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Inorg. Chem. 1996, 35, 404-414
N,N′-Ethylenedi-L-cysteine (EC) and Its Metal Complexes: Synthesis, Characterization, Crystal Structures, and Equilibrium Constants Yuejin Li,† Arthur E. Martell,*,† Robert D. Hancock,§ Joseph H. Reibenspies,† Carolyn J. Anderson,‡ and Michael J. Welch‡ Department of Chemistry, Texas A&M University, College Station, Texas 77843-3225, The Edward Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Missouri 63110, and Department of Chemistry, University of the Witwatersrand, Johannesburg, South Africa ReceiVed NoVember 17, 1994X N,N′-ethylenedi-L-cysteine (EC) and its indium(III) and gallium(III) complexes have been synthesized and characterized. The crystal structures of the ligand and the complexes have been determined by single-crystal X-ray diffraction. EC‚2HBr‚2H2O (C8H22Br2N2O6S2) crystallizes in the orthorhombic space group P21212 with a ) 12.776(3) Å, b ) 13.735(2) Å, c ) 5.1340 (10) Å, Z ) 2, and V ) 900.9(3) Å3. The complexes Na[M(III)EC]‚2H2O (C8H16MN2O6S2Na) are isostructural for M ) In and Ga, crystallizing in the tetragonal space group P42212 with the following lattice constants for In, (Ga): a ) 10.068(2) Å, (9.802(2) Å), b ) 10.068(2) Å, (9.802(2) Å), c ) 14.932(2) Å, (15.170(11) Å), Z ) 4 (4), and V ) 1513.6(5) Å3, (1457.5(11) Å3). In both metal complexes, the metal atoms (In and Ga) are coordinated by six donor atoms (N2S2O2) in distorted octahedral coordination geometries in which two sulfur atoms and two nitrogen atoms occupy the equatorial positions, and the axial positions are occupied by two oxygen atoms of two carboxylate groups. The structures of the complexes previously predicted by molecular mechanics are compared with the crystal structures of the Ga(III) and In(III) complexes obtained experimentally. In contrast to the oxygen donors in phenolate-containing ligands, such as 1,2-ethylenebis((o-hydroxyphenyl)glycine) (EHPG) and N,N′-bis(o-hydroxybenzyl)ethylenediamineN,N′-diacetic acid (HBED), the thiolate donors of EC enhances affinity for In(III) relative to Ga(III). The following stability sequence has been obtained: In(III) > Ga(III) . Ni(II) > Zn(II) > Cd(II) > Pb(II) > Co(II). Evidence was also obtained for several protonated and hydroxo species of the complexes of both divalent and trivalent metals, where the corresponding protonation constants (KMHL) decrease with increasing stability of the chelate, MLn-4, where Mn+ represent the metal ion.
Introduction In a recent paper,1 we reported a molecular mechanics simulation of the structures of the gallium(III) and indium(III) complexes of EC (N,N′-ethylene-di-L-cysteine), 1. Now we are able to report the crystal structures of the Ga(III) and In(III) complexes of this ligand and to compare the results with the simulated structures. Although EC has been known for a long time,2 and multidentate ligands containing thiolate donor groups have been of recent interest, we were surprised to learn that proton and metal ion affinities of EC have not been reported. The protonation constants and stability constants of the Ga(III) and In(III) chelates have recently been cited, but no experimental details were given.1 In this paper the protonation of and metal ion complexation by EC in aqueous solution, including the complexation of a series of divalent metal ions, are described. Many donor groups, such as phenolate, which impart high stabilities for the Fe(III) and Ga(III) chelates, do not do so in the case of In(III). The stability constants of complexes of racEHPG (1,2-ethylenebis((o-hydroxyphenyl)glycine), 2, with log K’s of Fe(III), Ga(III), and In(III) of 35.54, 33.89, and 26.68,3 and those of the corresponding metal complexes of HBED (N,N′-(o-hydroxybenzyl)ethylenediamine-N,N′-diacetic acid), 3, †
Texas A&M University. Washington University School of Medicine. University of the Witwatersrand. X Abstract published in AdVance ACS Abstracts, December 15, 1995. (1) Anderson, C. J.; John, C. S.; Li, Y.; Hancock, R. D.; McCarthy, T. J.; Martell, A. E.; Welch, M. J. Nucl. Med. Biol. 1995, 22, 165. (2) Crooks, H. M. In The Chemistry of Penicillin; Princeton University Press: Princeton, NJ, 1949; p 460. (3) Bannochie, C. J.; Martell, A. E. J. Am. Chem. Soc. 1989, 111, 4735. ‡ §
0020-1669/96/1335-0404$12.00/0
are 39.01, 38.51, and 27.76,4 respectively. The stability constants for indium(III) complexes are lower than those of iron(III) and gallium(III) by as much as 7-11 orders of magnitude.5 Indium(III) and Ga(III) are similar in their affinities for ligands with different donor groups. Estimated,6 and experimental7 (4) Ma, R; Motekaitis, R. J.; Martell, A. E. Inorg. Chim. Acta 1994, 224, 151. (5) Hancock, R. D.; Martell, A. E. Chem. ReV. 1989, 89, 1875.
