Correlation of Water Exchange Rate with Isomeric Composition in Diastereoisomeric Gadolinium Complexes of Tetra(carboxyethyl)dota and Related Macrocyclic Ligands

The solution structure and dynamics of metal-bound water exchange have been studied in a series of diastereoisomeric gadolinium complexes of tetra(carboxyethyl) derivatives, of 1,4,7,10-tetraazacyclodecane. The structures of the (RRRS), (RSRS), and (RRSS) ligands and the Eu, Gd, and Tb complexes of the (RRRR) isomer have been determined by X-ray crystallography. Luminescence measurements on the Eu and Tb complexes revealed an integral hydration state (q = 1) in each case for the Eu isomers, whereas nonintegral values were measured for the (RRRR) and (RRRS) Tb isomers (e.g., [(RRRR)-Tb·1]^-, q = 0.60). The ratio of the twisted and regular monocapped square antiprismatic isomers has been measured in solution by ¹H NMR for the Eu and Tb complexes and followed the order, (RRRR) > (RRRS) > (RSRS) > (RRSS). Water exchange rates in the gadolinium complexe have been determined by ¹⁷O NMR and were fastest for the (RRRR) isomer [τ_m = 68 ns (298 K)] and correlated very well with the proportion of the twisted square anti-prismatic isomer. The rate of water exchange in the (RRRR) Gd complexes is likely to be sufficiently fast so as not to limit the overall relaxivity in higher MW conjugates.