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Ternary Complexes between Cationic Gd III Chelates and Anionic Metabolites in Aqueous Solution: An NMR Relaxometric Study
Author(s) -
Botta Mauro,
Aime Silvio,
Barge Alessandro,
Bobba Gabriella,
Dickins Rachel S.,
Parker David,
Terreno Enzo
Publication year - 2003
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200204475
Subject(s) - chemistry , aqueous solution , titration , malonate , inorganic chemistry , molecule , stability constants of complexes , titration curve , cationic polymerization , gadolinium , chelation , nuclear magnetic resonance spectroscopy , proton nmr , ligand (biochemistry) , crystallography , stereochemistry , organic chemistry , receptor , biochemistry
The 1 H and 17 O NMR relaxometric properties of two cationic complexes formed by Gd III with a macrocyclic heptadentate triamide ligand, L 1 , and its Nmethylated analogue, L 2 , have been investigated in aqueous media as a function of pH, temperature and magnetic field strength. The complexes possess two water molecules in their inner coordination sphere for which the rate of exchange has been found to be sensibly faster for the Nmethylated derivative and explained in terms of electronic effects (decrease of the charge density at the metal center) and perturbation of the network of hydrogen‐bonded water molecules in the outer hydration sphere. The proton relaxivity shows a marked dependence from pH and decreases of about six units in the pH range 6.5 to 9.0. This has been accounted for by the displacement of the two water molecules by dissolved carbonate which acts as a chelating anion. The formation of ternary complexes with lactate, malonate, citrate, acetate, fluoride and hydrogenphosphate has been monitored by 1 H NMR relaxometric titrations at 20 MHz and pH 6.3 and the value of the affinity constant, K , and of the relaxivity of the adducts could be obtained. Lactate, malonate and citrate interact strongly with the complexes (log K ≥3.7) and coordinate in a bidendate mode by displacing both water molecules. Larger affinity constants have been measured for GdL 2 . Acetate, fluoride and hydrogenphosphate form monoaqua ternary complexes which were investigated in detail with regard to their relaxometric properties. The NMR dispersion (NMRD) profiles indicate a large contribution to the relaxivity of the adducts from water molecules belonging to the second hydration shell of the complexes and hydrogen‐bonded to the anion. A VT 17 O NMR study has shown a marked increase of the rate of water exchange upon binding which is explained by coordination of the anion in an equatorial site, thus leaving the water molecule in an apical position, more accessible for interactions with the solvent molecules of the second hydration shell which facilitate the exchange process.