Lanthanum-139 nuclear magnetic resonance studies of polyaminocarboxylate-lanthanum complexes in aqueous solution

JOURNAL

OF MAGNETIC

RESONANCE

66,274-282

(1986)

Lanthanum-139 Nuclear Magnetic ResonanceStudies of Polyaminocarboxylate-LanthanumComplexesin AqueousSolution CARLOSF.G.C.GERALDES*ANDA.DEANSHERRY The University of Texas at Dallas, P.O. Box 830688, Richardson, Texas 750834488 Received July 18, 1985; revised September 16, 1985 Several complexes of La(II1) with aminocarboxylate and polyaminocarboxylate ligands in aqueous solution have been studied using ‘39La chemical-shill and linewidth measurements. The ligand-induced shifts of the 139LaNMR resonance were found to depend, at least to first order, on additive substituent contributions, and were empirically related to the number and type of coordinated oxygen and nitrogen atoms. A previously proposed relationship between the number of coordinated carboxylate oxygen atoms and observed ‘391a shifts does not seem to hold for the polyaminocarboxylate complexes and we propose a new empirical relationship which allows an accurate prediction of the number and types of l&and donors in these systems.The 13% linewidths also provide qualitative information about the symmetry of each La3+ chelate. The results show that ‘39La NMR can be a powerful technique for elucidating the structure and dynamics of lanthanide complexes in solution. 0 1986 Academic Ress, Inc.

The study of the coordination properties of Ca*+ and Mg*+ with nitrogen- and oxygen-containing ligands is of clear biological importance. LanthanideJIII) cations have been proposed as models for Ca*’ binding sites in proteins (Z-4) and used as shift and relaxation probes of molecular structure and conformation in solution (2, 4, 5). However, cation NMR is also useful, as shown by 25Mg and 43Ca NMR studies (6). Lanthanum- 139 has a natural abundance of 99.9 l%, a magnetic moment of 2.7615 nuclear magnetons and a spin Z = $ It has a receptivity of 336 relative to i3C, but also has a relatively high electric quadrupole moment of 0.21 X 1O-24 cm*, leading, in most cases, to broad resonances as a consequence of quadrupolar relaxation (7). Thus, relative to other naturally occurring isotopes like 25Mg and 43Ca, ‘39La has by far the most favorable NMR properties, including a large chemical-shift range (up to 1200 ppm) and relatively large ligand-induced line-broadening effects (8). Several ‘39La NMR studies have been reported previously. Spin-lattice relaxation times, T, , have been used in studies of protein dynamics in solution (9-ZZ), and relaxation and ‘39La chemical-shift data have been found useful in the study of La3+ ion coordination by a variety of small ligands in aqueous salt solutions, mixed solvents, and acetonitrile (8, 12-17). The present work reports ‘39La NMR spectra of a variety of aqueous lanthanum complexes with aminopolycarboxylate and polyaminopolycarboxylate ligands conl To whom correspondence should be addressed at the Department of Chemistry, University of Coimbra, 3000 Coimbra, Portugal.

0022-2364186 $3.00 wt Q 1986 by Academic

F’ma Inc. AU rights of reproduction in any form mscrvd.

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taining both nitrogen and oxygen atoms (neutral hydroxylic or charged carboxylate) as potential metal binding sites with the aim of further testing proposed generalizations about ligand-induced shifts and relaxation effects (18-20). EXPERIMENTAL

Lanthanum(II1) chloride heptahydrate was obtained from Research Organic/Inorganic. All ligands were obtained from commercial sources, except NOTA and DOTA, which were synthesized according to literature procedures (21). Deuterium oxide (99.8%) was obtained from Sigma. The samples for NMR measurements were prepared by mixing the appropriate volumes of stock solutions of lanthanum(II1) chloride and the ligand. pH adjustments were made with HCl or NaOH. All NMR samples contained 0.1 M La3+ but no other salts were added to maintain constant ionic strength. The lanthanum-139 NMR spectra were recorded at 28.19 MHz on a JEOL FX200 FT spectrometer, using a pulse repetition rate of 100 ms, 4 K data points, and a spectral width of 50 kHz. The lock was established on the deuterium signal of the D20:H20 (20:80 v/v) solvent mixture used. Experimental errors for ‘39La chemical shifts and linewidths ranged from OS- 10 ppm and lo- 100 Hz, respectively, depending upon the observed linewidths. RESULTS

