NMR-STUDY OF SOLUTION STRUCTURES AND DYNAMICS OF LANTHANIDE(III) COMPLEXES OF DOTA

The low-temperature limiting NMR spectra (H-1 and C-13) of 12 LnDOTA- complexes (Ln = La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, and Lu) support the presence in solution of two isomeric forms. The relative abundance of the two isomers changes markedly along the lanthanide series with no apparent trend; NMR data suggest that the structure of the major isomer of LaDOTA- corresponds to that one of the minor isomer of the corresponding LU3+ complex. The solution structures of the two isomers are assigned on the basis of a fitting procedure of calculated to experimental chemical shifts of the resonances of the proton spectrum of YbDOTA-. The Yb3+-hydrogen distances utilized in these calculations are determined by exploiting the Curie spin mechanism contribution to the longitudinal and transverse proton relaxation rates. The solution structures of the two isomers differ for the layout of the acetate arms, the cyclododecane ring conformation being identical in both species. The interconversion between the two isomers occurs as the temperature is increased and this process may be independent of the rearrangement involving the cyclododecane ring, as shown from the C-13-VT NMR spectra of the Nd3+ complex and 2D-EXSY experiments on the Yb3+ Complex respectively.