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

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Journal of the American Chemical Society

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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 [τm = 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.


Copyright (2021), ACS


*At the time of publication, Mark Woods was affiliated with Durham University.

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