© 1996 American Chemical Society
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values of log K1 for Ga(III) and In(III) with unidentate ligands CH3COO-, RS-, and saturated nitrogen donors, indicate the affinities of Ga(III) and In(III) for the donor atoms present on EC (Fe(III) is included for comparison): metal ion
In(III)
Ga(III)
Fe(III)
log K1(CH3COO-)a log K1(NH3)a log K1(RS-)a
3.4 3.9* 9.6
3.8* 3.7* 8.7*
4.0 3.8* 8.6*
a Experimental values: 25 °C, ionic strength zero; see ref 7. Estimated (*) values: see refs 5 and 6. R is HOCH2CH2-.
One should note that mercapto groups are not “soft” in the hard and soft acid base sense,6 but should be regarded as possibly “borderline”, and form complexes of high stability with such “hard” metal ions as Ga(III). In(III) and Ga(III) differ in size;8 the ionic radius of Ga(III) is 0.55 Å, and that of In(III) is 0.80 Å (octahedral coordination). While the intrinsic strength of binding of these two ions to the individual donor atoms will be similar, the difference in size can lead to very different ligand selectivities, derived from steric effects.9 The lower stability of complexes of the larger In(III) ion than of the smaller Ga(III) ion with o-hydroxybenzylate groups can be attributed to (1) the slightly lower affinity that In(III) has for negatively charged oxygen donor groups (indicated by log K1(OH-) values of7 In(III) ) 10.0 and Ga(III) ) 11.4), and (2) the size of the chelate rings. The six-membered chelate rings formed favor coordination of small metal ions such as Ga(III) and disfavor complexation of large metal ions such as In(III).5,9 Indium-111 (T1/2 ) 67.9 h) is a widely used radionuclide for nuclear medicine imaging. 111In has a convenient half-life and it decays by electron capture with the emission of two photons at 173 keV (89%) and 247 keV (94%) which are ideal for gamma scintigraphy.10 The 2.8 d half-life makes 111In suitable for imaging with radiolabeled monoclonal antibodies (MAbs), which clear the circulation slowly. The most common bifunctional chelates used in complexing 111In for labeling of MAbs include analogs of DTPA (diethylenetriaminepentaacetic acid), 4.11,12 Bifunctional chelates of HBED, 2, have also been conjugated to MAbs and investigated in ViVo.13,14 One of the major problems with radiolabeled intact MAbs as radiopharmaceuticals is that a significant fraction of the radiometal is taken up and retained in the liver. Extensive work by our groups at Texas A&M and Washington University has shown that 111In- or 67Ga-labeled ligands show different liver clearance patterns depending on the stability of the metal-ligand complex.1,15,16 In(III) or Ga(III) complexes that are of greater thermodynamic stability clear out of the liver and into the intestines, whereas less stable complexes remain trapped in the liver, possibly because of exchange with intracellular liver (6) Hancock, R. D.; Martell, A. E. AdV. Inorg. Chem., in press. (7) Smith, R. M.; Martell, A. E.; Motekaitis, R. J. Critical Stability Constants Database, Beta Version; NIST: Gaithersburg, MD, 1993. (8) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. (9) Hancock, R. D. Progr. Inorg. Chem. 1989, 37, 187. (10) For a review, see: Anderson, C. J.; Schwarz, S. W.; Welch, M. J. in Handbook on Metals in Clinical and Analytical Chemistry; Seiler, H. G., Sigel, A., Sigel, H., Eds.; Marcel Dekker, Inc.: New York, 1994; pp 401-407. (11) Hnatowich, C. J.; Layne, W. W.; Childs, R. L. Int. J. Appl. Radiat. Isot. 1982, 33, 327. (12) Meares, C. F.; McCall, M. J.; Reardan, D. R.; Goodwin, D. A.; Diamanti, C. I.; McTigue, M. Anal. Biochem. 1984, 142, 68. (13) Mathias, C. J.; Sun, Y.; Connett, J. M.; Philpott, G. W.; Welch, M. J.; Martell, A. E. Inorg. Chem. 1990, 29, 1475. (14) Mathias, C. J.; Sun, Y.; Welch, M. J.; Connett, J. M.; Philpott, G. W.; Martell, A. E. Bioconj. Chem. 1990, 2, 204. (15) Madsen, S. L.; Bannochie, C. J.; Martell, A. E.; Mathias, C. J.; Welch, M. J. J. Nucl. Med. 1990, 31, 1662. (16) Madsen, S. L.; Bannochie, C. J.; Welch, M. J.; Mathias, C. J.; Martell, A. E. Nucl. Med. Biol. 1991, 18, 289.
