pH-dependent modulation of relaxivity and luminescence in macrocyclic gadolinium and europium complexes based on reversible intramolecular sulfonamide ligation.

A series of macrocyclic Eu, Gd, and Tb complexes has been prepared in which the intramolecular ligation of a beta-arylsulfonamide nitrogen is rendered pH-dependent, giving rise to changes in the hydration state, q, at the lanthanide center. In complexes based on DO3A, variation of the p-substituent in the arylsulfonamide moiety determines the apparent protonation constant log K(MLH) with values of 5.7, 6.4, and 6.7 for the -CF(3), -Me, and -OMe substituents, respectively. Introduction of three beta-carboxyalkyl substituents, alpha to three ring nitrogens, inhibits displacement of the bound water by added protein and also suppresses intermolecular binding by endogenous anions (lactate, HCO(3)(-)). Measurements of the pH dependence of the form and intensity of the Eu complexes revealed that intramolecular carboxylate coordination occurred competitively. This was reduced either by enhancing the electron density at the sulfonamide nitrogen or by enlarging the chelate ring from 7--8. Amplification of the relaxivity changes in the pH range 8--5 occurred on protein binding, and over the pH range 7.4--6.8 a 48% change in relaxivity was defined for [Gd.3a] (298 K, 65.6 MHz) in 50% human serum solution.

[GdPCP2A(H(2)O)(2)](-): a paramagnetic contrast agent designed for improved applications in magnetic resonance imaging.

A novel ligand based on a pyridine-containing macrocycle bearing two acetic and one methylenephosphonic arms (PCP2A) has been synthesized. An efficient synthesis of PCP2A is based on the macrocyclization reaction between 2,6-bis(chloromethyl)pyridine and a 1,4, 7-triazaheptane derivative bearing a methylenephosphonate group on N-4. The Gd(III) complex of PCP2A displays characteristic properties which make it a very promising contrast agent for improved applications in magnetic resonance imaging. In fact it shows (i) a very high stability constant (log K(GdPCP2A) = 23.4) which should guarantee against the in vivo release of toxic free Gd(III) ions and free ligand molecules and (ii) a relaxivity that is about 2 times higher than the values reported for contrast agents currently used in the clinical practice. Its high relaxivity is the result of the presence of two water molecules in the inner coordination sphere and a significant contribution from water molecule(s) hydrogen bonded to the phosphonate group. Moreover, the inner sphere water molecules are involved in an exchange with the bulk water which is relatively fast. This property is important for the attainment of an even higher relaxivity once the molecular reorientation rate of the [GdPCP2A(H(2)O)(2)](-) moiety is lengthened by means of conjugation to a macromolecular substrate.

Ternary Gd(III)L-HSA adducts: evidence for the replacement of inner-sphere water molecules by coordinating groups of the protein. Implications for the design of contrast agents for MRI.

Two novel gadolinium(III) chelates based on the structure of the heptadentate macrocyclic 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A) ligand have been synthesized and their relaxometric and luminescent properties thoroughly investigated. They contain two water molecules in the inner coordination sphere in fast exchange with the bulk solvent and bear either a p-bromobenzyl or a p-phosphonatomethylbenzanilido substituent for promoting further interaction with macromolecular substrates. Upon interaction with human serum albumin the expected relaxation enhancement is not observed owing to displacement of the two inner-sphere water molecules of the complexes by a donor atom (likely from a carboxylate group) on the protein and possibly the phosphate anion of the buffered solution, respectively. We modeled the observed behavior by measuring the decrease of the relaxation rate of the water protons upon addition of malonate anion to aqueous solutions of the complexes. Conversely, no change in the hydratation state of the Gd(III) center for both complexes has been observed when the substrate for the formation of the macromolecular adduct is represented by poly-beta-cyclodextrin.

Non-covalent conjugates between cationic polyamino acids and GdIII chelates: a route for seeking accumulation of MRI-contrast agents at tumor targeting sites.

Three novel Gd chelates containing on their external surface pendant phosphonate and carboxylate groups, which promote the interaction with the positively charged groups of polyornithine and polyarginine, have been synthesized. Their solution structures have been assessed on the basis of 1H- and 31P-NMR spectra of the Eu and Yb analogues. A thorough investigation of the relaxometric (1H and 17O) properties of the Gd chelates has been carried out and the observed relaxivities have been accounted for the sum of three contributions arising from water molecules in the first, second, and outer coordination layers, respectively. It has been found that the occurrence of a tight second coordination coating renders the dissociation of the water molecule directly coordinated to the Gd ion more difficult. The binding interactions between the negatively charged Gd chelates and the positively charged groups of polyornithine (ca. 140 residues) and polyarginine (ca. 204 residues) have been evaluated by means of the proton relaxation enhancement (PRE) method. Although the binding interaction decreases markedly in the presence of competitive anions in the solution medium, the affinity is strong enough that in blood serum it is possible to meet the conditions where most of the chelate is bound to the polyamino acid substrate. On this basis one may envisage a novel route for a MRI location of tumors as it is known that positively charged polyamino acids selectively bind to tumors having a greater negative charge than non-tumor cells.

Inhibition of the catecholase activity of biomimetic dinuclear copper complexes by kojic acid.

The inhibition of the catechol oxidase activity exhibited by three dinuclear copper(II) complexes, derived from different diaminotetrabenzimidazole ligands, by kojic acid [5-hydroxy-2-(hydroxymethyl)-gamma-pyrone] has been studied. The catalytic mechanism of the catecholase reaction proceeds in two steps and for both of these inhibition by kojic acid is of competitive type. The inhibitor binds strongly to the dicopper(II) complex in the first step and to the dicopper-dioxygen adduct in the second step, preventing in both cases the binding of the catechol substrate. Binding studies of kojic acid to the dinuclear copper(II) complexes and a series of mononuclear analogs, carried out spectrophotometrically and by NMR, enable us to propose that the inhibitor acts as a bridging ligand between the metal centers in the dicopper(II) catalysts.