DTPA Ligand Research Articles

Vibrational anisotropy decay resolves rare earth binding induced conformational change in DTPA.

Elucidating the relationship between metal-ligand interactions and the associated conformational change of the ligand is critical for understanding the separation of lanthanides via ion binding. Here we examine DTPA, a multidentate ligand that binds lanthanides, in its free and metal bound conformations using ultrafast polarization dependent vibrational spectroscopy. The polarization dependent pump-probe spectra were analyzed to extract the isotropic and anisotropic response of DTPA's carbonyl groups in the 1550-1650 cm-1 spectral region. The isotropic response reports on the population relaxation of the carbonyl stretching modes. We find that the isotropic response is influenced by the identity of the metal ion. The anisotropy decay of the carbonyl stretching modes reveals a faster decay in the lanthanide-DTPA complexes than in the free DTPA ligand. We attribute the anisotropy decay to energy transfer among the different carbonyl sites - where the conformational change results in an increased coupling between the carbonyl sites of metal-bound DTPA complexes. DFT calculations and theoretical simulations of energy transfer suggest that the carbonyl sites are more strongly coupled in the metal-bound conformations compared to the free DTPA. The stronger coupling in the metal bound DTPA conformation leads to efficient energy transfer among the different carbonyl sites. Comparing the rate of anisotropy decay across the series of metal bound DTPA complexes we find that the anisotropy is sensitive to the charge density of the central metal ion, and thus can serve as a molecular scale reporter for lanthanide ion binding.

Molecular studies regarding colon cancer cell line (HCT116) treated by new ruthenium and palladium complexes

Ru(H2dtpa) (H2O) and Pd (DME)Cl2 were prepared, and their structures were confirmed by different structural tools. [Ru(H4dtpa) (cysteine)] Cl was separated as a model for the interaction of Ru (III) complex containing DTPA ligand with its corresponding most active amino acid and the geometrical parameters for the ternary complex and their constituent ligands are evaluated. Cytotoxicity was examined for both complexes against different cancer cell lines (HCT116, A549 and Mcf7). They were tested against a normal human lung cell line (W138). In addition, both complexes ensured cytocidal, not cytostatic effect. Several molecular tools were utilized in order to figure out the mechanism of action that two complexes followed to inhibit human colon cancer (HCT116) cell line. Gene expression analysis, cell cycle assay, genomic DNA analysis, and apoptosis were a molecular techniques that proved both Ru(H2dtpa) (H2O) and Pd (DME)Cl2 complexes were promising and effective chemotherapeutic agents against colon carcinoma in this current study. So, authors recommended more in vivo future studies should be done to make sure of complexes bioactivity and safety before they would be used commercially.

Construction of copper (II) affinity- DTPA functionalized magnetic composite for efficient adsorption and specific separation of bovine hemoglobin from bovine serum

In this work, the copper (II) affinity-DTPA (Diethylene triamine pentacetate acid) functionalized magnetic composite was prepared and as an outstanding separation platform to isolate specific bovine hemoglobin (BHb) from bovine serum. Firstly, the Fe3O4 microspheres were prepared via solvothermal method, followed by silanization with TEOS (Tetrathoxysilane) and coating with APTMS (3-Aminopropyl trimethoxysilane). Then incorporating the DTPA ligands via the amino/anhydride reaction. Finally, the target adsorbents (Fe3O4@SiO2@NH2@DTPA@Cu2+ microspheres) were obtained via charging with copper ions. The resulting materials could selectively and specifically isolate BHb with a high maximum capture capacity of 911.3 mg g−1. The absorbed BHb was easily eluted with a recovery of 82.4% by using 0.1 M of Na2CO3-Histidine as eluent. And the real sample analysis of Fe3O4@SiO2@NH2@DTPA@Cu2+ microspheres was also tested using bovine serum, which was identified using MALDI-TOF Ms and SDS-PAGE analysis. The excellent results demonstrate that Fe3O4@SiO2@NH2@DTPA@Cu2+ microspheres would be a good bioseparation material for biomacromolecules like histidine-tagged proteins.