AND

DISCUSSION

ChemicalShuts The observed L39La chemical shifts for 19 lanthanum(II1) complexes are presented in Table 1. The ‘39La resonances of La(NTA) and La(NTAh3- have been observed previously (I 9) and our data agree well with those published results. In all cases, except with the IDA complexes, there was slow exchange between free and l&and-bound La3+ and, therefore, the values presented are limiting shifts for the respective complexes. Figure 1 shows examples of some of the spectra obtained. In the case of IDA, the observed ligand-induced shilt (Lig IS) increases monotonically with IDA concentration (Fig. 2). The variation is linear up to a L/M ratio of 2 corresponding to formation of strong 1: 1 and 1:2 complexes. The 1:3 complex is not fully formed until the L/h4 ratio reaches 6, in agreement with a lower stability constant for the tris complex (22). T:his weak 1:3 complex has been observed before in ESR titrations of Gd3’ with IDA (23). It is interesting to notice that the complexes formed by La3+ with dipicolinate (DP), which has the same number and types of ligand donor atoms as IDA, are in slow exchange. A previous proton NMR study (24) has also shown that the ligand exchange rate of Ln(DP)+ is much slower than Ln(IDA)‘. This is most likely a consequence of the more rigid steric configuration imposed by the aromatic ring of DP relative to the more flexible IDA chelate. The La3+chelate exchange rates depend not only on the rigidity of the ligand but also on the number of coordinating nitrogen and negative oxygen atoms (compare, for example, the fast and slow exchange situations ofLa(CMOS)+(CMOS = carboxymethoxysuccinate) and La(NTA) (20), respectively). The chemical shifts of i3% are mainly determined by the paramagnetic shielding term (7), up, which can be written as aP =

-

$.(e2h2/

m2c2)-$E-(r-3)Pu

111

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TABLE

1

Lanthanum-139 Chemical Shifts, A”, and Linewidths, AY,,~~, for Various Lanthanum Complexes’ in HzOd at 340 K, pH 7.2, [La3+] = 0.1 M Complex L3W2Qn13+

La(IDA)+ La(IDA)2La(IDAhLa(DP)+ WW2-

La(DP),‘La(NTA) La(NTA)*‘La(HIDA) La(HIDA)r’La(EDDA)+ La(EDDA)rLa(HEDTA)La(EDTA)La(CDTA)La(DTPA)‘La(TTHA)3La(NOTA) La(DClTA)-

A (ppm) 0 672 112+ 211+ 64k 123k 205 f 104 + 219 + 109k 191k 166+ 206 f 230 + 193k 235 f 258 f 224 f 223 f 323 f

1 3 3 1 2 3 0.5 1 1 2 3 3 5 5 10 4 2 5 1

h/2

(H-4

78 2660 5440 5230 2030 4660 5780 610 4170 2690 4350 4300 6420 12500 7600 15000 6300 5570 10000 3120

log & (Ref. (22)) 5.88 4.09 3.10e 7.98 5.81 4.27 9.87 7.39 8.00 5.98 7.04 4.73 13.82 16.34 16.35 2 1.60* 18.2’ g g

a Positive values to high frequency of [La(H20)J3’ as reference. b The reported linewidths have not been corrected for small differences in viscosity between the various samples at 340 K. ’ IDA = iminodiacetate; DP = pyridine- 1,6-dicarboxylate (dipicolinate); NTA = nitrilotriacetate; HIDA = N-hydroxyethyliminodiacetate; EDDA = ethylenediaminediacetate; HEDTA = N-hydroxyethylethylenediaminetriacetate; EDTA = ethylenediaminetetraacetate; DTPA = diethylenetriaminepentaacetate; TTHA = triethylenetetraaminehexaacetate; CDTA = truns-cyclohexanediaminetetraacetate; NOTA = 1,4,7-triazacyclonoanetriacetate; DOTA = 1,4,7, IO-tetraazacyclododecanetetraacetate. d In D20-H20 (20:80 v/v). e Value for the P?+ complex. /Value for the Nd3+ complex. 8 Not available in the literature.

where e, n, m, and c have the usual meaning, A E is the average excitation energy, (re3) is the radial expectation value of an electron in an outer p or d orbital, and P, is a sum of overlap integrals of outer p or d orbit& of La3+ and outer orbit& of coordinating ions or solvent molecules. Although a quantitative interpretation of experimental shifts is difficult, some empirical correlations of observed shift trends with structural features may be attempted. For example, recent studies (18-20) of Lig IS in several La(III)-(hydroxy)carboxylate complexes, La(NTA) and La(NTAh3- led to the empirical rule that substitution of water molecules in the primary coordination sphere of La3+ by a negatively charged oxygen atom of a carboxylate group results in a constant downfield shift of about 30 ppm, whereas substitution of a water ligand by a neutral hydroxyl, ether oxygen, or nitrogen atom has a negligible effect. The ap

