Jacques Reuben

Nuclear Magnetic Resonance Studies of Solutions of the Rare‐Earth Ions and Their Complexes. III. Oxygen‐17 and Proton Shifts in Aqueous Solutions and the Nature of Aquo and Mixed Complexes

A nuclear magnetic resonance survey has been undertaken in order to gain a better insight into the nature of the complex species present in aqueous solutions of the rare‐earth ions. Oxygen‐17 and proton chemical shifts of water are reported and the effects of concentration and added anions upon them discussed. Nuclear magnetic resonance evidence is presented, showing that the shifts observed in solutions of the perchlorate salts are due to a single hydrated species persisting over a wide range of concentrations and that no “inner‐sphere” complex is formed with perchlorate. It appears, also, that all the water molecules in the first solvation shell are equivalent with respect to their contribution to the shift, owing to their relatively short residence time. The sign, magnitude, and temperature dependence of the oxygen‐17 shifts as contrasted to those of protons support the suggestion that the former are mainly due to contact interaction, whereas the latter are dominated by dipolar interaction with the unp...

Nuclear Magnetic Resonance Studies of Solutions of the Rare‐Earth Ions and Their Complexes. IV. Concentration and Temperature Dependence of the Oxygen‐17 Transverse Relaxation in Aqueous Solutions

The concentration and temperature dependence of oxygen‐17 linewidths in solutions of rare‐earth perchlorates have been investigated. Empirical models are proposed to account for the observed behavior. An equation relating the apparent relaxation rate to the concentration and to the NMR parameters of the nuclei under conditions of fast chemical exchange has been derived. It was used in the interpretation of the experimental results. Lower limits have been estimated for the rate constants of water exchange between the aquo complexes of Tb3+, Dy3+, Ho3+, Er3+, and Tm3+ and the solvent. These are found to be in the range 0.3–2.6 × 107 sec−1 and to follow the trend of the ionic radii. The upper limit for the activation enthalpy of the exchange process is estimated to be 5 kcal/mole. The upper limits of the electron relaxation times are in the range 2.8–5.0 × 10−13 sec. The temperature dependence of the oxygen‐17 transverse relaxation rates in these systems is discussed in terms of the microdynamic behavior of ...

The lanthanides as spectroscopic and magnetic resonance probes in biological systems

The trivalent lanthanide ions, by virtue of the multitude of their spectroscopic and magnetic properties, are suitable as probes in studies of macromolecular environment, structure, and interactions. The approach is illustrated with examples from optical absorption, fluorescence, circularly polarized luminescence, nuclear magnetic resonance, X-ray diffraction, and other physical techniques. In order to evaluate the significance of the approach, the activation and inhibition of enzymatic reactions by lanthanides is reviewed.

Line broadenings induced by lanthanide shift reagents: Concentration, frequency, and temperature effects

Abstract The effects of reagent concentration, resonance frequency, and sample temperature on the line-broadenings induced by the lanthanide shift reagent Yb(fod)3 in the proton spectrum of pinacolone, (CH3)3 CCOCH3, have been investigated and analyzed in detail. For comparison the effects of other members of the Ln(fod)3 series have also been examined. It is found that a term containing the modulation by chemical exchange of the chemical shift difference between the coordinated and free substrate molecules makes a significant contribution to the observed line-broadenings leading to their nonlinear dependence on the fraction of complexed substrate. Also the ratio between the line-broadenings of the two different proton-containing groups is a nonlinear function of this fraction. The analysis of the results yields the relaxation rates of the protons in the coordinated state and their mean residence time as well as the electron spin relaxation time of the central lanthanide ion. Details are given of the approach to the data analysis and interpretation.

Criteria and algorithms for the characterization of weak molecular complexes of 2:1 stoichiometry from nuclear magnetic resonance data. Applications to a shift reagent system

Abstract Methods are described for the analysis of NMR data for systems in which complex formation with 2:1 stoichiometry takes place. Criteria for the reliability of the parameters obtained from the analysis are formulated. From simulation of typical cases it appears that normally two sets of experiments, one with each of the components held at a fixed concentration and the other varied, will provide an adequate description of the system within certain limits determined by the nature of the system. This approach is then applied to the DMSO-Eu(fod)3 system in carbon tetrachloride. It is found that the limiting shifts of the 1:1 and 2:1 adducts, respectively, are 447 ± 11 and 275 ± 65 Hz, and the dissociation constants are 0.75 ± 0.06 and 24.0 ± 1.7 mM. These parameters reproduce the data to within 2.4%.