Effect of Mg and Ca on the Stability of the MRI Contrast Agent Gd–DTPA in Seawater

Gadolinium diethylenetriaminepentaacetic acid (Gd–DTPA) is widely applied as a contrast enhancer in medical MRI. As Gd–DTPA is only minimally captured in wastewater treatment plants (WTPs) or degraded by UV light and other oxidative processes, concentrations in rivers have increased globally by orders of magnitude following its introduction in 1987. The complex also seems impervious to estuarine scavenging and is beginning to emerge in coastal waters, yet it is unknown how its stability is changed by competition for the DTPA ligand from major seawater cations. We performed potentiometric titrations at seawater ionic strength (0.7 M NaClO4) to determine dissociation constants of the five DTPA carboxylic acid groups, as well as stability constants of Mg, Ca, and Gd complexes with the fully deprotonated and single-protonated ligand. These are in general agreement with literature values at low ionic strength and confirm that complexes with Ca are more stable than with Mg. A new finding, that the DTPA complexes of Mg and Ca appear to be hydrolyzed at elevated pH, implies that their coordination in these chelates is less than hexadentate, enabling additional competition with Gd from dinuclear Mg and Ca species. Side-reaction coefficients for trace-metal-free seawater, calculated from our results, suggest that the higher abundance of Mg and Ca may significantly destabilize Gd–DTPA in coastal waters, causing dissociation and release of as much as 15% of the organically complexed Gd from the ligand. This effect could enhance the particle-reactivity and bioavailability of anthropogenic Gd in sensitive estuarine habitats, indicating an urgent need to further study the fate of this contaminant in marine environments.

Identification of complexes involving thallium(I) and thallium(III) with EDTA and DTPA ligands by electrospray ionization mass spectrometry.

Thallium is considered to be an environmental threat; however, its hazardous properties depend on its oxidation state. Tl(III) is approx. 1000-times more toxic compared with Tl(I), therefore identification of each species is essential in order to properly evaluate the associated health hazard. Electrospray ionization mass spectrometry (ESI-MS) allows determination of speciation in solution due to its soft mode of ionization while selective complexation with ligands can distinguish the Tl species. Selective complexation of Tl(I) and Tl(III) ions requires the use of two selective complexing agents and selection of appropriate conditions for this process. Tl(I) and Tl(III) ions as well as two ligands (EDTA and DTPA) were used to form binary (single ion + single ligand), ternary (one ion + both ligands) and quaternary systems (both ions and both ligands) under different pH conditions (7 and 8). These mixtures were subjected to the determination of Tl species using ESI-MS operating in positive and negative ion mode. Tl(I) complexes with DTPA were identified at pH7 and 8, whereas, in the case of EDTA, the complexes were detected only at pH8. In contrast, Tl(III) formed distinct complexes with EDTA at pH7 and 8, while with DTPA the complexes were detected only at pH8. Analysis of the quaternary system (which contained both ions and both ligands) revealed that Tl(I) formed complexes with EDTA, while Tl(III) formed complexes with DTPA at pH7 and 8. The obtained results confirmed that the increase in the solution complexity allowed simultaneous identification of different complexes in solutions containing both Tl species. The initial analyses carried out for binary and ternary solutions facilitated the simultaneous determination of specific complexes (Tl(I) with EDTA and Tl(III) with DTPA) in the quaternary system.