proteins. Maintaining high binding strength of ligands to Ga(III) and In(III) may be necessary to prevent exchange with natural receptors for trivalent metal ions such as the blood protein transferrin or other intercellular proteins in the liver.17 Experimental Section Materials. All commercially available reagents used were of analytical grade. All solutions were prepared with distilled water that was deionized by means of an ion-exchange column from which oxygen and carbon dioxide were removed by boiling and subsequent cooling under argon. The metal salts were standardized complexometrically by either EDTA (ethylenediaminetetraacetic acid), 5, titration with an appropriate indicator18,19 or passage through Dowex 50W-X8 cationexchange resin. In the latter case, the eluted acid was titrated with standard KOH solution. The base used for potentiometric titration was carbonate-free KOH solution, which was prepared from CO2-free commercial concentrate (Baker “Dilut-it” ampules) and was standardized against oven-dried potassium hydrogen phthalate. A CO2-free atmosphere for the base was maintained at all times. Synthesis and Characterization of the Ligand. Ethylenebis-Lcysteine, 1, was prepared via the reduction of L-thiazolidine-4-carboxylic acid by sodium in liquid ammonia.2 Anal. Calcd (found) for C8H16N2O4S2: C, 35.81 (35.42); H, 6.01 (5.96); N, 10.44 (10.15). Mass spectrum (FAB): m/z 267 (M - H)-. 1H NMR (in D2O/DCl): 4.55 ppm (t, -CH-); 3.72 ppm (s, -NCH2CH2N-); 3.31 ppm (m, -CH2S-). Synthesis of Crystalline H4L‚2HBr‚2H2O, I. Single crystals of H4L‚2HBr‚2H2O were grown as colorless needles from a hot aqueous 48% HBr solution upon slow cooling. Synthesis of Crystalline Na[InEC]‚2H2O, II. To an aqueous solution of EC (54 mg, 0.20 mmol, 5 mL), was added 44 mg (0.20 mmol) of InCl3. The solution was refluxed for 20 min after the pH value of the solution was adjusted to 7.5 with NaOH. The resulting solution was filtered. Cubic shaped, colorless single crystals of Na[InEC]‚2H2O were grown from the filtrate on cooling to room temperature. Mass spectrum(FAB): m/z 379 (M - H)-; 401 (M + Na - H)-; 437 (M + Na + 2H2O - H)-. Electronic spectral band, λmax (max): 219 nm (1.61 × 104 dm3 mol-1 cm-1). Synthesis of Crystalline Na[GaEC]‚2H2O, III. With the same procedure as that described for Na[InEC]‚2H2O, crystals of Na[GaEC]‚2H2O were obtained. Mass spectrum(FAB): m/z 333 (M - H)-; 355 (M + Na - H)-; 391 (M + Na + 2H2O - H)-. X-ray Crystallography. Single-crystal X-ray diffraction analyses were performed on EC‚2HBr‚2H2O, Na[InEC]‚2H2O and Na[GaEC]‚2H2O. The details of the data collection and structure refinement, the atomic coordinates, and the bond distances and angles for each compound are given in Tables 1-5, respectively. A colorless needle (0.40 × 0.05 × 0.05 mm) for I, and colorless plates for II (0.25 × 0.25 × 0.25 mm) and III (0.05 × 0.15 × 0.20 mm) were each mounted on glass fibers with epoxy cement, at room temperature and cooled to 163 K for I and II and maintained at room temperature for III. Preliminary examinations and data collections were performed on a Rigaku AFC5R X-ray diffractometer (oriented graphite monochromator; Mo KR (λ ) 0.710 73 Å) radiation). Cell parameters were calculated from the least-squares fitting of the setting angles for 25 high-angle reflections (2θav > 20°) The ω scans for several intense reflections indicated acceptable crystal quality. Data were collected to 2 θ ) 50° at 163 K for I an II and at room temperature for III. Scan width, on θ, for the data collection were (1.628 + 0.3 tan θ)° for I, (0.945 + 0.3 tan θ)° for II, and (1.365 + 0.3 tan θ)° for III. Weak reflections were rescanned for a maximum of three scans to improve their individual counting statistics. Variable scan rates between 4 and 16°/min were employed. Three control reflections, collected every 150 reflections, showed no significant trends. Background measurements were made by the stationary crystal and (17) Madsen, S. L.; Welch, M. J.; Motekaitis, R. J.; Martell, A. E. Nucl. Med. Biol. 1992, 19, 431. (18) Vogel, A. I. A Textbook of QuantitatiVe Inorganic Anaysis, 3rd ed.; Longmans, Green and Co.: London, 1961. (19) Schwarzenbach, G.; Flaschka, H. Complexometric Titrations; Metheun: London, 1969.
