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Aqueous Lanthanide Shift Reagents. 1. Interaction of the Ethylenediaminetetraacetate Chelates with Carboxylates. pH Dependence, Ionic Medium Effects, and Chelate Structure Gabriel A. Elgavish and Jacques Reuben* Contribution f r o m the Isotope Department, The Weizmann Institute of Science, Rehouot, Israel. Received December 31 1975 ~
Abstract: The suitability of the ethylenediaminetetraacetate (EDTA) chelates of the trivalent lanthanides as aqueous shift and relaxation reagents for carboxylate substrates has been established in a study of their effects on the proton resonance of acetate. The praseodymium chelate forms mainly a 1:l acetato complex with a downfield intrinsic shift of 1.83 ppm (at 39 "C) and a dissociation constant of the order of 0.2 M, the latter depending on the ionic medium (LiCI concentration). The useful pH range is between 6 and 10, confined in the acidic side by the formation of acetic acid and on the basic side by the formation of hydroxo complexes of the chelates. Similar behavior is observed with Yb(EDTA)-, which induces upfield shifts (intrinsic shift 3.5 ppm), and with Gd(EDTA)-, which causes line broadenings. A structural change in the EDTA chelates along the lanthanide series manifests itself in the effect of LiCl concentration upon the observed shifts. It appears that the lighter lanthanides are pentachelated, whereas the heavier ones are hexachelated. Yet the acetato complexes are isostructural as judged by the relative magnitude and direction of the shifts induced by Pr(EDTA)- and by Yb(EDTA)-, which are in good agreement
with the theoretical predictions for pseudocontact shifts in lanthanide complexes. Recommendations are given regarding the use of the EDTA chelates or the aquolanthanides as aqueous shift reagents in structural studies as well as for achieving spectral resolution.
The application of lanthanide chelates, that are soluble in organic solvents, as shift reagents in the N M R spectroscopy of organic molecules is now well established.' More recently interest is focusing on the use of lanthanide ions as probes in biological systems2Systems of biological interest are normally studied in aqueous solutions and therefore the water soluble aquoions are usually employed as shift reagents.*J The aquolanthanides, however, are stable only on the acidic side of neutral pH, hydrolyzing and precipitating as the hydroxides a t higher p H value^.^ It has been suggested that water-soluble chelates of the lanthanides could be employed a t the higher p H values4 Lanthanides form water soluble chelates of extremely low dissociation constants with a series of aminopolycarboxy l a t e ~ The . ~ ethylenediaminetetraacetates (EDTA), which have been characterized crystallographically,5 are known to interact with additional ligands in solution.6 Therefore we embarked on a detailed investigation of the anionic Ln(EDTA)- chelates as aqueous shift reagents. In the first stages of these studies our attention will be directed toward the characterization of the complex species formed in solution as manifested in the chemical shifts and in the enhanced relaxation rates induced in the N M R spectra of a variety of substrates.' In this way the suitability of these chelates as shift reagents should also be revealed. Presented in this article are the results of our studies on the interaction of carboxylates (mainly acetate) with Pr(EDTA)-, which is a downfield shift reagent, with Yb(EDTA)-, which causes upfield shifts, and with Gd(EDTA)-, which is a relaxation reagent. W e were able to elucidate the details of the p H dependence of the induced shifts and line broadenings, to establish the effective stoichiometry, to delineate the structure and structural equilibria of these systems, and to obtain evidence for a structural change of the chelates along the lanthanide series. For comparison some experiments with aquopraseodymium were also carried out.
