Size-selective hydrophobic cavity

Cyclodextrins provide a hydrophobic micromedium in an aqueous phase. This characteristic is analogous to the reaction pockets of enzymes. Enzymes provide size-selective hydrophobic cavities and catalyze the reactions of bound substrates. As one might therefore expect, artificial enzymes based on cy-... 

MS detection is difhcult in MEKC because the ion source is contaminated by the surfactants. The addition of charged, water-soluble cyclodextrin (CD) as a modifier in MEKC was introduced to improve the resolution of PAHs by allowing for their differential partitioning between the micellar and the aqueous/CD phase. The chiral character of CD increased selectivity based on size and shape. Cyclodextrins, produced by the enzymatic digestion of starches, have a toroidal shape with a hydrophobic cavity and a hydrophihc exterior. Nonpolar solutes can enter the cavity and form... 

CDs are oligosaccharides with characteristic toroidal molecular shapes. Their outside surfaces are hydrophilic, while their cavities are hydrophobic. CDs tend to include molecules, which fit their cavities by hydrophobic interaction. The most widely used CDs consist of six (a-CD), seven (fi-CD), or eight (y-CD) glucopyranose units. The size of the cavity differs significantly among the a-, / -, and y-CDs. Selectivity results from the inclusion of a portion of a hydro-phobic solute into the cavity. The use of CDs is especially effective for the separation of aromatic isomers and aromatic enantiomers that have the chiral center close to the aromatic ring. Many CD derivatives have been developed for increased solubility in water as well as for modifying the cavity shape that provides different selectivities from un-derivatized CDs and are commercially available. 

We intended to design novel macrocyclic compounds as hosts that have the following characteristics. The macrocyclic compounds should be soluble in water and have hydrophobic cavities inside to form host-guest inclusion complexes with organic compounds by hydrophobic interactions in aqueous solution. The hydro-phobic cavities should be sufficiently rigid to show selectivity upon complexation. It is highly desirable that cavities of various sizes are available. 

Receptors 2.64 and 2.65 (derived from ds-l,3,5-tris(aminomethyl)cyclohexane) have both been incorporated into membrane ion-selective electrodes. The two hosts differ dramatically in their degree of rigidity and preorganisation, and the size of the cavity that they create and consequently display very different response characteristics. The more rigid 2.65 shows Hofmeister-like selectivity while the more flexible tris(2-aminoethylamine) derivative (2.64) proved to be distinctly anti-Hofmeister. The Hofmeister series was developed in 1888, based on the ability of ions to salt-out proteins and correlates the hydrophobic tendencies of various ions. 

Macrocyclic ligands such as crown ethers have been widely used for metal ion extraction, the basis for metal ion selectivity being the structure and cavity size of the crown ether. The hydrophobicity of the ligand can be adjusted by attachment of alkyl or aromatic ligands to the crown. Impressive results have been obtained with dicyclohexano-18-crown-6 as an extractant for Sr in [RMIM][(CF3S02)2N] IL/aque-... 

PCPs with well-defined pores and surface-isolated Lewis acid sites could potentially serve as size- or shape-selective heterogeneous catalysts, in a similar manner to zeolites.33 43 161-164 The two-dimensional PCP, [Cd(4,4 -bpy)2(H20)2] 2N03 4H20 , was the first example that showed catalytic properties for the cyanosilylation of aldehydes.33 Experimental data in the case of cyanosilylation of imines, which is also performed by the same compound, led to the conclusion that hydrophobic grid cavities bind to the substrate very efficiently to promote a rapid reaction, and that the heterogeneous reaction involves the selective activation of the imino nitrogen by the weak Lewis acid Cdn center.161 In this polymer, the NO3" anions exist in a coordination-free state. This situation contributes to increasing the Lewis acidity of the Cdn centers. 

A host of carriers, with a wide variety of ion selectivities, have been proposed for this task. Most of them have been used for the recognition of alkali and alkaline metal cations (e.g., clinically relevant electrolytes). A classical example is the cyclic depsipeptide valinomycin (Fig. 5.13), used as the basis for the widely used ISE for potassium ion (38). This doughnut-shaped molecule has an electron-rich pocket in the center into which potassium ions are selectively extracted. For example, the electrode exhibits a selectivity for K+ over Na+ of approximately 30,000. The basis for the selectivity seems to be the fit between the size of the potassium ion (radius 1.33 A) and the volume of the internal cavity of the macrocyclic molecule. The hydrophobic sidechains of valinomycin stretch into the lipophilic part of the membrane. In addition to its excellent selectivity, such an electrode is well behaved and has a wide working pH range. Strongly acidic media can be employed because the electrode is 18,000 times more responsive to K+ than to H+. A Nernstian response to potassium ion activities, with a slope of 59mV/pK+, is commonly observed... 

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