406 Inorganic Chemistry, Vol. 35, No. 2, 1996
Li et al.
Table 1. Summary of Crystallographic data for I, II, and III I empirical formula fw temp, K wavelength, Å cryst syst space group unit cell dimensb a, Å b, Å c, Å vol, Å3 Z density (calcd), g/cm3 abs coeff (µ), mm-1 final R indices [I > 2σ(I)]a R indices (all data)a
II
III
C8H22Br2N2O6S2 466.22 163(2) 0.71073 orthorhombic P21212
C8H16InN2NaO6S2 438.16 163(2) 0.71073 tetragonal P42212
C8H16GaN2NaO6S2 393.06 293(2) 0.71073 tetragonal P42212
12.776(3) 13.735(2) 5.1340(10) 900.9(3) 2 1.719 4.752 R1 ) 0.0363, wR2 ) 0.0756 R1 ) 0.0618, wR2 ) 0.1529
10.068(2) 10.068(2) 14.932(2) 1513.6(5) 4 1.923 1.889 R1 ) 0.0438, wR2 ) 0.1196 R1 ) 0.0540, wR2 ) 0.1274
9.802(2) 9.802(2) 15.170(11) 1457.5(1) 4 1.791 2.227 R1 ) 0.0674, wR2 ) 0.1554 R1 ) 0.1966, wR2 ) 0.3182
a R1 ) ∑||F | - |F ||/∑|F |. wR2 ) {∑[w(F 2 - F 2)2]/∑[w(F 2)2]}1/2, where w ) [∑2(F 2) + (aF 2)2 + (bF 2)]-1 where a ) 0.012, 0.054, and o c o o c o o o o 0.072 for I, II, and III, respectively, and b ) 0.936, 9.26, and 7.54 for I, II, and III, respectively. R ) β ) γ ) 90° for I, II, and III.
Table 2. Atomic Coordinates (×104) and Equivalent Isotropic Displacement Parameters (Å2 × 103) x Br(1) S(1) O(1) O(2) O(3) N(1) C(1) C(2) C(3) C(4) In(1) Na(1) S(1) O(1) O(2) O(3) O(4) N(1) C(1) C(2) C(3) C(4) Ga(1) Na(1) S(1) O(1) O(2) O(3) O(4) N(1) C(1) C(2) C(3) C(4)
y
z
U(eq)
2160(1) 7826(2) 5207(5) 5191(5) 4494(4) 5918(4) 6813(6) 5798(6) 4889(6) 5367(6)
I, H4L‚2HBr‚2H2O 5047(1) 310(1) 7426(2) -3921(6) 6532(4) -3077(12) 7986(4) -1200(10) 5876(4) 1332(5) 8926(4) -5384(14) 7352(7) -6334(19) 7838(6) -5586(18) 9462(5) -5559(15) 7480(6) -3010(17)
28(1) 40(1) 43(2) 33(2) 54(2) 30(2) 38(2) 31(2) 29(2) 28(2)
5000 0 4501(4) 2801(7) 1174(7) 1303(15) 3194(29) 4629(7) 3773(11) 3412(9) 4486(10) 2382(10)
II, Na[InEC)‚2H2O 0 1400(1) 0 1074(5) 2003(3) 529(2) -90(8) 1712(4) 1318(8) 2051(5) 1303(15) 0 3194(29) 5000 1363(8) 2595(4) 2909(10) 1464(7) 2052(9) 2289(7) 555(10) 3420(5) 1019(11) 2010(7)
28(1) 142(6) 55(1) 35(2) 41(2) 94(5) 296(30) 18(2) 33(2) 26(2) 26(2) 30(2)
0 5000 307(4) 2117(9) 3805(10) 3200(16) 3685(14) 298(12) 1116(15) 1506(13) 485(18) 2588(17)
III, Na[GaEc]‚2H2O 5000 -1481(1) 5000 -1034(8) 6841(4) -552(3) 4858(11) -1680(6) 6271(11) -2029(8) 3200(16) 0 6315(14) 0 6379(12) -2536(8) 7892(15) -1388(13) 7074(15) -2208(12) 5613(16) -3361(10) 6028(16) -1975(11)
23(1) 175(11) 38(1) 26(2) 36(3) 104(9) 60(5) 27(3) 42(5) 29(4) 41(4) 30(4)
a U(eq) is defined as one-third of the trace of the orthogonalized U ij tensor.