Experimental Section Solutions of lanthanide salts were prepared from the corresponding oxides (Research Chemicals, Phoenix, Ariz.) by treatment with analytical grade acids. Concentrations were determined by EDTA ti-
trations using Arsenazo as the end-point indicator. The EDTA solutions were prepared from the free acid by neutralization with four equivalents of lithium hydroxide. Fresh lanthanide-EDTA solutions were prepared in DzO prior to each set of measurements by mixing stoichiometricamounts of LnC13 and Li4EDTA. Note that in this case a threefold molar excess of LiCl'is present. In order to keep a constant Lif concentration LiCl was added as needed. The pH was adjusted by suitable addition of acid or base. No buffers were employed. All chemicals were analytical grade reagents and were used without further purification. Proton NMR spectra were recorded on a Varian T-60 spectrometer operating at the ambient probe temperature of 39 f 1 "C. The pH was measured with an accuracy of 0.01 units using a Metrohm glass electrode. Reported values are meter readings corrected for isotope effects.
Results and Discussion pH Dependence. In the system composed of acetic acid and Ln(EDTA)- there are a number of species that may exist in equilibrium with each other with relative concentrations governed in part by the pH. Thus, acetic acid may exist as the free acid or as acetate. Protonation of the Ln(EDTA)- chelates has been found to occur with pK values in the range 2.5-2.8.8 The existence of hydroxo complexes of the Ln(EDTA)- chelates has also been s ~ r m i s e dTherefore .~ the effect of hydrogen ion concentration on the chemical shift induced by Pr(EDTA)- and by Yb(EDTA)- in the proton resonance of acetate was studied in the p H range 2-12. The results obtained in the presence of Pr(EDTA)-, which induces downfield shifts, are presented in Figure 1. It is seen that the p H profile is bell shaped with a plateau in the range 6-10, and with two apparent pK's, one ca. 5 and the other ca. 11. A qualitatively similar behavior is observed with the upfield shifts induced by Yb(EDTA)-, the results for which are shown in Figure 2. It is clear from these results that a complex is formed between acetate and the Ln(EDTA)- chelates with the pK on the acidic side of the curve corresponding to the formation of acetic acid, which does not complex, and that on the basic side corresponding to the formation of a hydroxo complex of Ln(EDTA)-. It is not immediately obvious, however, whether the decrease of the induced shifts a t high p H results from the dissociation of the acetato complex or is due to a much smaller
Elgauish, Reuben
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Lanthanide E D T A Chelates as Shift Reagents
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0 5
I
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13
Figure 3. The pH dependence of the acetate line width in a solution containing 2 mM Gd(EDTA)-, 0.12 M acetate, and 2 M LiCI. The smooth curve was calculated with the parameters in Table I, taking into account field inhomogeneity effects as reflected in the line width of fert-butyl alcohol (3.3 Hz).
< .
, . 7 -
9 PH
E
I4
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Table I. NMR Parameters and Dissociation Constants of Complexes of Lanthanide Chelates
PH Figure 1. The pH dependence of the acetate shift (downfield relative to tert-butyl alcohol) and of the acetate species: acetic acid (HA), free acetate (A), and the acetato complex of Pr(EDTA)-(MA) in solutions containing 0.2 M Pr(EDTA)- and 0.032 M acetate. The smooth dotted curves are calculated (see text) with the parameters in Table I. The chemical shift between acetic acid and acetate was taken into account.
Pr(EDTA)Gd(EDTA)Yb(EDTA)-
0.19
-110 16206 +210
0.10
0.40
(2
3 x 10-4 f 0.5) x 3 x 10-5
2 x 10-4 1x 6X
These are the dissociation constants of the hydroxo complexes of the lanthanide N'-(2-hydroxyethyl)ethylenediaminetriacetates ( H E D T A ) calculated from the data in ref 10. This is the intrinsic line width of the acetato-Gd(EDTA)- complex.
Line broadenings in gadolinium complexes, however, are expected to be mainly a function of r-6 and of the fraction of complexed substrate (assuming rapid chemical exchange).',2 The p H profile of the line broadenings induced by Gd(EDTA)- in the resonance of acetate is shown in Figure 3. It is seen that the behavior is qualitatively similar to that observed with the shifts. The observed shifts and line broadenings were fitted with a model that involves the following equilibria: H+AeHA
M+A M
MA ,,