Proton and deuteron relaxation in aqueous solutions of gadolinium (III)

In previous proton relaxation studies of the gadolinium ion, the estimation of the mean residence time of a proton in the aqueous complex has been ambiguous. It is shown that this ambiguity can be resolved by comparison of deuteron and proton relaxation times. The method is applied to aqueous solutions of GdCl3. (AIP)

NMR Relaxation Times of Adsorbed Ammonia

NMR relaxation times of NH3 adsorbed on modified silica surfaces have been studied using a progressive‐saturation method. Measurements have been done for different surface coverages and frequencies. It has been found that the longitudinal and transverse relaxation rates of the adsorbate depend strongly on surface coverage. This has been explained by the existence of distribution of correlation times.The relaxation rates differed in some respects from those known for liquids and solids, in other respects they showed some similarity. Possible relaxation mechanisms are discussed.

On the relative location of the inhibitor-and calcium-binding sites in bovine trypsin as determined by nuclear magnetic resonance. Possible ambiguities in paramagnetic probe mapping studies.

Possible pitfalls in mapping studies utilizing the nuclear relaxation rates induced by paramagnetic probes are pointed out. In cases in which a distance is sought between a paramagnetic ion and a small molecule (e.g. substrate, inhibitor, etc.), both bound non-covalently to a macromolecule, heterogeneity in the system with respect to the binding of either of them may result in ambiguous conclusions. It is shown that the trypsin-gadolinium (III)-inhibitor system is heterogeneous, as revealed in the dependence of the water and inhibitor proton line-widths upon both the Gd3+ and the enzyme concentrations and in the effects of added Ca2+ on the line-widths. The results imply that in published work (Abbott et al. (1975) Biochemistry 14, 4935) the distance from a weak rather than from the strong metal ion binding site of trypsin (EC 3.4.21.4) may have been determined.

Chapter 39 Bioinorganic chemistry: Lanthanides as probes in systems of biological interest

Publisher Summary This chapter reviews the bioinorganic chemistry of the lanthanides, and discusses the physical methods to study biological systems with lanthanides. It also describes the principles and provides examples of utilizing optical absorption, fluorescence, circularly polarized luminescence, nuclear relaxation rates, nuclear magnetic resonance (NMR) chemical shifts, and NMR of lanthanum-139. The electronic energy levels of lanthanide aquo-ions are only slightly perturbed upon complex formation. Nevertheless these perturbations, particularly those associated with changes in the ligand-field symmetry, are often spectroscopically discernible. The interactions of the dipole–dipole type transmitted through space kind give rise to diverse phenomena, such as intensity enhancement of lanthanide fluorescence and shifts and enhanced relaxation rates in the resonances of ligand nuclei. In this respect, macromolecules—in contrast to small ligands—offer the possibility of having close spatial proximity between a lanthanide and a molecule or a molecular fragment without actual association between the two.

Chapter 38 Shift reagents and NMR of paramagnetic lanthanide complexes

Publisher Summary This chapter is discusses the theory, experiment, and application of the nuclear magnetic resonance (NMR) of paramagnetic lanthanide complexes and shift reagents. The effect of paramagnetic ions or their complexes on the NMR spectra of ligand molecules has been known since the early days of NMR spectroscopy. Lanthanides can be used as shift reagents both in aqueous solutions and in organic solvents. The tripositive lanthanide ions form complexes in aqueous solution with negatively charged ligands and with neutral molecules containing a favorable arrangement of several polar groups. In addition, the chapter explains nuclear relaxation in lanthanide complexes. Nuclear relaxation in paramagnetic complexes is brought by the time dependent terms of the nuclear spin Hamiltonian. The magnitude of the relaxation effect is governed by the intensity of the electron-nuclear interaction and by the rate at which the interaction is interrupted.