We have synthesized a new macrocyclic ligand, N,N'-Bis[(6-carboxy-2-pyridyl)methyl]-1,7-diaza-12-crown-4 (H(2)bp12c4), designed for complexation of lanthanide ions in aqueous solution. The X-ray crystal structure of the Gd-III complex shows that the metal ion is directly bound to the eight donor atoms of the bp12c4 ligand, the ninth coordination site being occupied by an oxygen atom of a carboxylate group of a neighboring [Gd(bp12c4)](+) unit, while the structure of the Lu-III analogue shows the metal ion being only eight-coordinate. The hydration numbers obtained from luminescence lifetime measurements in aqueous solution of the Eu-III and Tb-III complexes suggest an equilibrium in aqueous solution between a dihydrated (q = 2), ten-coordinate and a monohydrated (q = 1), nine-coordinate species. This has been confirmed by a variable temperature UV-vis spectrophotometric study on the Eu-III complex. The structure of the complexes in solution has been investigated by H-1 and C-13 NMR spectroscopy, as well as by theoretical calculations performed at the DFT (B3LYP) level. The results indicate that the change in hydration number occurring around the middle of the lanthanide series is accompanied by a change in the conformation adopted by the complexes in solution [Delta(lambda lambda lambda lambda) for q = 2 and Lambda(delta lambda delta lambda) for q = 1]. The structure calculated for the Yb-III complex (Lambda(delta lambda delta lambda)) is in good agreement with the experimental structure in solution, as demonstrated by the analysis of the Yb-III-induced paramagnetic H-1 shifts.