Calixarenes hydrophobicity

Another approach to functionalize calixarenes and resorcinarenes with peptides consists in modification of lower rims. Such a modification requires introduction of all potential binding sites within substituents since derivatives do not take advantage of the inclusion properties of the calixarene hydrophobic cavity. Additionally, the lower rim is much more narrow than the upper rim, and, therefore peptides are arranged in close proximity. As a consequence, it is difficult to avoid adverse intramolecular non-covalent interactions. In fact, derivatives 12 and 13 exhibit... 

In an effort to restrict the location of semiconductor nanoparticles in LB films and inhibit aggregation, the formation of CdS in LB films of calixarenes was investigated [195]. Limiting areas of 3.0 nm and 1.8 nm were obtained on 0.5 mM CdCli, compatible with the cross-sectional areas of the calixarenes. Y-type LB fdms were prepared at 25 mN m on glass, quartz, and silicon. The substrates had been made hydrophobic by treatment with a silane vapor. After H2S treatment overnight in sealed jars, UV absorbance spectra and XPS data were obtained. The absorption edge for the CdS particles formed in the calixarene LB films transferred at pH 5.5 was 3.3 eV as compared with 2.7 eV for films formed in cad-... 

The specific structure of [(H20)5Ni(py)]2+ was observed in the complexes with the second-sphere coordination of calix[4]arene sulfonate.715 There are two different [(H20)5Ni(py)]2+ cations in the complex assembly. In one the hydrophobic pyridine ring is buried in the hydrophobic cavity of the calixarene with the depth of penetration into the calixarene cavity being 4.3 A (Figure 9). The second independent [(H20)5Ni(py)]2+ cation is intercalated into the calixarene bilayer. 

It has already been mentioned that metal complexes with confined binding pockets often display unusual chemical reactivities (see Section II). Thus, complexes of substituted hydrotris (pyrazolyl)borates, in which the substituents serve to from a hydrophobic binding pocket, have already been shown to exhibit enhanced chemical reactivity when compared with their unmodified analogs (282,283). Likewise, cyclodextrin and calixarene-based metallocavitands have been used as catalysts for selective organic transformations, and even as catalysts for reactions that... 

In principle, there are four basic strategies to compensate for the repulsive effects between the hydrophobic fullerene surface and water (a) encapsulation in the internal hydrophobic moiety of water-soluble hosts like cyclodextrins (Andersson et al., 1992 Murthy and Geckeler, 2001), calixarenes (Kunsagi-Mate et al., 2004) or cyclotriveratrylenes (Rio and Nierengarten, 2002) (b) supramolecular or covalent incorporation of fullerenes or derivatives into water-soluble polymers (Giacalone and Martin, 2006) or biomolecules like proteins (Pellarini et al., 2001 Yang et al., 2007) (c) suspension with the aid of appropriate surfactants and (d) direct exohe-dral functionalization in order to introduce hydrophilic moieties. 

Results obtained by ES/MS confirm that the stability of calixarene/cation complexes depends upon the medium. The calixarene in solution presents a strong affinity for cesium, whereas in the gas phase, it displays a stronger affinity for sodium. Moreover, the stability of calixarene/Na+ complexation in a solvent phase is increased by the presence of water in the dilution system (up to 40% in acetonitrile), whereas other alkali complexes are destabilized by the presence of water. Finally, affinity for sodium, which is weak in the solution for calixarenes bearing benzo moieties, considerably increases in the gas phase. These results confirm the interpretation of the MD simulations in an aqueous phase, which lead the authors to conclude that cesium-over-sodium selectivity is governed by the hydration of the sodium cation in the complex, and by the higher hydrophobicity of the complexation site leading to an enhancement of selectivity for cesium over sodium 49... 

In mixtures with the hydrophobic dicarbollide anion, the extraction of Ca, Sr, and Ba by calixarenes with oxygen donor atoms improves, allowing the separation from alkali ions.131 The competition by alkali ions is lower than in crown ether extraction systems. Separation factors S with CA2 in nitrobenzene are log S = 7 (Ca), 5.5 (Sr), and 5.4 (Ba). 

The ESIPT of 2-(2 -hydroxyphenyl)-4-methyloxazole (HPMO) (27) has been explored by Douhal and co-workers [166] for its probe characteristics in a variety of organized media which include cyclodextrin, calixarene, micelle, and HSA. The incorporation of HPMO into hydrophobic cavities in an aqueous medium involves the rupture of its intermolecular hydrogen bond to water and formation of an intramolecular hydrogen bond in the sequestered molecule. Upon excitation (280-330 nm) of this entity, a fast intramolecular proton-transfer reaction of the excited state produces a phototautomer (28), the fluorescence of which (Xm = 450 170 nm) shows a largely Stokes-shifted band. Because of the existence of a twisting motion around the C2—C bond of this phototautomer, the absorption and emission properties of the probe depend on the size of the host cav-... 

In aqueous solution, water-soluble calixarenes have the potential to bind much more strongly to organic guests than in lipophilic media because of the hydrophobic effect (Section 1.9). Unfortunately, p-alkylcalixarenes are not water-soluble and must be derivatised in order to take them up into aqueous... 

Figure 7.40 Structure of the spherical lanthanide p-sulfonatocalix[4]arene assembly (a) partial space filling view along the pseudo-fivefold axis. Pyridine N oxide and one calixarene are shown in stick mode. S03 groups line the surface of the sphere, aryl rings define the hydrophobic shell and the polar core comprises 30 water molecules and two Na+ ions, (b) cut away view showing an [Na(H20)6] 2 cluster within the core. (Reprinted with permission from AAAS from [52]).

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