NMR, EPR, and INDO Studies on the complexes of dopamine with Cu2+, Mn2+, and Fe3+ in aqueous solution

JOURNAL

OF MAGNJSIC

RESONANCE

55,

39-50 (1983)

NMR, EPR, and INDO Studies on the Complexes of Dopamine with Cu2+,Mn2+, and Fe3’ in AqueousSolution L. BURLAMACCHI, A. LAI, M. MONDUZZI, AND G. SABA Istituto Chimica Fisica e Industriale, Universitci di Cagliari, Via Ospedale 72, 09100 Cagiiari, Italy Received January 3, 1983; revised May 9, 1983 The spin-lattice relaxation rates of dopamine in the presence of Cu*+, Mn*+, and Fe3+ have been investigated by FT NMR spectroscopy in aqueous solution. The SolomonBloembergen equation is found to hold in the case of Mn*+ complex while a failure of its applicability was observed in Cu” and Fe” complexes. Rationalization of the relaxation rates with the aid of EPR spectroscopy allowed the estimation of the pertinent molecular correlation time. Furthermore INDO MO calculations fiunished detailed information on the mechanism of spin density onto the ligand. INTRODUCTION

NMR paramagnetic relaxation rates have often been used to elucidate molecular geometries in solution, metal-ion coordination numbers, and dynamical properties of inorganic and biological systems (I, 2). Most of these investigations relied on the assumption that the predominant effect on the relaxation of the observed nuclei of a ligand molecule was a dipolar interaction with the metal ion so that the well-known equations first introduced by Solomon (3) and Bloembergen (4) applied. It was found, however, that generally the molecular geometries and hydration numbers obtained by this method were different from those calculated from existing X-ray diffraction data (5, 6). This failure was ascribed to the fact that leakage of unpaired spin density from the metal ion onto the ligand can significantly contribute to the dipolar electronnucleus interaction and, hence, the Solomon-Bloembergen equations were modihed accordingly (7-Z I). It turned out that especially with carbon nuclei this mechanism may be the major determinant of T1 relaxation (6, 7); consequently, it was pointed out that information obtained by an assumed linear dependence of T;’ on the inverse sixth power of the metal ion observed nucleus distance should be considered with an extreme caution (12). Recently, Kowaleski et al. (13, 14), from nonempirical MO calculations, found that neglecting the l&and-centered dipolar term for a number of metal ions amounts to underestimating the effective distance between the unpaired electron and the interacting nucleus. In the literature a limited number of investigations were reported where this more sophisticated approach was employed, mostly to study the interactions of paramagnetic metal ions and solvent molecules such as Hz0 or NH3 (5, 8, 13, 14). In a few cases simple organic ligands with a unique binding site to the metal ion were investigated (12). In previous work NMR relaxation spectroscopy was applied to the study of the systems Ni*+ and Co’+ dopamine in aqueous solution (15, 16). This molecule and 39

0022-2364183 $3.00 Copyright 0 1983 by Academic Press. Inc. All rights of reproduction m any form resewed.

40

BURLAMACCHI

ET AL.

other catecholamines together with their complexes with some of the first row transition metals gained an increasing interest because of their possible involvement in neuronal activity (17, 18). In this work the investigation is extended to the study of the interaction of dopamine with Cu*+, Mn*+, and Fe3+ by means of 13C NMR relaxation and.EPR spectroscopy, with the aid of INDO MO calculations. THEORETICAL

SECTION

The effects of a paramagnetic ion on the nuclear spin-lattice relaxation rate (PRR) are generally described in terms of the reduced and modified (3,4, 1 I, 12) SolomonBloembergen (SB) equation where both the metal-centered and ligand-centered dipolar interactions are considered: R 1M

UUML=

VGh

=

52 S(S + l)g2~Zy~~~(r~ + p*P)

w

[II

where (RL)ML and (RJL are the observed relaxation rates in the presence and in the absence of the metal ion, n is the number of coordinated ligands per paramagnetic ion, and p is the concentration ratio of the metal ion to the total l&and, rM is the metal-nucleus distance, p and r are, respectively, the total unpaired spin density and the radius of the 2s or 2p orbital in a carbon atom (r = 0.638 A); the correlation time T, is defined by -I = 7;;: + 7;’ + 7,’ 7, VI where rM is the mean lifetime of the ligand in the coordination sphere of the metal ion, T, is the reorientational correlation time of the complex, r, is the electron spin relaxation time, and the remaining symbols have the usual meaning. If the l&and-centered dipolar contribution is negligible the following relationship holds:

