Complexation of Calix[4]crown-5-Ether with -Amino Acids and Their Molecular Recognition via UV/Vis Spectroscopy

The Association equilibrium constants of complexes between some -amino acids (Alanine, Valine, Leucine, Arginine, Aspartic acid, Glutamic acid, Histidine and Lysine) with 1, 3-dimethoxy, 2, 4dithiolCalix[4]crown-5-ether in chloroform and methanol mixed solvent (50%: 50%, V:V) have been investigated by means of UV/Vis spectrophotometry titration. On the basis of complexation formation values, it has been shown that amino acid to ligand ratio is exclusively 1: 1. The stability constant values of the formed complexes in descending order are as: Ala > Val > Arg > Lys > Leu > Asp > Glu > His. The variations in stability constants occurred between a given amino acid and Calixcrown in terms of various interactions. The structures of three different optimized conformers have been optimized with semi-empirical and ab initio computations. The conformers' stabilities are in following order: cone > partial-cone >1, 3-alternate. Structures of complexes formed between the crown-5-ether moiety of Calix[4]arene and protonated Arginine and Lysine were determined through minimization of complex formation energies. It has been indicated that the complex stability depends on number of amine groups in the alkyl chain as well as the number of methylene groups between the amine groups.