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A] La3+( ag ]

B) La(IDAI; A C) La(HIDA] i

Lob

E)

1000 600

200

0 -200

-600

St ppm) FIG 1 ‘39La NMR spectra of La’+ complexes in aqueous solution; [La3’] = 0.10 M, t = 70°C; pH = 7.2 excep; ii (C). (A) LaC&; (B) La)+:IDA = 1:2; (C) La3+:HIDA = l:l, pH 4.3; (D) La3+:DTPA = 1:l; (E) La’+:DOTA = 1:0.66; Laf and Lab are the resonances for free and DOTA-bound La’+, respectively.

proximate additivity of the Lig IS’s in polycarboxylate ligands (18-20) seems to open promising possibilities to determine the mean number of carboxylate groups that coordinate the La3+ ion. Although the absence of a measurable ‘39La Lig IS due to coordination of a neutral oxygen atom in aqueous La3+ solutions is well documented experimentally (8, Z820), the proposed negligible effect of nitrogen atom coordination, which was based on data derived only from La(NTA) and La(NTA)z3- complexes (19), was found not to be generally true, as shown later (8) in a study of La3+ complexes with ethylenediamine (en) and diethylenetriamine (dien). A nearly linear relationship was found between the ‘39La chemical shifts of these ligands in complexes with different stoichiometries and the number of ligand N atoms coordinated to La3+ in acetonitrile (8). In the cases of La(en)43’ and La(dienh3+ , which presumably have no coordinated solvent molecules, the shift induced per coordinated nitrogen atom is as high as +50 ppm.

278

GERALDES 300

I

I

AND SHERRY

I

/

I

I

6

I

./--•

-4

P

-3

; 3 2

-2

f 2. I

I 0

I

2

3

I 4

I

I

5

6

[IDA] / [~a~+] FTC. 2. Ligand induced shift, A (O), and linewidth at half peak height, AY,,z (0), of the 13% resonance with increasing IDA concentrations in water at 70°C; [La”] = 0.10 M, pH 7.2.

The shifts observed for the La3+ complexes with IDA, DP, and NTA (each containing only one nitrogen donor atom) are systematically somewhat higher than the Lig IS values calculated by assuming a shift contribution of +30 ppm from each coordinated oxygen and a negligible contribution from the nitrogen atom. The nitrogen is certainly coordinated to La3+ in these complexes as shown, for example, by an observed contact contribution to the “N paramagnetic shifts of “N-enriched paramagnetic Ln(NTA) complexes (25), or by NMR studies of paramagnetic Ln(DP)33- complexes (26). Our result indicates the contribution of each nitrogen to the Lig IS in these complexes, although much smaller than +50 ppm, is certainly not negligible. A similar situation is found for La(HIDA) and La(HIDA)z3- at pH 7.2 assuming that the hydroxyl group is ionized and coordinated to La3+. The ‘39La NMR spectrum of La(HIDA) changes when the pH is dropped to 4.3 (Fig. 2C). In addition to the resonance observed at higher pH, a broader resonance (Au,,2 = 4000 Hz) appears as a shoulder near + 13 ppm in slow exchange with the first, corresponding presumably to partial formation of a less symmetric La(HIDA)+ species containing a protonated, noncoordinated hydroxyl group. The importance of the contribution of coordinated nitrogen atoms to the total Lig IS becomes clear for the lanthanum complexes with the polyaminopolycarboxylate ligands. For example, the observed shift of +193 ppm for La(EDTA)- (Table 1) is much larger than the value of + 120 ppm calculated from the empirical rule, 30 ppm for each of the four carboxyl groups. Let us assume the following empirical equation, based on data from the literature for oxygen (19) and nitrogen (8) coordination: Lig IS = 5Om + 3Op0-

PI

where pi and po- are the average number of coordinated nitrogen and negatively charged oxygens, respectively. The validity of such a relation can be tested by consid-

