Effects on Metal Binding of Circular NH- - -OC Intramolecular Hydrogen Bonding in Hydrophilic Pseudocavities of p-tert-Butylcalix[4]arenes Carrying Amino Acid Moieties

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Abstract
Calix[4]arenes bearing amino acid moieties were prepared by the reaction of carboxyl-protected amino acids with the acid chlorides of O-carboxymethylcalix[4]arenes. The binding ability toward metal ions was estimated by solvent extraction. These calixarenes showed selective extraction capability toward Na+ and Ag+ ions, which increased in the order 2a < 2b < 2d < 2c. The complexing ability was also affected by the nature of the solvents and was higher in ethyl acetate than dichloromethane. 1H NMR studies showed that the complexation of 2c with Na+ ion induces a change of the orientation of the amide groups from a network-like pattern with circular N−H- - -OC intramolecular hydrogen bonding of 2c to a pattern in which carbonyl groups converge on the Na+ ion. This change was also supported by a measurement of CD spectra.

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