CONFORMATIONAL-ANALYSIS OF ALKALI-METAL SALTS OF A SYNTHETIC CARBOXYLIC IONOPHORE BY NMR-SPECTROSCOPY IN COMBINATION WITH X-RAY CRYSTALLOGRAPHY

The solution structures of a linear synthetic carboxylic polyether ion ophore, HO[CH2CH2O(1,2-C6H4)O](4)-CH2(1,2-C6H4)COOH, 1, and its alkali metal salts have been studied by H-1 and C-13 NMR spectroscopies (H-1 -H-1 COSY, NOESY, C-13-H-1 COSY, HMQC, HMBC, and long range C-13 J res olved 2D-NMR). The ionophore 1 selectively transports K+, Rb+, and Cs over Na+ and Li+. The H-1 NMR spectra of 1 and the Li and Na salts ar e simple, whereas those of K, Rb, and Cs salts are more highly chemica l shift resolved but are very similar to each other. Conformational an alyses of K, Rb, and Cs salts were carried out on the basis of the 3D structures in the crystal using vicinal three-bond coupling constants, NOE, and chemical shifts, which depend on the torsion angles of 1,2-e thylenedioxy, the distances between methylene and aromatic methyne pro tons, and the magnetic anisotropy mainly resulting from the aromatic r ings, respectively. These data show good correlation with those expect ed from the structures in the crystal. The backbone bending observed i n the crystals was also confirmed with chemical shifts and (3)J(H-C) o f C-13 NMR spectra of the K, Rb, and Cs salts. These results show that the conformation of K, Rb, and Cs salts which is similar to that in t he crystal is dominant in a liquid membrane, but the Li and Na salts r emain highly mobile in solution. These results are discussed in connec tion with the ion-transport ability of 1 through the liquid membrane.