Syntheses, structural determination, and binding studies of binuclear nine-coordinate (MnH)4[Ho 2 III (Dtpa)2] · 12H2O and polynuclear nine-coordinate {(MnH)[HoIII(Egta)] · 3H2O} n

Two novel rare earth metal coordination compounds, (MnH)4[Ho 2 III (Dtpa)2] · 12H2O (I) (Mn = methylamine, H5Dtpa = diethylenetriamine-N,N,N′,N″,N″-pentaaceticacid) and {(MnH)[HoIII(Egta)] · 3H2O} n (II) (H4Egta = ethyleneglycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid) have been successfully synthesized through direct heating reflux and characterized by FT-IR spectroscopy, thermal analysis, and single-crystal X-ray diffraction techniques (CIF files CCDC nos. 890878 (I) and 1457061 (II)). Complex I shapes a binuclear nine-coordinate structure with distorted tricapped trigonal prismatic conformation and crystallizes in the triclinic crystal system with space group P1. The central Ho3+ ion is coordinated by three nitrogens and six oxygens from two octadentate Dtpa ligands. The cell dimensions are as follows: a = 9.8420(10), b = 12.1319(13), c = 13.2931(15) A, α = 89.223(2)°, β = 68.4280(10)°, γ = 72.1280(10)° and V = 1396.0(3) A3. Complex II adopts a polynuclear nine-coordinate structure with pseudo-monocapped square antiprismatic conformation and crystallizes in the monoclinic crystal system with space group C2/c. The central Ho3+ ion is coordinated by two nitrogens and seven oxygens from two octadentate Egta ligands. The crystal data are as follows: a = 38.4755(13), b =13.5569(5), c = 8.7343(3) A, β = 100.135(3)° and V = 4484.8(3) A3.

The Effect of Pressure and Temperature on Separation of Free Gadolinium(III) From Gd-DTPA Complex by Nanofiltration-Complexation Method

Nowdays, the utilization of rare earth elements has been carried out widely in industry and medicine, one of them is gadolinium in Gd-DTPA complex is used as a contrast agent in a magnetic resonance imaging (MRI) diagnostic to increase the visual contrast between normal tissue and diseased. Although the stability of a given complex may be high enough, the complexation step couldnot have been completed, so there is possible to gadolinium(III) in the complex compound. Therefore, the function of that compounds should be dangerous because of the toxicity of gadolinium(III) in human body. So, it is necessarry to separate free gadolinium(III) from Gd-DTPA complex by nanofiltration-complexation. The method of this study is complexing of Gd2O3 with DTPA ligand by reflux and separation of Gd-DTPA complex from gadolinium(III) with a nanofiltration membrane on the variation of pressures(2, 3, 4, 5, 6 bars) and temperature (25, 30, 35, 40 °C) and determined the flux and rejection. The results of this study are the higher of pressures and temperatures, permeation flux are increasing and ion rejections are decreasing and gave the free gadolinium(III) rejection until 86.26%.

Probing the structure-relaxivity relationship of bis-hydrated Gd(DOTAla) derivatives.

Two structural isomers of the heptadentate chelator DO3Ala were synthesized, with carboxymethyl groups at either the 1,4- or 1,7-positions of the cyclen macrocycle. To interrogate the relaxivity under different rotatational dynamics regimes, the pendant primary amine was coupled to ibuprofen to enable binding to serum albumin. These chelators 6a and 6b form bis(aqua) ternary complexes with Gd(III) or Tb(III) as estimated from relaxivity measurements or luminescence lifetime measurements in water. The relaxivity of [Gd(6a)(H2O)2] and [Gd(6b)(H2O)2] was measured in the presence and absence of coordinating anions prevalent in vivo such as phosphate, lactate, and bicarbonate and compared with data attained for the q = 2 complex [Gd(DO3A)(H2O)2]. We found that relaxivity was reduced through formation of ternary complexes with lactate and bicarbonate, albeit to a lesser degree then the relaxivity of Gd(DO3A). In the presence of 100-fold excess phosphate, relaxivity was slightly increased and typical for q = 2 complexes of this size (8.3 mM(-1) s(-1) and 9.5 mM(-1) s(-1), respectively, at 37 °C, 60 MHz). Relaxivity for the complexes in the presence of HSA corresponded well to relaxivity obtained for complexes with reduced access for inner-sphere water (13.5 and 12.7 mM(-1) s(-1) at 37 °C, 60 MHz). Mean water residency time at 37 °C was determined using temperature-dependent (17)O-T2 measurements at 11.7 T and calculated to be (310)τM = 23 ± 1 ns for both structural isomers. Kinetic inertness under forcing conditions (pH 3, competing DTPA ligand) was found to be comparable to [Gd(DO3A)(H2O)]. Overall, we found that the replacement of one of the acetate arms of DO3A with an amino-propionate arm does not significantly alter the relaxometric and kinetic inertness properties of the corresponding Gd complexes; however, it does provide access to easily functionalizable q = 2 derivatives.

