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 1 H 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 17 O 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.