Receptors guanidinium-based

Schmuck, C., How to improve guanidinium cations for oxoanion binding in aqueons solution The design of artificial peptide receptors , Coord. Chem. Rev. 2006, 250, 3053-3067. 

Despite these disappointing results, a great deal of activity has focused on guanidinium-based hosts. The majority of recent systems are acyclic, however, analogous to podand hosts for cations. These species combine ease of synthesis (no high dilution required) and the fast complexation/decomplexation 

Bicyclic guanidinium derivatives have also been incorporated into bis(guanidinium) receptors capable of recognising tetrahedral oxoanions, particularly polyphosphate residues of important biological molecules. The flexibility of receptor 4.31 allows the two guanidinium arms to swing into a mutually perpendicular arrangement in order to effectively complex polyphosphates even in water. 

The binding of citrate by 4.28 has been investigated by the indicator displacement assay (IDA) method (cf. EDTA titrations. Section 3.1.3). Binding of an organic dye indicator with comparable structure to citrate competes with the substrate for the same binding site. The binding of the indicator 

Different enantioselectivity has been observed for adenosine monophosphate nucleotides complexed with 11, where one naphthoyl unit has been sub- 

Despite its ionic structure 12 is almost insoluble in water. The affinity toward amino acids was studied by extraction experiments, where the enantiomers of Phe, Val, and Trp were extracted into CD2C12 from aqueous solutions. The amounts of amino acids extracted were determined by NMR integration . As expected, the aromatic amino acids were preferred (40% extraction efficiency for L-Phe and L-Trp). In the organic phase, no valine was detected. Moreover, almost no D-enantiomers were extracted by (S,S)-12 (D-Trp 0.5%,D-Phe 2%). 

When trying to elucidate the structure of complexes 14a with L-Trp and D-Trp, only ill-defined spectra have been available. No NOE signals could be detected between naphthalene and the indole side chain, although for the complex with L-Trp a few contacts could be attributed to the proximity between the 

The lasalocid derivatives 20a,b and 21a,b were less effective enantioselective carriers than the crown-ether-based compounds. Interesting behavior was observed on studying the complexation properties of 21b. Although this receptor showed a remarkable selectivity for L-Trp, negligible enantioselectivity for Phe was observed. 

These ligands were shown to extract many AT-acyl a-amino acids from aqueous phosphate buffer (pH=7.4) into chloroform and also their preformed tetra-butylammonium salts. L-Enantiomers of the amino acids AT-Ac-Ala, AT-Ac-Phe, AT-Ac-Val, and N-Ac-Trp were extracted with sevenfold preference by ligand 24. Analogously, ligand 25 was found to be even more efficient, as the L-enan-tiomeric preference was tenfold. Ligands 26 and 27, however, were much less efficient. 

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The sulfate anion is geometrically similar to phosphate, though less basic. In spite of the inherent difficulty for its recognition, there are a few examples in the field of guanidinium-based receptors for sulfate binding. 

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