(&M)I 1’6 [ 1 =(rM)i

(RIM),

(di

t31

’

If, on the contrary, the unpaired spin density contributions are not negligible and ifthe relaxation rates of at least three different carbon atoms of the ligand are available, the SB equation can be rearranged to give F(p) = p: + A/g + Bp: = 0

[41

where A =

(RIM)*

-

(&M)3[h)3/hh16

WIM)Z

-

(Rd3[(d3/(d216 (RIM)I

- U’bd3K~d3/hh16b-d3 6

VW2

-

@d3[(rddh)d6

[ 1 (rM’M)2

-

In principle, any molecular geometry for a given complex can be tested either by applying Eq. [3] or when it fails by application of Eqs. [4], provided that the distribution of the unpaired spin density on the ligand is known.

COMPLEXES

OF

41

DOPAMINE

EXPERIMENTAL

Dopamine hydrochloride (DA), Mn(NO& . 4Hz0, Cu(NO& 3Hz0, Fe(NO& . 9H20, and 99.75% DzO were purchased from Merck. Each sample was pD adjusted using NaOD or DCl. All the samples in 40 solution were prepared in a dry box under N2 atmosphere and then protected from atmospheric oxygen with a layer of oil parafhn. The r3C FT NMR spectra were recorded at 32 1 1“C on a Varian FT-80A pulsed spectrometer at the operating frequency of 20 MHz, using 10 mm sample tubes. The temperature was measured with a precalibrated variable temperature control unit. The i3C spectra were hydrogen decoupled by means of square wave modulation of the decoupler carrier centered in the proton field. The chemical shifts were measured from p-dioxane, added as internal inert reference. The partially relaxed spectra were obtained by the inversion recovery sequence, averaging 5 12 FIDs. The relaxation times were determined by a nonlinear three-parameter regression with standard errors never greater than 7%. The PRR were obtained from the average of at least three determinations on fresh samples of DA 1 M in the presence of Cu*+ 2 X lo-* M at pD = 6, Mn*+ 5 X 10m3M at pD = 6 or Fe3+ 5 X 10B3 M at pD = 4. The EPR spectra were recorded on a Bruker Model 200 TT spectrometer operating at X band (9500 MHz). Samples with concentration and pHs similar to those used in the NMR spectra were employed. To obtain the carbon-metal distances the geometry of DA found in the solid state (19) was employed while the metal-oxygen distances were taken from the metalhexa-aqua complexes, i.e., 1.94 A for Cut+ (20, 21), 2.2 A for Mn*+ (20), and 2.02 A for Fe3+ (22). l

RESULTS

AND

DISCUSSION

Figure 1 illustrates some typical 13C nuclear magnetic spectra of DA in aqueous solution of Cu*+, Mn’+, and Fe3+, respectively, with the assignments taken from a previous work (23). Table 1 summarizes the PRR defined according to the second term of Eqs. [I]. Figure I and the data of Table 1 both indicate the formation of a complex between DA and each of the metal ions. In particular these paramagnetic reagents are seen to afhect the ligand carbons in pairs following the order (RIM)3,4 ti (RIM)2,J > (&& while the aliphatic carbons experience only outer sphere effects, since their PRR differ only a little from that of pdioxane added as an inert control, This suggests that at the experimental pDs only the catecholic function is involved in the binding to the metal ion. Similar results were obtained for the complexes of DA with Ni*+ and Co*+ in aqueous solution (25, 16). Because of the above results the subsequent analysis was limited to the fastest relaxing aromatic carbon atoms, their PRR being corrected by subtraction of the average of (&,.& and (RI,&. In fact, this correction, if not applied, does not modify the results of the analysis appreciably. Cu(II)-DA By choosing a model where DA chelates the metal ion “via” the catecholic oxygens the applicability of the SB equation can be tested. In Fig. 2a the 13C PRR are plotted

A

R

5.2 1

OH

6

OH

4 5 6

0 1

3 2

8

(2 7 CtiZ 8 ai.2 I NH2

I

I

150

50

b(PPm

of 1 M DA at pD = 5 (A); I M DA in the presence of 2 X IO-’ M Cd’ at pD = 6 (B); 1 it4 DA in the presence of 5 X 10e3 M MI? at pD = 6 (C); 1 M DA in the presence of 5 X lo-’ M Fti’ at pD = 4 (D); R is the resonance of pdioxane added as internal reference. FIG.

