Model-free nuclear magnetic resonance study of intermolecular free energy landscapes in liquids with paramagnetic Ln3+ spotlights: Theory and application to Arg-Gly-Asp

Abstract

We propose an easily applicable method for investigating the pair distribution function of a lanthanide Ln(3+) complex LnL (L = ligand) with respect to any solvent or solute molecule A carrying observable nuclear spins. Let r be the distance of Ln(3+) to the observed nuclear spin I. We derive a simple expression of the experimental value of the configurational average of 1/r(6) in terms of longitudinal paramagnetic relaxation (rate) enhancements (PREs) of the spin I measured on a standard high-resolution NMR spectrometer and due to well-chosen concentrations of LnL complexes in which Ln(3+) is a fast-relaxing paramagnetic lanthanide or the slowly-relaxing gadolinium Gd(3+). The derivation is justified in the general case of a molecule A which is by turns in a bound state where it follows the complex and a free state where it moves independently. It rests on the expression of the underlying PRE theory in terms of the angle-dependent pair distribution function of LnL and A. The simplifications of this theory in the high-field regime and under the condition of fast exchange between bound and free states are carefully discussed. We also show that original information on the angle dependence of the molecular pair distribution function can be gained from the measured paramagnetic dipolar shifts induced by complexed fast-relaxing Ln(3+) ions. The method is illustrated by the case study of the anionic Lnttha(3-) = [Ln(3+)(ttha)](3-) (ttha(6-) = triethylene tetraamine hexacetate) complex interacting with the biologically important tripeptide Arg-Gly-Asp (RGD) which carries peripheral ionic groups. The usefulness of an auxiliary reference outer sphere probe solute is emphasized.

Quantitative interpretation of the very fast electronic relaxation of most Ln3+ ions in dissolved complexes

“…It is clear that the values of T eff 1 vary by less than 40% with respect to the purely transient case (k S = 0), except in the very particular situation of a pure cubic static term of order 6. Note that for any pure static ligand-field with H lig, S = 320 cm −1 , the Redfield equation (21) leads at zero field to the unphysical value T Redfield 1e = 0.06 × 10 −3 ps, which is three orders of magnitude shorter than the simulated and experimental values.…”

“…However, their fast electronic relaxation is very useful since it allows one to determine the geometry of their LnL complexes (L = ligand) from the induced paramagnetic dipolar shifts of the resonance frequencies of the ligand nuclei [18][19][20] and to efficiently investigate the intermolecular interactions of these complexes with solvent and solute species. 21 For a given ligand, the geometries of the LnL complexes of the various Ln 3+ ions are generally very similar so that any LnL complex inducing appropriate shifts can serve as structural representative of the whole family. a) Author to whom correspondence should be addressed.…”

Characterisation and application of ultra-high spin clusters as magnetic resonance relaxation agents

“…5 In the present paper, we explored a different way for the characterization of the molecular clusters by variation of the lanthanide ions also mentioned in ref. 8. The molecular architecture allows the synthesis of a series of Fe 10 Ln 10 clusters with different electronic properties and therefore hyperfine couplings.…”

Prototropic Exchange Governs T1 and T2 Relaxivities of a Potential MRI Contrast Agent Nanozeolite Gd−LTL with a High pH Responsiveness

“…[43][44][45] However, the presently studied suspension is rather complex and addition of most compounds results in precipitation. As a compromise, we followed a modified procedure of Gossuin et al 44 by measuring transverse relaxation rates of the methyl protons of methanol for samples of Gd-LTL-L in mixtures of methanol and water.…”