stationary counter technique at the beginning and end of each scan for 0.50 of the total scan time. Lorentz and polarization corrections were applied to 877, 846, and 1525 reflections for I, II, and III, respectively. A semiempirical absorption correction was applied to I and II, and an empirical absorption correction was applied to III.20 A total of 965, 835, and 1303 reflections for I, II and III were used in further calculations. (20) PROCESS, Program for Data Reduction for Rigku AFC diffractometers. Molecular Structure Corp., The Woodlands, Texas.
Table 3. Bond Lengths (Å) and Angles (deg) for Ia S(1)-C(1) O(2)-C(4) N(1)-C(3) C(2)-C(4) C(2)-N(1)-C(3) N(1)-C(2)-C(1) C(1)-C(2)-C(4) O(2)-C(4)-O(1) O(1)-C(4)-C(2)
1.794(9) 1.182(9) 1.509(9) 1.515(12) 113.1(6) 111.7(7) 113.1(7) 124.9(9) 110.7(7)
O(1)-C(4) N(1)-C(2) C(1)-C(2) C(3)-C(3)1
1.319(9) 1.507(9) 1.508(11) 1.505(13)
C(2)-C(1)-S(1) N(1)-C(2)-C(4) C(3)1-C(3)-N(1) O(2)-C(4)-C(2)
114.8(6) 107.4(7) 108.3(8) 124.4(8)
a Symmetry transformations used to generate equivalent atoms: (1) -x + 1, -y + 2, z.
Table 4. Bond Lengths (Å) and Angles (deg) for IIa In(1)-O(1)1 In(1)-N(1) In(1)-S(1)1 Na(1)-O(2)2 Na(1)-O(3)2 Na(1)-O(1) Na(1)-Na(1)3 O(1)-C(4) O(3)-Na(1)3 N(1)-C(3) C(2)-C(4) O(1)1-In(1)-O(1) O(1)-In(1)-N(1) O(1)-In(1)-N(1)1 O(1)1-In(1)-S(1)1 N(1)-In(1)-S(1)1 O(1)1-In(1)-S(1) N(1)-In(1)-S(1) S(1)1-In(1)-S(1) C(1)-S(1)-In(1) C(4)-O(1)-Na(1) C(4)-O(2)-Na(1) C(2)-N(1)-C(3) C(3)-N(1)-In(1) N(1)-C(2)-C(4) C(4)-C(2)-C(1) O(2)-C(4)-O(1) O(1)-C(4)-C(2)
2.264(7) 2.281(7) 2.452(3) 2.300(9) 2.45(2) 2.979(7) 3.207(14) 1.274(13) 2.45(2) 1.483(11) 1.527(14) 156.3(3) 72.7(2) 88.6(3) 86.7(2) 160.6(2) 106.1(2) 83.5(2) 115.87(14) 95.0(3) 76.8(6) 109.2(7) 115.7(7) 109.6(5) 109.1(8) 108.8(8) 123.3(10) 117.8(9)
In(1)-O(1) In(1)-N(1)1 In(1)-S(1) Na(1)-O(2) Na(1)-O(3) Na(1)-O(1)2 S(1)-C(1) O(2)-C(4) N(1)-C(2) C(1)-C(2) C(3)-C(3)1 O(1)1-In(1)-N(1) O(1)1-In(1)-N(1)1 N(1)-In(1)-N(1)1 O(1)-In(1)-S(1)1 N(1)1-In(1)-S(1)1 O(1)-In(1)-S(1) N(1)1-In(1)-S(1) C(4)-O(1)-In(1) In(1)-O(1)-Na(1) Na(1)3-O(3)-Na(1) C(2)-N(1)-In(1) C(2)-C(1)-S(1) N(1)-C(2)-C(1) N(1)-C(3)-C(3)1 O(2)-C(4)-C(2)
2.264(7) 2.281(7) 2.452(3) 2.300(9) 2.45(2) 2.979(7) 1.823(11) 1.254(13) 1.480(11) 1.547(14) 1.52(2) 88.6(3) 72.7(2) 77.1(4) 106.1(2) 83.5(2) 86.7(2) 160.6(2) 111.2(7) 149.2(3) 81.7(7) 100.1(5) 115.2(7) 108.2(7) 109.7(6) 118.8(9)
a Symmetry transformations used to generate equivalent atoms: (1) -x + 1, -y, z; (2) -x, -y, z; (3) y, x, -z.