Simple separation and characterization of lanthanide–polyaminocarboxylic acid complexes by HILIC ESI-MS

Lanthanide complexes with EDTA and DTPA ligands were separated and characterized by HILIC ESI-MS for the first time.

Peptoid‐Based Rare‐Earth (Group 3 and Lanthanide) Transporters

AbstractGroup 3 and lanthanide metals have been complexed by peptoid transporters [polycationic oligo(N‐substituted glycines)]. To achieve this, the rare‐earth‐complexing DTPA ligand was coupled with the peptoids, and afterwards lanthanide salts were added. The photopysical and biological properties of these complexes were investigated.

ChemInform Abstract: The Future of Biomedical Imaging: Synthesis and Chemical Properties of the DTPA and DOTA Derivative Ligands and Their Complexes

AbstractReview: 231 refs.

Theoretical insights into covalency driven f element separations

Through Density Function Theory (DFT) calculations, we set out to understand the structures and stabilities of the aqueous phase complexes [M(III)(DTPA)-H(2)O](2-) (M = Nd, Am) as well as the changes in Gibbs free energy for complexation in the gas phase and aqueous solution. All bonding analyses suggest that the preference of the DTPA(5-) ligand for Am over Nd is mainly due to electrostatic and covalent interactions from the oxygen atoms with the nitrogen chelates providing an additional, yet small, covalent interaction. These results question the exclusive use of hard and soft acids and bases (HSAB) concepts for the design of extracting reagents and suggest that hard-soft interactions may play more of a role in the separations process than previously thought.

Gadolinium MRI Contrast Agents Based on Triazine Dendrimers: Relaxivity and In Vivo Pharmacokinetics

Four gadolinium (Gd)-based macromolecular contrast agents, G3-(Gd-DOTA)(24), G5-(Gd-DOTA)(96), G3-(Gd-DTPA)(24), and G5-(Gd-DTPA)(96), were prepared that varied in the size of dendrimer (generation three and five), the type of chelate group (DTPA or DOTA), and the theoretical number of metalated chelates (24 and 96). Synthesis relied on a dichlorotriazine derivatized with a DOTA or DTPA ligand that was incorporated into the dendrimer and ultimately metalated with Gd ions. Paramagnetic characteristics and in vivo pharmacokinetics of all four contrast agents were investigated. The DOTA-containing agents, G3-(Gd-DOTA)(24) and G5-(Gd-DOTA)(96), demonstrated exceptionally high r1 relaxivity values at off-peak magnetic fields. Additionally, G5-(Gd-DOTA)(96) showed increased r1 relaxivity in serum compared to that in PBS, which was consistent with in vivo images. While G3-(Gd-DOTA)(24) and G3-(Gd-DTPA)(24) were rapidly excreted into the urine, G5-(Gd-DOTA)(96) and G5-(Gd-DTPA)(96) did not clear as quickly through the kidneys. Molecular simulation of the DOTA-containing dendrimers suggests that a majority of the metalated ligands are accessible to water. These triazine dendrimer-based MRI contrast agents exhibit several promising features such as high in vivo r1 relaxivity, desirable pharmacokinetics, and well-defined structure.

