phytate binding inside the binding pocket of Escherichia coli phytase, based on the crystal structure. Note the numerous hydrogen-bonding interactions, (b) Calculated structure of anandamide inside a self-assembled, calix[4]arene-derived capsule. Note the hydrogen bonds that hold the capsule together and the lack of specific interactions between the host stmcture and the guest. (Figure reproduced with permission from reference 34. (2007) National Academy of Sciences, USA.)...

Theoretically, calix[4]arene phosphites can exist in six conformations. X-ray structural analyses, NMR spectroscopic studies, and comparison with related monophosphates and monoarsenites showed that, depending on the substituent R, preferentially conformation a or b is adopted (Scheme 2.101) [109]. Both can interconvert simply by rotating the substituted ring from an up to a down position. 

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