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ering complexes for which there is independent structural data in solution (27). For instance, in the case of La(NOTA), for which there is evidence that the La3+ cation is coordinated to three nitrogens and an average of 2.5 carboxylate oxygens (21) we find very good agreement between the experimental Lig IS of +223 ppm and the calculated +225 ppm value from Eq. [2]. Similarly, good agreement is observed for La(EDTA)- assuming pi = 2 and po- = 3.2 (28). The Lig IS value for La(DOTA),of +323 ppm agrees very well with proton NMR studies in solution of paramagnetic Ln(DOTA)- species (29) and the X-ray crystal structure of Na[Eu(DOTA)] * 5HzO 1(30),which show that the lanthanide cation is coordinated to four nitrogen and four lcarboxylate oxygens. We now attempt to interpret the other Lig IS data of Table 1, based upon Eq. [2]. In caseswhere no independent structural information in solution exists, the conclusions can be ambiguous, as the two unknowns (pi and pe) cannot be determined from only one measured Lig IS. However, in La(EDDA)+ the observed shift correlates very well with coordination of all nitrogens and carboxylate groups. The measured Lig IS for La(EDDA)2- indicates that not all eight coordinating groups of the two EDDA ligands are bound to the La3+ ion, in agreement with its small stability constant (Table l), and in contradiction with a proposal from the literature (31). A comparison of the Lig IS’s of La(EDTA)- with those of La(HEDTA)- and L.a(CDTA)-, which have the same number and type of coordinating atoms, indicates that in the later complexes all the carboxylates and the ionized hydroxyl group of HEDTA are coordinated to La3+. This agrees with proton NMR studies on La(CDTA)- (27,32) and La(HEDTA)(27, 33). The Lig IS’s for La(DTPA)*- and La(TTHA)3- are much smaller than the values expected from coordination by all ligand donor atoms, which indicates that some of the nitrogens and/or carboxylates in these complexes are not bound to La’+. The Lig IS value of +258 + 4 ppm measured for La(DTPA)*- is compatible with the proposed structure of this complex in solution, deduced from proton NMR studies (34); a heptadentate structure in which the unbound ligand group is the middle nitrogen or its acetate group, or an equilibrium between these two forms. The total coordination number of the lanthanide cation would be eight, as there is evidence for one inner sphere water molecule in the Eu(DTPA)*- (35) and the Gd(DTPA)*- (,36) complexes. Finally, in the case of La(TTHA)3- there is agreement that the TTHA6- ligand cannot donate all 10 possible donor atoms and most likely IzL(TTHA)~- has more than one uncoordinated carboxylate. This general conclusion is supported by the low value of +224 f 2 ppm for the Lig IS, and by observed specific interactions of paramagnetic La(TTHA)3-- complexes with positively charged systems, like protonated amino groups of amino acids (37) and Na+ ions (38). More specifically, two proposals have been presented for the structure of La(TTHAP- in solution; one with a La3+ coordination number of eight (39) using all four nitrogens and four carboxylates (having two uncoordinated carboxylates) and a second involving hexacoordination by two nitrogens and four carboxylates (40). The Lig IS value of +224 + 4 ppm measured for La(TTHA)3- is most compatible with the latter structure as the former structure should yield a ‘39La shift of +320 ppm. Additional support for this structure comes from the observation that the stability constant of La(TIT-IA)3- is intermediate between the stabilities of La(EDTA)- and La(DTPA)*-.

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Linewidths

For a quadrupolar nucleus like ‘39La the resonance linewidth at half height, Avi,~, is given by Eq. [ 3 ] (7), which applies to small complexes in solution undergoing rapid tumbling. Au

37? (21+ 3) .(f.y)y “2 = -iil’Z2(2Z - 1)

+$.

131

In this equation (e2qzzQ/h) is the quadrupolar coupling constant, Q is the nuclear quadrupole moment, qrz is the largest component of the electric field gradient at the nucleus, a is the asymmetry parameter for q(0 < a < I), and T, is the correlation time. 7, = rR under conditions of rapid tumbling, which applies for small lanthanide(II1) Complexes in solution (41). The rotational Correlation time rR can be approximated by the diffusion equation of Stokes 47rr3q rR = 3kT