Stability of 47Sc-complexes with acyclic polyamino-polycarboxylate ligands.

The aim of this study was to evaluate acyclic ligands which can be applied for labeling proteins such as monoclonal antibodies and their fragments with scandium radionuclides. Recently, scandium isotopes (47Sc, 44Sc) are more available and their properties are convenient for radiotherapy or PET imaging. They can be used together as “matched pair” in theranostic approach. Because proteins denaturize at temperature above 42 °C, ligands which efficiently form complexes at room temperature, are necessary for labelling such biomolecules. For complexation of scandium radionuclides open chain ligands DTPA, HBED, BAPTA, EGTA, TTHA and deferoxamine have been chosen. We found that the ligands studied (except HBED) form strong complexes within 10 min and that the radiolabelling yield varies between 96 and 99 %. The complexes were stable in isotonic NaCl, but stability of 46Sc-TTHA, 46Sc-BAPTA and 46Sc-HBED in PBS buffer was low, due to formation by Sc3+stronger complexes with phosphates than with the studied ligands. From the radiolabelling studies with n.c.a. 47Sc we can conclude that the most stable complexes are formed by the 8-dentate DTPA and EGTA ligands.

EDTA and DTPA modified ligands as sequestering agents for uranyl decorporation

Synthesis of modified EDTA and DTPA ligands and determination of their binding affinities for the uranyl cation are described. Thanks to a screening method, based on a chromophoric complex displacement procedure, chelating properties were studied in aqueous media under various pH conditions for evaluation of their in vivo uranyl-removal efficacy. Each ligand showed a more or less pronounced affinity for uranium. Specific ligands based on EDTA or DTPA analogues containing sulfocatecholamide (CAMS) were found to exhibit a significant affinity towards uranyl ion in acidic, neutral or basic conditions.

Sensitization of Eu(III) luminescence by donor-phenylethynyl-functionalized DTPA and DO3A macrocycles

Abstract The synthesis and complexation to Eu(III) of two macrocyclic ligands functionalized donor-phenylethynyl-moieties as sensitizer is presented. The two ligands based on DTPA bis-amide ([Eu(L1)]) and DO3A ([Na][Eu(L2)]) have been chosen because of their increased stability in water for potential applications as luminescent bioprobes for one- or two-photon excited scanning microscopy. The DTPA and DO3A ligands possess oxygen or sulfur donor atoms, respectively, connected to a triethyleneglycol (PEG) chain to ensure the water solubility. The optical properties were studied and reveal that ([Eu(L1)]) present a lower brightness than the ([Na][Eu(L2)]) counterpart because of its inner sphere water molecule coordination that quenches the emission. On the other hand, [Na][Eu(L2)] exhibits excellent spectroscopic properties with high quantum yield ϕ = 0.284, and brightness B = 10,220 M−1cm−1(defined as ɛ.ϕ). Unfortunately, the two-photon cross-section of this last complex was measured to be only 4 GM limiting its potential applications to linear microscopy.

Syntheses and crystal structures of nine-coordinate K2[EuIII(dtpa)(H2O)]·5H2O and Na2[TbIII(Httha)]·6H2O complexes

