(DOTA-bis(amide)}lanthanide complexes: NMR evidence for differences in water-molecule exchange rates for coordination isomers

Two derivatives of 1,4,7,10-tetraazacyclododecane with trans-acetate and trans-amide side-chain ligating groups have been prepared and their complexes with lanthanide cations examined by multinuclear NMR spectroscopy. These lanthanide complexes exist in aqueous solution as a mixture of slowly interconverting coordination isomers with H-1 chemical shifts similar to those reported previously for the major (M) and minor (m) forms of the tetraacetate ([Ln(dota)](-)) and tetraamide ([Ln(dtma)](3+)) complexes. As in the [Ln(dota)](-) and [Ln(dtma)](3+) complexes, the m/M ratio proved to be a sensitive function of lanthanide size and temperature. An analysis of 1H hyperfine shifts in spectra of the Yb3+ complexes revealed significant differences between the axial (D-1) and non-axial (D-2) components of the magnetic susceptibility tensor anisotropy in the m and M coordination isomers and the energetics of ring inversion and m double left right arrowM isomerization as determined by two-dimensional exchange spectroscopy (EXSY). O-17 shift data for the Dy3+ complexes showed that both have one inner-sphere water molecule. A temperature-dependent O-17 NMR study of bulk water linewidths for solutions of the Gd3+ complexes provided direct evidence for differences in water exchange rates for the two coordination isomers. The bound-water lifetimes (tau (298)(M)) in the M and m isomers of the Gd3+ complexes ranged from 1.4-2.4 mus and 3-14 ns, respectively. This indicates that 1) the inner-sphere water lifetimes for the complexes with a single positive charge reported here are considerably shorter for both coordination isomers than the corresponding values for the [Gd(dtma)](3+) complex with three positive charges, and 2) the difference in water