In complexation of 18-crown-6 with glycine, L-alanine, L-serine, L-threonine, L-asparagine, L-glutamine, L-histidine, L-phenylalanine, and diglycine in D 2 O at 298 K, side groups of the amino acids are not involved in additional interactions with the macrocycle, except the OH group of threonine.All the known applications of crown ethers are based on their unique capability for selective complexation with cations and neutral molecules. The choice of the guest molecule by host macromolecules is determined by the complementarity principle and is the basis for many biochemical processes such as enzymatic catalysis, membrane transport, antigen3 antibody interactions, etc. [1,2]. The principle of molecular recognition determines the use of crown compounds and their derivatives for separation of biomolecules (e.g., amines and amino acids) [335].Published data [6 38] show that amino acids and peptides form with 18-crown-6 1 : 1 complexes. A 19 F NMR study shows that complexation of 18-crown-6 with b-fluoro-a-amino acids in acid solutions is due to H bonding of the NH 3 + group of the amino acid with alternating oxygen atoms of the macroring [7]. The potentiometric titration data and results of computer simulation, given in [8], also support this complexation mechanism. First, the curves of titration of D,L-phenylalanine in methanol in the presence of 18-crown-6 and without it confirm that the NH 3 + group is the center of the complexation with the crown ether. Second, in the case of blocking of the NH 3 + group (e.g., with acetate ion), no complexation occurs. Third, the results of computer simulation show that the hydrogen atoms of the amino group are in short contact with the macrocycle: three hydrogen bonds are formed, and three oxygen atoms of 18-crown-6 are involved in the electrostatic interaction with the NH 3 + group. However, participation of side groups of amino acids in the complexation with 18-crown-6 was not discussed, and data on this problem are lacking.Previously we made a thermodynamic study of complexation of 18-crown-6 with L-amino acids and oligopeptides in aqueous solution at 298.15 K [9, 10]. The results were interpreted assuming participation of side groups of some amino acids in additional interactions with the macrocyclic ring of the ether. To confirm this assumption, we studied in this work the mechanism of complexation of 18-crown-6 with a series of amino acids (glycine, L-alanine, L-serine, L-threonine, L-asparagine, L-glutamine, L-histidine, L-phenylalanine) in water by 13 C NMR spectroscopy and correlated the coordination shifts with the previously obtained thermodynamic parameters of the complexation.The shifts Dd C of the signals of the carbon nuclei in the amino acids incorporated in the complex with 18-crown-6 relative to the free amino acids are listed in the table. According to [11], at these concentrations the amino acids do not form associates and the interactions of species are long-range and are accompanied by destruction of the solvation shells and decrease in the hydration numbe...