The structures were solved by direct methods.21 Full-matrix leastsquares anisotropic refinement on F2 for all non-hydrogen atoms,22 yielded R1 of 0.036, 0.044, and 0.067 for I, II, and III, respectively, at convergence. Carbon-, nitrogen-, and sulfur-bound hydrogen atoms (21) Sheldrick, G. SHELXS-86 Program for Crystal Structure Solution; Institut fur Anorganishe Chemie der Universitat: Gottingen, Germany, 1986.
N,N′-Ethylenedi-L-cysteine and Its Metal Complexes
Inorganic Chemistry, Vol. 35, No. 2, 1996 407
Table 5. Bond Lengths (Å) and Angles (deg) for III Ga(1)-O(1) Ga(1)-N(1) Ga(1)-S(1)1 Na(1)-O(2) Na(1)-O(4)2 Na(1)-O(3) Na(1)-O(1)2 Na(1)-Na(1)3 O(1)-C(4) O(3)-Na(1)3 N(1)-C(2) C(1)-C(2) C(3)-C(3)1
2.102(9) 2.115(11) 2.308(4) 2.281(13) 2.40(2) 2.95(2) 2.994(10) 3.14(2) 1.31(2) 2.95(2) 1.45(2) 1.53(2) 1.53(3)
Ga(1)-O(1)1 Ga(1)-N(1)1 Ga(1)-S(1) Na(1)-O(2)2 Na(1)-O(4) Na(1)-O(3)2 Na(1)-O(1) S(1)-C(1) O(2)-C(4) O(4)-Na(1)3 N(1)-C(3) C(2)-C(4)
2.102(9) 2.115(11) 2.308(4) 2.281(13) 2.40(2) 2.95(2) 2.995(10) 1.82(2) 1.22(2) 2.40(2) 1.47(2) 1.52(2)
O(1)-Ga(1)-O(1)1 O(1)1-Ga(1)-N(1) O(1)1-Ga(1)-N(1)1 O(1)-Ga(1)-S(1)1 N(1)-Ga(1)-S(1)1 O(1)-Ga(1)-S(1) N(1)-Ga(1)-S(1) S(1)1-Ga(1)-S(1) C(1)-S(1)-Ga(1) C(4)-O(1)-Na(1) C(4)-O(2)-Na(1) Na(1)3-O(4)-Na(1)
163.4(5) 89.1(4) 78.3(4) 99.5(3) 168.2(3) 90.6(3) 86.8(3) 104.8(2) 94.3(5) 74.9(8) 110.6(11) 81.4(8)
O(1)-Ga(1)-N(1) O(1)-Ga(1)-N(1)1 N(1)-Ga(1)-N(1)1 O(1)1-Ga(1)-S(1)1 N(1)1-Ga(1)-S(1)1 O(1)1-Ga(1)-S(1) N(1)1-Ga(1)-S(1) C(4)-O(1)-Ga(1) Ga(1)-O(1)-Na(1) Na(1)3-O(3)-Na(1) C(2)-N(1)-C(3) C(3)-N(1)-Ga(1)
78.3(4) 89.1(4) 81.6(7) 90.6(3) 86.8(3) 99.5(3) 168.2(3) 109.8(9) 151.8(5) 64.3(6) 115.3(12) 109.6(9)
C(2)-N(1)-Ga(1) C(2)-C(1)-S(1) N(1)-C(2)-C(1) N(1)-C(3)-C(3)1 O(2)-C(4)-C(2) O(2)-C(4)-Na(1) C(2)-C(4)-Na(1)
98.8(9) 112.3(10) 108.7(12) 108.8(10) 122.(2) 46.6(9) 155.1(12)
N(1)-C(2)-C(4) C(4)-C(2)-C(1) O(2)-C(4)-O(1) O(1)-C(4)-C(2) O(1)-C(4)-Na(1)
109.4(12) 109.8(14) 123.(2) 115.1(14) 79.5(8)
were placed in idealized positions with isotropic thermal parameters fixed at 0.08 Å2. No extinction correction was applied. Absolute configurations for all three structures were determined by examination of the Flack23 absolute structure parameters (0.08(4), 0.1(2), and 0.02(8) for I, II, and III, respectively. Neutral atom scattering factors and anomalous scattering correction terms were taken from ref 24. General Techniques. 1H NMR spectra in D2O solution were measured on a Varian XL-200 spectrometer. The pD’s of the D2O solutions of the ligand were measured with a combination microelectrode. The -log [H+] measurements were made after the calibration of the microelectrode with standard aqueous HCl solutions and the final pD was calculated by the equation pD ) pH + 0.40.25 Sodium 3-(trimethylsilyl)propane-1-sulfonate (DSS) was used as an internal standard. Negative-ion fast atom bombardment (FAB) mass spectra were determined with a VG Analytical 70S high resolution, double focusing, sectored (EB) mass spectrometer equipped with a VG Analytical 11/250J data system. Elemental analyses were carried out by Galbraith Laboratories, Inc., Knoxville, TN. UV absorption spectra were recorded with a Perkin-Elmer Model 553 fast scan spectrophotometer interfaced with a Perkin-Elmer R-1000 recorder. Potentiometric Determinations. The potentiometric apparatus consisted of a Corning Research pH meter Model 130 fitted with glass (Sargent Welch) and calomel reference (Fisher) electrodes, a waterjacketed titration cell, a 10 mL capacity Metrohm piston buret which delivers standard KOH titration solution directly to the air-sealed cell through a buret tip which is secured to the cell cap with a clamp and O-rings, and a thermostated constant temperature bath set at 25.0 ( 0.1 °C. A stream of purified argon was used as the inert atmosphere in the titration cell and to degas all solutions before titrations. Oxygen and carbon dioxide were excluded from the reaction mixture by maintaining a slight positive pressure of purified argon in the titration (22) Sheldrick, G. SHELXL-93 Program for Crystal Structure Refinement; Institut fur Anorganishe Chemie der Universitat: Gottingen, Germany, 1993. (23) Flack, H. D. Acta Crystallogr. 