1. ‘%Z NMR

spectra

42

COMPLEXES

43

OF DOPAMINE

TABLE 1 THE PARAMAGNETIC RELAXATION RATES RIM OF IN THE PRESENCE OF Cuz+, Mn2+, AND Fe”+

DA

carbons

Cu-DA pD = 6

Mn-DA pD = 6

Fe-DA pD = 4

3 4 2, 5 I 6 7 8

799 777 276 141 162 44 45

3781 3779 463 187 171 76 98

2464 2682 1386 646 169 531 368

Note. From the known formation constants of Cu2+-DA (37-39) and Fe’+-DA (40) complexes a 1:1 stoichiometry is derived at our experimental conditions, while for the Mn2+DA complex a similar stoichiometq is assumed.

vs the inverse sixth power of the carbon-metal ion distances. It can be seen that these two quantities are not linearly correlated. This result reflects the failure in reproducing the ratios of the PRR by the corresponding ratios of the Cu*+-carbon distances (Fig. 2a) and therefore stresses the nonapplicability of the point dipole approximation. A measure of the deviations from the SB approximation was attained with the aid of EPR spectroscopy. The EPR spectrum of the Cu’+-DA system at room temperature shows four resolved hyperflne lines with unsymmetric line broadening (Fig. 3a). The hypertine coupling constant is 80 G, t%irly greater than the hyperhne coupling constant for the hexaquo complex (37 G), whose spectrum is practically absent. This means that at the concentrations and pH examined Cu2+ is completely bound to DA. It is well known that the electron spin relaxation in solution arises from the modulation of A, g, and the spin-rotational coupling tensors (24). A complete analysis, which is beyond the scope of this work, would require the knowledge of these parameters. However, comparison with the computer simulated spectra using known parameters for similar systems, gave the values 5 X lo-’ and 8 X lo-” set for r, and T,, respectively. Introduction of the latter value in the approximated SB equation showed that the PRR of C, and C, lay on the “theoretical” straight line of Fig. 2a and consequently they are dominated by a metal-centered dipolar mechanism. The remaining pairs C2, C5 and Ci , C,, located farther and farther from the paramagnetic center, show an increasing ratio of the ligand centered with respect to the metal-centered contributions. This is not surprising since a considerable leakage of unpaired spin density from Cu2+ onto the aromatic ligand nuclei (particularly for carbon atoms) is expected (12, 25). Mn(II)-DA Again we can test the applicability of the SB equation investigating the functional dependence of the PRR on the calculated distances of the carbon atoms from the metal ion. Figure 2b illustrates this dependence and evidences the existence of a linear

44

BURLAMACCHI

ET AL.

a

nG4 (se1)

I

CU-DA

2.0

1.0

rit

IO 3 ri1

-6

b

n&h4

(set-‘)

3 4

P

Mn-DA

3000

C 25

1.46

1.4 5

C (6

1.85

1.79

t

2000 -

0.0

10

2.0

M FIG. 2.

-6

r-6 x10

3

(Al

‘%I pammagnetic relaxation rates R IM vs the inverse sixth power rz (see text for details).

correlation. This behavior can be interpreted assuming (i) a negligible small unpaired spin density on the carbon atoms (SB limit), (ii) a linear dependence of this quantity on the (rM)-6. In both the cases the ratio of the paramagnetic relaxation times of any two different carbon nuclei equals the sixth power of the ratio of the corresponding distances. To investigate the dynamical properties of this complex (i.e., the reorientational correlation time) a choice must be made between the two hypotheses.

COMPLEXES

OF

DOPAMINE

45

Fe-DA

FIG.

2-Continued.

