Guest complexes sulfates

The crystal structure of a host-guest complex between triethylammonium sulfate and a calix[4]arene derived from resorcinol (Figure 23) has been reported by Mann and coworkers [36]. The host was found to exist in an approximately conical form. Alternating hydrophilic and hydrophobic layers were foimd in the crystal. 

Kubik and co-workers have developed a series of highly effective anion receptors based upon cyclic peptides. Cyclic hexapeptide receptors such as 17 consist of alternately linked L-proline and 6-aminopicolinic acid subunits [25]. A 1 1 binding stoichiometry for 17 and the sodium salt of ben-zenesulfonate was confirmed by a Job plot but in the case of the halide and sulfate sodium salts 2 1 host/guest complexes were found. This was confirmed by electrospray mass spectrometry and in the case of iodide a crystal structure of the 2 1 complex was obtained where the iodide was sandwiched between two cyclic hexapeptide receptors. 

Kaniansky et al. have investigated the CZE separations of inorganic anions (chloride, bromide, iodide, sulfate, nitrite, nitrate, fluoride, phosphate) based on their host-guest complex equilibria with a-cyclodextrin (a-CD) [22]. a-CD employed as a host was found to influence selectively the effective mobilities of the anions. The anions were detected by conductivity after CE separation. The developed method has been successfully applied in the anions in milk samples with satisfactory results. 

Fig. 9. a Structure of the [18]crown-6 dimethyl sulfate complex 16) b details of the infinite chain of alternating host and guest molecules 16)... 

The effects of solvation/desolvation equilibria on the thermodynamics of complexation can be discerned in the pattern of formation constant values across a series of azaciyptand hosts or anionic guests. In the dianionic series thiosulfate, selenate, sulfate, formation constants, to a first approximation, decrease as the hydration energy of the anion increases, reflecting the enthalpic cost of desolvation. 

To our surprise, we found that 5 binds anions such as halides or sulfate even in 80% D2O-CD3OD, which is remarkable considering that, despite the large excess of water molecules in these mixtures, the hydrogen-bonding interactions between the anions and the peptide are still efficient [22], Assignment of the structure of the anion complexes of 5 revealed the reason for the unusual receptor properties of this peptide. We could show that 5 preferentially forms 2 1 sandwich-type complexes with suitable anions in which the guests are bound in a cavity located be-... 

---