1993, A39, 876. (24) International Tables for Crystallography; Wilson, A. J. C., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992; Vol. C, Tables 6.1.1.4 (pp 500-502), 4.2.6.8 (pp 219-222), and 4.2.4.2 (pp 193-199). (25) Glasoe, P. K.; Long, F. A. J. Phys. Chem. 1960, 64, 188.
Figure 1. Potentiometric equilibrium curves of 1.00 × 10-3 M EC and EC complexes with divalent metal ions (as indicated). t ) 25.0 °C, µ ) 0.10 M (KCl), and a ) moles of base added per mole of ligand. cell. The last traces of oxygen were removed from the argon by passing it through an alkaline pyrogallol solution. The pH meter was calibrated to read p[H] directly prior to each potentiometric equilibrium experiment with a freshly prepared solution of standard dilute acid (HCl) at an ionic strength adjusted to 0.10 M with KCl. The day-to-day changes observed were on the order of Ga(III) . Ni(II) > Zn(II) > Cd(II) > Pb(II) > Co(II).
The protonated forms of the divalent metal complexes predominate in acid solution as indicated by the data in Table 8 and by the species distribution curves (for example : Cd(II) complexes in Figure 9) . The first two protonation sites are assigned to the mercapto groups. A probable arrangement of ligand donor groups of the diprotonated form is shown in formula 10, in which the two asymmetric carbon atoms with the L configuration are indicated by an asterisk and the metal ion is coordinated by two amino and two carboxylate groups. Comparison with the analogous chelates of EDDA, N,N′ethylenediaminediacetic acid, 11, indicates that the arrangement of donor groups in formula 10 may apply to the Co(II), Pb(II), and Cd(II) diprotonated complexes of EC because their stabilities are close to those of EDDA. But significantly lower stability constants are observed for the diprotonated Ni(II) and Zn(II) complexes of EC (Table 9) compared to the stabilities of the EDDA complexes. The stability constants of corresponding diprotonated complexes of EHPG are also significant lower than those of EDDA complexes. A different arrangement of donor groups of the diprotonated form must be involved in the Ni(II) and Zn(II) complexes. Participation of the mercapto donor groups of EC in metal ion coordination greatly increases the stability constant especially in the case of Ni2+ and Zn2+, which have high affinity for S- donors. The stability constant of 1024.83 for Ni(II)-EC complex is the highest one among the divalent metal complexes studied in this work and of the other ligands listed in Table 9. The stability sequence of monoprotonated divalent metal complexes (Table 8) is in agreement with the concept that the higher the stability of the complex, the lower is the protonation constant. This is logical since the greater the interaction between a ligand donor group and a metal ion, the lower is its tendency to combine with a competing positive ion. It is noted that the stability constants of the hexadentate complexes of EC are greater than the overall stability constants of bis-tridentate complex of cysteine for all divalent metal ions listed in Table 9. The stability constants of related ligands are compared in Table 9. Both In(III) and Ga(III) react strongly with the ligand and at considerably lower pH than do the divalent metal ions. Inspection of the lower buffer regions of the p[H] profiles (Figure 8) indicate the presence of significant concentrations of proto-
414 Inorganic Chemistry, Vol. 35, No. 2, 1996
Li et al.