As a matter of fact it is possible to estimate the correlation time TV from the geometry of the molecular complex and from the knowledge of the unpaired spin densities on the aromatic carbon atoms. Unfortunately Mn2+ is not a shifI reagent and therefore no information is experimentally obtainable on the unpaired spin delocalization. Taking as a first approximation hypothesis (i) as the correct one from the slope of the line in Fig. 2b a value of 5.3 X lo-” for r, is calculated. EPR spectra of Mn’+-DA show the usual sextet pattern clearly attributable to the hexaquo manganous ion, whose intensity decreases with increasing DA concentration. Opposite to Fe3+ and Cu2+ manganous ion is thus incompletely bound to DA at the concentrations examined. ‘Glassy spectra show most of the absorption centered at ges = 2, flanked by broad wing to low field and some absorption at gee 4.3 and 6 (Fig. 3b). This allows the statement that the mean D term is 4.000 MHz and that T, is at least one order of magnitude longer than the iron-DA complex (see later). We thus have that rr dominated in Eqs. [2] and the value of 7, = 5.3 X lo-” set must be identified with the reorientational time. This time leads to a Debye-Stokes calculated mean radius for the complex a N 4 A which seems a reasonable value for the hypothesized complexation model. Since this reorientation time is not too d&rent from that of the Cu2+-DA complex, hypothesis (ii) can be neglected, in line with the findings of other investigations on the complexes of the manganese ion (26, 27), where no spin delocalization was observed. Fe(III,LDA The general trend observed in the Cu-DA complex is also found in the Fe3+-DA complex (Fig. 2~). Thus similar conclusions can be drawn as to the applicability of the SB limit. Making use of the reorientational correlation times found for manganese

46

BURLAMACCHI

ET AL.

2

a

----z$

d i

b

e

&I x10

I

a@ 1000

I I if---rI

!

lil...

gaff=43 3ooo

geff=* 5ooo

g&lo GalJS.5

geff’4.3

1000

g&f=*

3oocl

5000

FIG. 3. X-band EPR spectra of Cu”-DA at 298 K water solution (a); water-20% glycerol solution of Mn2+-DA complex at 90 K (b); water-20% glycerol solution of Fe(NO& at 90 K (c); water-2oW glycerol solution of Fe3+-DA complex at 90 K, pH = 4.5 (d); and the same as (d) at pH = 7.5 (e).

and copper complexes, a theoretical straight line with no physical meaning is obtained, since the relaxation rates for C3, Cd are predicted to be much higher than those experimentally found. Therefore it seems that the main modulation of the dipolar interaction arises from the electronic relaxation time. If, as a first approximation, we assume as negligible the contribution due to the unpaired spin density on C3, Cd relaxation rates, we can derive from the slope of the SB straight line (Fig. 2c) a value of 2.4 X 10-l’ set that therefore corresponds to the electronic relaxation time. The reliability of this value was checked with the aid of EPR spectroscopy. The EPR spectra of Fe3+-DA solutions at room temperature appear undetectably broad. The electron spin relaxation for Fe3+ and for Mn*+ is dominated by the time dependence under Brownian rotation of the zero-field splitting (zfs) anisotropy. The theory, described elsewhere (28, 29), predicts that the linewidth is a function of the product (0.3). TV,where (D:D) is the inner product of the zfs tensor. Any change in the ionic surroundings able to decrease the ligand field symmetry will thus increase

COMPLEXES

47

OF DOPAMINE

the electron spin relaxation rate and cause line broadening. Furthermore the absence of any EPR spectrum attributable to free Fe3+ ions (single peak -600 G broad) shows that in this case iron is totally bound to DA. As for many other transition metal ions, since the linewidth is not measurable, 7, is not experimentally determinable in solution. To solve this problem the zfs term was estimated from frozen solutions at 90 K, in which a glassy state was obtained by addition of 20% glycerol. Figures 3c and d show the EPR spectra of free Fe(NOJh and of the iron-DA complex at pH = 4.5. While for the free ions, part of the absorption appears at g& = 2, for the Fe-DA complex only a weak absorption is observed at and above geff = 4.3. Analysis of 3dS ions in glasses was previously done (30, 31) and estimation of a mean zfs term for various systems was made (32-35). Summing up, the absence of any absorption at geff = 2 for the iron-dopamine complex and the presence of absorptions at g,E = 4.3, 6, and 10 allows one to make a rough estimate of a mean zfs term D which should be not less than the X-band Zeeman energy (D 9 10.000 MHz) (32). Assuming 7, = 8 X lo-” in analogy with the copper complex, a mean 7, value of < I X 10-l ’ is obtained. In this calculation several assumptions were made, including (a) the Fe-DA complex has the same rotational correlation time as the copper complex; (b) the zfs distribution in a glassy matrix reflects that of the liquid (32), and (c) a mean value of 7, over the different spin transitions is used. Although some of these seem to be rather rough approximations, it appears quite evident that 7, -C 7r and that the electron spin relaxation is dominating in Eqs. [2].