The title complexes, K2[EuIII(dtpa)(H2O)]·5H2O (H5dtpa = diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid), Na2[TbIII(Httha)]·6H2O (H6ttha = triethylenetetramine-N,N,N′,N′,N″,N″-hexaacetic acid), were prepared, and their compositions and structures were determined by elemental analyses and single-crystal X-ray diffraction techniques. The crystal of K2[EuIII(dtpa)(H2O)]·5H2O belongs to triclinic crystal system and \( P\bar 1 \) space group. The crystal data are as follows: a = 8.3540(17), b = 10.147(2), c = 15.059(3) α = 84.63(3)A, β = 82.02(3)°, γ = 83.96(3)°, V = 1253.1(4)A3, Z = 2, R = 0.0325 and wR = 0.1013 for 4407 observed reflections with I ≥ 2σ(I). The [EuIII(dtpa)(H2O)]2− has a nine-coordinate pseudo-monocapped square antiprismatic structure, in which the nine coordinate atoms, three N and six O are from one dtpa ligand and one water molecule. The crystal of the Na2[TbIII(Httha)]·6H2O belongs to monoclinic system and P21/c space group. The crystal data are as follows: a = 10.3976(10), b = 12.7908(13), c = 23.199(2) A = 90.914(2)°, V = 3084.9(5)A3, Z = 4, R = 0.0309 and wR = 0.0704 for 5429 observed reflections with I ≥ 2σ(I). In the [TbIII(Httha)]2−, the Tb3+ ion is nine-coordinated yielding a pseudo-monocapped square antiprismatic conformation, in which the ttha ligand coordinates to the central Tb3+ ion with four N atoms and five O atoms. There is a free non-coordinate carboxyl group (−CH2COOH) that can be modified by some biological molecules having target function.

Syntheses and structures of seven-coordinate K3[Cd(Dtpa)], K2[Cd(H2O)4][Cd(Edta)(H2O)]2 · 2H2O, and Na2[Cd(H2O)4][Cd(Edta)(H2O)]2 · 2H2O complexes

The title complexes, K3[Cd(Dtpa)] (H5Dtpa = diethylenetriamine-N,N,N,N′,N′-pentaacetic acid, (I)), K2[Cd(H2O)4][Cd(Edta)(H2O)]2 · 2H2O (H4Edta = ethylenediamine-N,N,N′,N′-tetraacetic acid, (II)), and Na2[Cd(H2O)4][Cd(Edta)(H2O)]2 · 2H2O (III), were prepared, and their compositions and structures were determined by elemental analyses, IR spectra, and single-crystal X-ray diffraction techniques, respectively. In complex I, the Cd is seven-coordinated by one Dtpa ligand yielding a pseudo-monocapped trigonal prism conformation, and the complex crystallizes in the triclinic crystal system with the Pi space group. The crystal data are as follows: a = 8.7300(17), b = 9.1200(18), c = 15.110(3) A, α = 95.52(3)°, β = 96.59(3)°, γ = 99.63(3)°, V = 1170.0(4) A3, Z = 2, ρ = 1.754 g/cm3, μ = 1.519 mm−1, F(000) = 616, R = 0.0644 and wR = 0.1712 for 3842 observed reflections with I ≥ 2σ(I). For complex II, in the [Cd(Edta)(H2O)]2− complex anion the Cd2+ ion is seven-coordinated by one Edta ligand and one water molecule, yielding a pseudo-pentagonal bipyramid conformation. In the [Cd(H2O)4]2+ cation, the bridged Cd is six-coordinated, yielding an almost standard octahedral conformation. The complex crystallizes in the monoclinic system with P21/n space group. The crystal data are as follows: a = 9.098(3), b = 16.442(6), c = 12.023(4) A, β = 91.053(6)°, V = 1798.3(12) A3, Z = 2, ρ = 2.098 g/cm3, μ = 2.086 mm−1, F(000) =1124, R = 0.0406 and wR = 0.1152 for 3680 observed reflections with I ≥ 2σ(I). In complex III, the conformations of Cd2+ ions are similar to those of the potassium salt complex, and the complex also crystallizes in the monoclinic crystal system with the P21/n space group. The crystal data are as follows: a = 9.134(7), b = 16.500(13), c = 12.075(10) A, β = 91.054(12)°, V = 1820(2) A3, Z = 2, ρ = 2.015 g/cm3, μ = 1.856 mm−1, F(000) = 1092, R = 0.0363 and wR = 0.0879 for 3707 observed reflections with I ≥ 2σ(I).

