Paolo De Bernardin

A new water soluble Zn-salophen derivative as a receptor for α-aminoacids: Unexpected chiral discrimination†

A new water soluble zinc-salophen complex with appended D-glucose moieties was synthesized and characterized. Its binding properties toward six aminoacids were investigated by UV-vis spectrophotometric titrations. Association constants showed to be unfavorably affected by increasing steric hindrance of the side chain. Quite surprisingly, different association constants were measured for the L and D enantiomers. These data suggest that aminoacids are bound via two interactions, zinc-carboxylate coordination and two hydrogen bonds between the ammonium group of the aminoacid and two oxygen atoms of one D-glucose moiety. Such conclusion is supported by the result of semiempirical (PM3) calculations. Notably, the K(L)/K(D) value of 9.6 observed for the association of phenylalanine rivals with the highest values found for the chiral recognition of aminoacids in water.

Ion-Pair Recognition by Metal)Salophen and Metal)Salen Complexes

The development of heteroditopic receptor systems that can simultaneously bind cationic and anionic species is one of the most challenging research topics in supramolecular chemistry, attracting the attention of a large number of research groups worldwide. Such an interest is due especially to the fact that the overall receptor–ion-pair complex is neutral and this can be advantageous in many situations, such as salt solubilization and extraction, and membrane-transport applications. Receptors designed for ion-pair complexation are molecules comprising well-known anion-binding motifs and familiar cation-binding sites. An important family of compounds that can use metal Lewis-acidic centres for anion recognition and that can be easily derivatized to introduce an additional binding site for the cation is metal–salophen and metal–salen complexes. This short review shows that the high versatility of salen and salophen ligands and of the corresponding metal complexes allows, through simple modifications of the basic skeleton, the obtention of highly efficient receptors for ion pairs.

Paramagnetic Relaxation Enhancement Experiments: A Valuable Tool for the Characterization of Micellar Nanodevices

Micellar incorporation of hydrophobic molecular receptors is a promising strategy to obtain efficient nanodevices that work in water. In order to fully evaluate the potential of this approach, information on the localization and orientation of the receptor inside the micelle are necessary. Systematic studies undertaken on a uranyl-salophen receptor incorporated into CTABr and CTACl micelles show that nuclear magnetic resonance paramagnetic relaxation enhancement (NMR-PRE) experiments are particularly suitable to provide this type of information. The effect on the measurements of surfactant concentration, nature of the surfactant polar head, and ionic strength is also reported. Notably the normalization procedure applied to the obtained data can be considered of general application, thus enabling the comparison of information collected for different types of supramolecular micelle/receptor systems.