Multiple-frequency EPR spectra of two aqueous Gd3+ polyaminopolypyridinecarboxylate complexes: a study of high field effects

In the search for highly efficient magnetic resonance imaging contrast agents, polyaminopolypyridinecarboxylate complexes of Gd3+ have shown unusual properties with both very rapid and very slow electron spin relaxation in solution observed by electron paramagnetic resonance. Since the relationship between the molecular structure and the electron spin properties remains quite obscure at this point, detailed studies of such complexes may offer useful clues for the design of Gd3+ compounds with tailored electronic features. Furthermore, the availability of very high frequency EPR spectrometers based on quasi-optical components provides us with an opportunity to test the existing relaxation theories at increasingly high magnetic fields and observation frequencies. We present a detailed EPR study of two gadolinium polyaminopolypyridinecarboxylate complexes, [Gd(tpaen)]- and [Gd(bpatcn)(H2O)], in liquid aqueous solutions at multiple temperatures and frequencies between 9.5 and 325 GHz. We analyze the results using the model of random zero-field splitting modulations through Brownian rotation and molecular deformations. We consider the effect of concentration on the line width, as well as the possible existence of an additional g-tensor modulation relaxation mechanism and its possible impact on future experiments. We use 17O-NMR to characterize the water exchange rate on [Gd(bpatcn)(H2O)], and find it to be slow (~ 0.6x10^6 s-1).

A Multinuclear NMR Study on the Structure and Dynamics of Lanthanide(III) Complexes of the Poly(amino carboxylate) EGTA4- in Aqueous Solution

Alain Borel, André Merbach

The structures and intramolecular dynamics of [Ln(EGTA)(H2O)]- (Ln = La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, Lu3+ and EGTA4- = 3,12-bis(carboxymethyl)-6,9-dioxa-3,12-diazatetradecanedioate(4-)) in aqueous solution have been investigated by variable temperature 1H and 13C NMR. The quantitative analysis of the proton hyperfine shifts and considerations about the stereochemical nonrigidity indicate the occurrence of a structural change along the lanthanide series with the crossover between Sm and Eu. Changes of coordination number from 10 to 9 to 8 are proposed to occur across the series. The experimental data from 17O NMR, EPR and NMRD studies for the Gd3+ complex are treated using a self-consistent theoretical model in a simultaneous multiple parameter least-squares fitting procedure (Powell, D. H.; Ni Dhubhghaill, O. M.; Pubanz, D.; Helm, L.; Lebedev, Y. S.; Schlaepfer, W.; Merbach, A. E. J. Am. Chem. Soc. 1996, 118, 9333). An intermolecular dipole-dipole electronic relaxation mechanism that has very recently been described for Gd3+ complexes (Powell et al., op. cit.) is included in the data treatment. The high water exchange rate of [Gd(EGTA)(H2O)]- of kex298= (3.1 ± 0.2) × 107 s-1 is explained in terms of a limiting dissociative exchange mechanism (DV* = +10.5 ± 1.0 cm3mol-1), favored by the steric constraints on the water binding site that are revealed by the study on the solution structure.

1997

EPR on aqueous Gd3+ complexes and a new analysis method considering both line widths and shifts

Alain Borel, Lothar Helm, Eva Jakab Toth, André Merbach

We performed variable temperature (0–100°C), concentration and frequency (9.425, 75, 150 and 225 GHz) continuous wave electron paramagnetic resonance (EPR) measurements on three different Gd(III) compounds: [Gd(H2O)8]3+, [Gd(DOTA)(H2O)]− (DOTA: 1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10-tetraazacyclododecane) and [Gd(DTPA-BMA)(H2O)] (DTPA-BMA: 1,5-[bis(N-methylcarbamoyl)methyl]-1,3,5-tris(carboxymethyl)-1,5-diamino-3-azapentane) in aqueous solution. A simultaneous analysis of peak-to-peak widths and dynamic frequency shifts provides access to the transverse electronic relaxation, which is described using a transient zero field splitting (ZFS) mechanism with a spin rotation contribution. Our simultaneous analysis procedure involves numerical calculations using the full relaxation matrix and yields results in acceptable agreement with experimental data for reasonable values of the ZFS parameters (trace of the square of the ZFS Hamiltonian Δ2=1019–1020 s−2 depending on the complex, correlation time of the fluctuations τv298=10−11–10−10 s). We also discuss the relationship between our approach and recent developments found in the literature.

2000

Nuclear Spin Relaxation Parameters of MRI Contrast Agents – Insight from Quantum Mechanical Calculations

Lothar Helm, Oleg Yazyev

Nuclear magnetic relaxation in the presence of paramagnetic centres has gained increasing interest in recent years partly due to its importance for contrast agents in magnetic resonance imaging. Rational design of new more efficient agents is possible as a result of a better understanding of the underlying relaxation mechanisms. Quantum chemical calculations together with molecular dynamics simulations allow obtaining fundamental parameters such as quadrupole coupling constants and hyperfine interaction tensors directly at a molecular level. Recent results are presented on gadolinium(III) ions in aqueous solution and on [Gd(DOTA)(H2O)]–, a commercial MRI contrast agent. Isotropic hyperfine coupling constants can be calculated for 17O and 1H nuclear spins of water molecules in the first and second coordination sphere of Gd3+. It is also shown that the commonly used point-dipole approximation for the dipolar interaction between the electron and the nuclear spin is in general valid for 1H spin but not for the directly bound 17O spin. The calculated quadrupole coupling parameters allow a direct determination of the rotational correlation time of complexes from the 17O nuclear spin relaxation.

Wiley-Blackwell2008