Structural, Kinetic, and Thermodynamic Characterization of the Interconverting Isomers of MS-325, a Gadolinium(III)-Based Magnetic Resonance Angiography Contrast Agent

The amphiphilic gadolinium complex MS-325 ((trisodium-{(2-(R)-[(4,4-diphenylcyclohexyl) phosphonooxymethyl] diethylenetriaminepentaacetato) (aquo)gadolinium(III)}) is a contrast agent for magnetic resonance angiography (MRA). MS-325 comprises a GdDTPA core with an appended phosphodiester moiety linked to a diphenylcyclohexyl group to facilitate noncovalent binding to serum albumin and extension of the plasma half-life in vivo. The chiral DTPA ligand (R) was derived from L-serine, and upon complexation with gadolinium, forms two interconvertible diastereomers, denoted herein as isomers A and B. X-ray crystallography of the tris(ethylenediamine)cobalt(III) salt derivative of isomer A revealed a structure in the polar acentric space group P32. The structure consisted of three independent molecules of the gadolinium complex in the asymmetric unit along with three Delta-[Co(en)3]3+ cations, and it represents an unusual example of spontaneous Pasteur resolution of the cobalt cation. The geometry of the coordination core was best described as a distorted trigonal prism, and the final R factor was 5.6%. The configuration of the chiral central nitrogen of the DTPA core was S. The Gd-water (2.47-2.48 A), the Gd-acetate oxygens (2.34-2.42 A), and the Gd-N bond distances (central N, 2.59-2.63 A; terminal N, 2.74-2.80 A) were similar to other reported GdDTPA structures. The structurally characterized single crystal was one of two interconvertable diastereomers (isomers A and B) that equilibrated to a ratio of 1.81 to 1 at pH 7.4 and were separable at elevated pH by ion-exchange chromatography. The rate of isomerization was highly pH dependent: k1 = (1.45 +/- 0.08) x 102[H+] + (4.16 +/- 0.30) x 105[H+]2; k-1 = (2.57 +/- 0.17) x 102[H+] + (7.54 +/- 0.60) x 105[H+]2.

Syntheses of rare earth metal complexes with aminopolycarboxylic acids and study on structural changes: Nine-coordinated mononuclear K2[DyIII(dtpa)(H2O)]·6H2O and binuclear K4[HoIII 2(dtpa)2]·4H2O

The crystal and molecular structures of mononuclear K2[DyIII(dtpa)(H2O)]·6H2O (H5dtpa = diethylenetriaminepentaacetic acid) and binuclear K4[HoIII 2(dtpa)2]·4H2O complexes have been determined by single-crystal X-ray structure analyses. The crystal of K2[DyIII(dtpa) (H2O)]·6H2O complex belongs to the triclinic crystal system, space group. The [DyIII (dtpa)(H2O)]2− complex anion has a nine-coordinate pseudo-monocapped square antiprismatic structure, in which the nine coordinated atoms (three N and six O) are from one dtpa ligand and one water molecule. The crystal of K4[HoIII 2(dtpa)2]·4H2O complex belongs to triclinic crystal system, space group. The [HoIII 2(dtpa)2]4− complex anion is composed of two identical parts and every HoIIIO6N3 has a nine-coordinate, distorted monocapped square antiprismatic structure, in which the dtpa ligand coordinates to one central HoIII ion with three N atoms and five O atoms and to the other HoIII ion with one O atom in the same coordinated carboxylic group. Although the DyIII and HoIII ions have close ionic radii (1.052 and 1.041 Å) and electronic configuration (f 9 and f10) they form mononuclear and binuclear nine-coordinate complexes, respectively. Perhaps there is a critical ionic radius for mononuclear and binuclear structures, namely the dtpa complexes of rare earth metals with larger ionic radii than 1.052 Å of DyIII ion adopt a mononuclear nine-coordinate structure, while those with smaller ionic radii than 1.041 Å of HoIII ion adopt a binuclear nine-coordinate structure.