Paramagnetic Relaxation Enhancements in Acetate and Its Fluorine Derivatives Interacting with Gd³⁺: Complex Formation, Structure, and Transmetallation

Abstract

The relative spatial distribution and motion with respect to Gd(3+) of the (1)H and (19)F nuclei in the acetate ion and its fluorine derivatives are studied in D(2)O solutions through the paramagnetic relaxation rate enhancements (PREs) of these nuclei. We derive general theoretical expressions of the longitudinal PRE in terms of the analytical concentrations of metal and ligands, formation constants of the complexes, metal-nucleus distances, and coordination lifetimes of the ligands. The observed formation constants of the 1 metal: 1 ligand complexes markedly decrease with increasing number of fluorine atoms, the electronegativity of which reduces the negative partial charge of the coordinating COO(-) group. The coordination lifetimes are very short at the scale of the relaxation times of the protons of metal bound acetate, that is, shorter than about 10 μs. The average distance of the acetate protons from Gd(3+) is in fair agreement with independent crystallographic determination. The release of free Gd(3+) from the very stable Gddtpa (dtpa=diethylene-triaminepentaacetate) complex caused by the competition of Zn(2+) for dtpa, is evidenced by an increase of the PREs with Zn(2+) concentration. The observed PRE increase is consistent with the known equilibrium constants governing the speciation involving Gd(3+), Zn(2+), and dtpa. The present case study illustrates a method which easily yields experimental tunable properties suitable to test the ongoing theories of lanthanide Ln(3+) complexation in solution.

Ligation driven 19F relaxation enhancement in self-assembled Ln(iii) complexes

“…The strong binding and relaxation enhancements observed with 4 represent a 20-fold improvement over those previously observed for ionic coordination species. 14,[25][26][27] Similar relaxation rate observations were made with trifluoromethyl-lactate coordinated Gd-DO3A chelate species (R 1 B 100 s À1 at 7 T), however, extremely poor aqueous binding was observed (K a 6-12 M À1 ), likely due to the strong electron-withdrawing nature of the fluorine atoms weakening its coordinating capability. 24 In our case, the introduction of an ethoxy group between the fluorine and coordinating carboxylate groups serves to lessen this effect and promote the strong binding observed.…”

“…16,[18][19][20][21][22] Ion pairing or reversible coordination between coordinating anions or fluorides and a paramagnetic moiety can lead to increased contact time between the ions, shortening 1 H and, to a lesser extent, 19 F relaxation times. 14,19,[22][23][24][25][26][27] Reports of fluorinated carboxyl-based ligands coordinating with heptadentate paramagnetic chelates have shown some promise in this area, with R 1 values B100 s À1 at 7 T observed for fluorinated L-lactate systems; these are, however, limited by their impracticably low binding affinities (K a o 12 M À1 ). 24 The ligation chemistry of lanthanides is rich and has been associated with the development of sensors and complexes of sensitised emission.…”