1 nRIM k3ec-11 3000:-

l

pD

3.7

B

pD

4.5

.

pD

7.0

0

FIG. 4. The 13Cpammagnetic relaxation rates RIM vs the inverse sixth power r$ for Fe3+-DA complex at three different pDs.

48

BURLAMACCHI

ET AL.

It is interesting to note that with increasing pH, the EPR fine splitting lines, observed in glassy Fe-DA solution, converge toward a single peak centered at gti = 4.3 (Fig. 3e). A plausible interpretation of this fact is that with increasing pH equilibrium is shifted toward a single complex with well defined ligand symmetry. The disappearance of the ges = 10 and 6 transitions should account for a smaller D term, thus inducing a shorter 7,. The results of a similar NMR analysis as a function of the pD are shown in Fig. 4. Since, as discussed above, T, represents the correlation time for the complex, shortening of T, with increasing pDs must be reflected in a decrease of the PRR of the various carbon atoms. In fact the experimental evidence (Fig. 4) strongly suggests the correctness of this hypothesis.

INDO Analysis of the PammagneticRelaxationRates In the theoretical section it was shown that for at least three differently relaxing nuclei of a ligand molecule it is possible to relate the observed relaxation rates to the unpaired spin densities on the carbon atoms with the elimination of the unknown correlation time. If the metal ions were shift probes one could estimate the spin densities from the NMR spectra and check the reliability of the model chosen for the complex by testing the consistence of Eqs. [4]. If this requisite is not fulfilled, as is the case of the metal ions under investigation, but accurate molecular orbitals are known for the metal complexes, unpaired spin densities about the benzene ring could be calculated. While these calculations are impracticable at this level of molecular complexity, it is nevertheless instructive to investigate this relationship at the more

0.01

Qll

0.01

--,

Fe-DA Cu-

o.o<

DA

O.O(

FIG.

text).

5. Dependence of IQ) for Cu”-DA

and Fe”-DA

systems on the dihedral angle C-C-O-H

(see

COMPLEXES

OF

DOPAMINE

49

approximate INDO level (36). In this approach the cationic radical model of DA was employed with the bond lengths and angles reported in the experimental section and a C-O bond value of 1.30 A, intermediate between a single and a double bond. The total unpaired spin densities on the aromatic carbon nuclei for various C-CO-H torsions were introduced in Eqs. [4] relatively to Cu2+ and Fe3+ complexes. The results, shown in Fig. 5, indicate that the values of p: , p$, and pi satisfying Eqs. [4] occur for torsional angles around 90”, that is, for a u delocalization mechanism of the unpaired spin density. It is noteworthy that a similar mechanism was found for Ni2+-DA and Co2+-DA complexes from the analysis of the isotropic shifts. CONCLUSION

It was shown that the applicability of the Solomon-Bloembergen equation depends on the nature of the paramagnetic metal ion and that the interpretation of the paramagnetic relaxation rates must be performed accordingly. In particular, electron spin delocalization, while irrelevant in the manganous complex, becomes predominant in the remaining complexes. The complementary use of the NMR and EPR spectroscopies allowed a more accurate rationalization of the dynamical properties of the complexes. It was also shown that the use of INDO MO calculations is to be a useful tool for investigating the delocalization mechanisms of the unpaired spin densities from measurements of paramagnetic relaxation data. ACKNOWLEDGMENTS This work was partially calculations were performed llniversity of Ca8liari.

supported by the Consiglio Nazionale delle Ricer&e (CNR), Italy. on a UNIVAC 1100 computer at the Centro di Calcolo Elettronico

Ail the of the

REFERENCES

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17.

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