Table 10. Formation Constants and pM Values at Physiological pH for Ga(III) and In(III) Complexesa Ga(III) ligand
In(III) log KML
pM
log KML
pM
EC TACN-TM HBED EDTA transferrinb
31.5 34.2 38.51 21.0 20.3c, 39.6c,d
24.7 23.6 28.6 20.7 20.4
33.0 36.1 27.76 24.9 18.3c, 35.1c,d
26.2 23.9 17.9 22.2 18.3
a t ) 25.0 °C; µ ) 0.10 M (KCl); pM ) -log [M] at pH 7.4 and 100% excess ligand. b References 41 and 42. c Conditional constants at p[H] 7.4. d log βM2L.
nated chelates as well as nearly complete coordination of indium and gallium. The stability constants of the 1:1 ML chelates were derived from the p[H] titration of the competition reaction with EDTA for In(III)-EC and Ga(III)-EC, respectively (not shown). For the Ga(III) system, EDTA competes with EC below pH 6 but only predominates below p[H] 3. For In(III) the competition occurs in the p[H] range 3-7.5. The In(III)EDTA complex predominates at and below p[H] 5.0. The stabilities of a number of Ga(III) and In(III) chelates are compared in Table 10. Correlations of In3+ in ViVo behavior, measured by its binding to transferrin with stability constants (log K) are not possible because stability constants express the metal ion affinity for a completely deprotonated ligand, whereas the transferrin constants are conditional ones at physiological pH (taken here as 7.4). On the other hand the effectiveness of ligands in binding metal ions may be more suitably compared by considering pM (-log [M]) as a direct measure of the concentration of free metal ion present at the p[H] of interest. The pM values of Ga(III)-EC and In(III)-EC are 4 and 8 log units larger than those of Ga(III)-transferrin and In(III)transferrin, respectively (Table 10). Therefore the competition between EC and transferrin for either Ga3+ or In3+ is seen to be completely in favor of EC at p[H] 7.4, indicating that the complexes should be stable enough to avoid interference by transferrin. The radiolabled complexes of 111InEC and 67GaEC were recently confirmed to be stable in ViVo.1 The pM values for the Ga(III)-HBED complex is much higher than for the In(III) complex, while the pM value for In(III) is close to that
of In(III)-transferrin. This would indicate some extent of metal ion exchange with transferrin for the In(III)-HBED-transferrin system. Even though the stability constants1 for TACN-TM (1,4,7-tris(2-mercaptoethyl)-1,4,7-triazacyclononane) (N3S3) chelates are larger than those of the corresponding EC chelates, the pM values for Ga(III) and In(III)-TACN-TM complexes at physiological pH (7.4) are 1-2 log units less than those of In(III)-EC and Ga(III)-EC, respectively, because TACN-TM has high protonation constants, and therefore formation of its complexes are subject to stronger hydrogen ion competition. EC containing thiolate donors is a more effective multidentate ligand and potential radiopharmaceutical in the form of the 111In complex than the corresponding complexes of HBED or TACN-TM. High stability is an important prerequisite for potential 67Ga3+ or 111In3+ complexes used as radiopharmaceuticals. The fact that In(III) and Ga(III) complexes of EC are very stable in aqueous solution, do not dissociate at physiological p[H] (7.4), and do not exchange with transferrin to an appreciable extent, indicates that it may be an effective ligand for radiopharmaceutical applications. The high stabilities may reflect the strong bonding associated with the metal thiolate coordinate bonds, especially for the In(III) complex. However, the ligand EC produces anionic complexes when it chelates to In(III) or Ga(III). Such charged complexes are generally low in lipophilicity and may have poor membrane permeability. The present results may lead to further development of mercaptide-containing ligands that would be able to form highly lipophilic and stable complexes with In(III) and Ga(III). Acknowledgment. This work was supported by grants from the National Cancer Institute, U.S. Public Health Service, No. CA-42925, and The Robert A. Welch Foundation, No. A-259. Supporting Information Available: Complete crystal data and structure refinement (Table S1), atomic coordinates and equivalent isotropic displacement parameters (Table S2), bond lengths and angles (Table S3), anisotropic displacement parameters (Table S4), hydrogen coordinates and isotropic displacement parameters (Table S5), and packing diagrams of I, II, and III (19 pages). Ordering information is given on any current masthead page. IC941330L
