Cavitands hydrophobic cavities

Some of the earliest work to be carried out on cage compounds was by Donald Cram and co-workers [7]. Much of his initial work was concerned with bowlshaped species containing deep cavities, called cavitands, such as 2 (Fig. 4). Such hosts are capable of strong neutral guest complexation due to the deep hydrophobic cavity that they possess. 

The template plays an important role in the formation of capsule F because no directional non-covalent bonds such as H-bonding and M-L interactions are involved in the self-assembly process. In addition, an external hydrophobic template is necessary because aqueous solvent does not template the formation of F. Capsule F consists of two octa-acid, deep-cavity cavitands with a pseudo-conical hydrophobic cavity (Figure 6.6a). The eight carboxylic acid groups are located at the periphery of the cavitand, thereby inducing... 

Cucurbit[n]urils (n = 1 and 8) (38) have also been used for the molecular recognition of zwitterionic species in aqueous medium (Figure 16). These systems show a hydrophobic cavity with the urea-type carbonyls pointing to the corresponding entrances. This peculiarity implies binding properties with some resemblance to other cavitands (CDs, calixarenes, or resorcinarenes). The size of the cucurbituril receptor can be controlled by the number of monomers present in the structure, and they are usually named as CBn (with n being the number of repeating units). 

A bowl-shaped Pd Ls precursor 269 undergoes dimerization in the presence of a nine-residue peptide in NaNOs aqueous solution by Scheme 4.24 to give a 1 1 cage complex of the Pd Lg coordination capsule 504 with large hydrophobic cavity this capsule is in equilibrium with its cavitand analog [25]. According to the NMR data, the guest peptide molecule adopts a-helical conformation within the hydrophobic cavity of 504 [25]. 

Cucurbituril is a hexameric macrocyclic compound with the formula (C6H6N402)6 shaped like a pumpkin which belongs to the botanical family Cucurbitaceae. This macrocyclic cavitand has idealized symmetry Z>6h with a hydrophobic internal cavity of about 0.55 nm. The two portals, which are each laced by six hydrophilic carbonyl groups, have a diameter of 0.4 nm [Fig. 20.4.8(a)],... 

The four-wall water-soluble cavitand IV-5 (Scheme 12.16) forms a stable deep cavity that has a high affinity towards choline B-1 and acetylcholine B-2 (association con-stants>10 M ) in DjO. Interestingly, the trimethylammonium head of B-1 is pointed inward into the cavity of the host. In contrast, the long chain trialkylanunonium ions, such as dodecyltrimethylanunonium ion, are bound with the (helical folded) alkyl chain in the cavity, but with an association constant of >1(1 M . In addition to the favourable electrostatic interaction between the tetracarboxylate wider rim and the tetraalkylam-monium core, hydrophobic interactions dominate the binding in this case and reveal the bitopic character of both the host and the guest. Isothermal titration calorimetric data have shown that the complex formation is both enthalpically and entropically driven. 

Deep-cavity cavitands that dimerize into capsules via the hydrophobic effect, in the presence of a suitable guest molecule and in aqueous solution, have been developed by the Gibb group [110,111], Such complexes possess hydrophilic outer coats, hydrophobic rims that favor self-assembly, and deep hydrophobic pockets (up to 1 nm wide to 2 nm long). They have been used to drive the formation of high-definition assemblies with a number of guest molecules, including steroid and hydrocarbon molecules [112]. Reactions within the capsule (eg, selective oxidation of substrates) and potential applications in hydrocarbon gas separation [111] have been also achieved or demonstrated [113]. 

In this case the driving force for complexation is the hydrophobicity of the analytes which prefer the liphophilic cavity to water solvation. For a given molecule the log Poet is a quantitative measure of its hydrophobicity. Only analytes with log Poet greater than 1.5-1.6 are detectable by cavitand-coated QCM in water (Table 3). This effect is clearly illustrated by the different behaviour of anisole and phenol. Both are aromatic derivatives bearing an electrodonating group but the more polar phenol (log Poet = 1.46) prefers water solvation to inclusion in the cavity of either 41 or 42. The opposite is observed for anisole (log Poet = 2.11). 

In conclusion the selective response of cavitand-coated QCM sensors in water to neutral analytes depends on the size and shape of the liphophilic cavity and on the hydrophobicity of the analyte. The presence of the strong hydrophobic effect gives rise to a great enhancement in sensitivity (at least one order of magnitude with respect to QCM in air), balanced by a reduction in selectivity, since specific interactions like CH-tc ones, are much weaker. 

Giles MD, Liu S, Emanuel RL, Gibb BC, Grayson SM (2008) Dendronized supramoleculeu nanocapsules pH independent, water-soluble, deep-cavity cavitands assemble via the hydrophobic effect. J Am Chem Soc 130 14430-14431... 

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