where r is the radius of a rigid sphere (to which the complex is approximated) tumbling in the solvent of viscosity 7. The previous equations show that, although a quantitative analysis of the linewidths is difficult to perform, Av,,~ depends upon the symmetry of the ligand field (i.e., affecting qzz and a) and upon the microdynamic behavior (T, 1, and r) of the complex. These features have been experimentally illustrated previously (8, 13, 18, 19). The linewidths listed in Table 1 reflect both the symmetry of each La3+ complex and their differences in 7~. The differences in rR between the various complexes should largely reflect differences in r3 since T)is not expected to change dramatically from one sample to another. Most notably is the very narrow linewidth observed for La(NTA) which increases nearly sevenfold in La(NTA)z3-. This increase reflects both the difference in r3 between the mono and bis complexes and the greater symmetry of La(NTA) versus La(NTA)23- (19). The slightly smaller linewidth observed for La(IDA)33- than for La(IDA)z- most certainly indicates that the former complex has higher symmetry. The DP complexes show a regular increase in linewidth between the mono, bis, and tris complexes, reflecting a steady progression in TR. If one corrects for r3 differences between the corresponding IDA and DP complexes, it also becomes clear that the larger, more rigid DP ligand forms more symmetrical complexes with La3+ than does IDA. Considering the tetradentate ligands of approximate equal size, NTA clearly forms the most symmetrical complex with La3’ followed by HIDA and finally EDDA. Among the polyaminopolycarboxylate ligand chelates, La(TTHA)3and La(DTPA)‘- are more symmetric than La(EDTA)-, which is much more symmetric than La(HEDTA)-. The linewidth of La(CDTA)- also indicates that this ligand provides a less symmetric l&and field than EDTA but part of this difference must be due to the larger size of CDTA. Finally, it is interesting to notice the much higher symmetry of La(DOTA)- relative to the other polyazamacrocyclic complex of similar size, La(NOTA). This must reflect the more complete “wrapping” of the larger DOTA around La3+ (29, 42), thereby forming a highly symmetric l&and field.

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CONCLUSIONS

The ‘39La NMR data obtained in this work for well characterized La3’ complexes in solution demonstrates the utility of the ‘39La resonance shift and linewidth as a structural tool. The limiting ligand-induced shift (Lig IS) is easy to obtain due to the high stability constants of the complexes studied and to the slow exchange conditions observed with all ligands except IDA. The shifts induced by each coordinating group are additive and, therefore, from the measured Lig IS, it is possible to obtain the average number of nitrogen and carboxylate groups directly bound to the lanthanide cation. The information thus obtained compares well and complements independent results using proton NMR (26-29, 31-34, 36-38), electronic spectroscopy (40), X-ray diffraction (30), thermodynamic studies (24), potentiometry (39), and luminescence methods (35,43-45). The shift induced by a negative oxygen has a constant value of +30 ppm, whereas the shift by a nitrogen atom in a monoamine derivative (+5 to +20 ppm) is much smaller than in a polyamino derivative, where it is approxirnately +50 ppm. This may reflect a greater covalency (larger overlap integral, see equation 1) of the La3+ -nitrogen bonds in the polyamino chelates than in the monoamino chelates. The order of observed ‘39La substituent induced deshielding effects, K z=-O- > 0 (neutral), coincides with the order of donor basicities and lanthanidedonor bond thermodynamic and kinetic stabilities (32). Therefore, the measured deshielding effect appears to be good measure of the covalent contribution to the La3+ligand bond, as previously observed for sulfur vs oxygen donors (8). The ‘39La linewidths give information about the relative symmetries of the La3+ chelates studied. The luminescence properties of Eu3+ and Tb3+ have been used successfully to study the coordination of these ions by small ligands in the crystalline state and in solution (35, 43-45). Among the various types of information obtained bly this technique, it has been possible to obtain information about the symmetry of various Eu3+ complexes with aminocarboxylates in solution from the structure of selected peaks in their luminescence spectrum (35). The classification of the Eu3+ chelates into high- and low-symmetry species (35) compares favorably with the symmetry conclusions reached here from the ‘39La linewidths except for the NOTA and DOTA complexes. Both Eu(NOTA) and Eu(DOTA)- are classified as high symmetry species by luminescence whereas the ‘39La linewidths indicate La(DOTA)- is highly symmetric and La(NOTA) has low symmetry. This may reflect real differences between the La(NOTA) and Eu(NOTA) structures as we have presented other evidence (21) that the smaller trivalent lanthanide cations fit into the cyclononane ring of NOTA much better than do the larger trivalent cations. The larger ring size of DOTA allows it to accommodate La3+ or Eu3+ equally well thereby forming highly symmetrical species in both cases (26, 29). ACKNOWLEDGMENTS This investigation was supported by Grant AT-584 from the Robert A. Welch Foundation and by NATO Travel Grant No. 0345/82. C.F.G.C.G. also acknowledges support from INIC, Portugal. REFERENCES 1. R. J. P. WILLIAMS, Quant. Rev. 24, 33 1 (1970). 2. L. LEE AND B. D. SYKES, Biochemistry 19,320s

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