Exploring the Limits of Ligand Rigidification in Transition Metal Complexes with Mono-N-Functionalized Pyclen Derivatives.

We report the synthesis of a new family of side-bridged pyclen ligands. The incorporation of an ethylene bridge between two adjacent nitrogen atoms was reached from the pyclen-oxalate precursor described previously. Three new side-bridged pyclen macrocycles, Hsb-3-pc1a, sb-3-pc1py, and Hsb-3-pc1pa, were obtained with the aim to assess their coordination properties toward Cu2+ and Zn2+ ions. We also prepared their nonreinforced analogues H3-pc1a, 3-pc1py, and H3-pc1pa as comparative benchmarks. The two series of ligands were characterized and their coordination properties were investigated in detail. The Zn2+ and Cu2+ complexes with the nonside-bridged series H3-pc1a, 3-pc1py, and H3-pc1pa were successfully isolated and their structures were assessed by X-ray diffraction studies. In the case of the side-bridged family, the synthesis of the complexes was far more difficult and, in some cases, unsuccessful. The results of our studies demonstrate that this difficulty is related to the extreme stiffening and basicity of such side-bridged pyclens.

Fluorinated Mn(III)/(II)-Porphyrin with Redox-Responsive 1 H and 19 F Relaxation Properties.

A new fluorinated manganese porphyrin, (Mn-TPP-p-CF3 ) is reported capable of providing, based on the Mn(III)/Mn(II) equilibrium, dual 1 H relaxivity and 19 F NMR response to redox changes. The physical-chemical characterization of both redox states in DMSO-d6 /H2 O evidenced that the 1 H relaxometric and 19 F NMR properties are appropriate for differential redox MRI detection. The Mn(III)-F distance (dMn-F =9.7-10 Å), as assessed by DFT calculations, is well tailored to allow for adequate paramagnetic effect of Mn(III) on 19 F T1 and T2 relaxation times. Mn-TPP-p-CF3 has a reversible Mn(II)/Mn(III) redox potential of 0.574 V vs. NHE in deoxygenated aqueous HEPES/ THF solution. The reduction of Mn(III)-TPP-p-CF3 in the presence of ascorbic acid is slowly, but fully reversed in the presence of air oxygen, as monitored by UV-Vis spectrometry and 19 F NMR. The broad 1 H and 19 F NMR signals of Mn(III)-TPP-p-CF3 disappear in the presence of 1 equivalent ascorbate replaced by a shifted and broadened 19 F NMR signal from Mn(II)-TPP-p-CF3 . Phantom 19 F MR images in DMSO show a MRI signal intensity decrease upon reduction of Mn(III)-TPP-p-CF3 , retrieved upon complete reoxidation in air within ~24 h. 1 H NMRD curves of the Mn(III)/(II)-TPP-p-CF3 chelates in mixed DMSO/water solvent have the typical shape of Mn(II)/Mn(III) porphyrins.

Rigidified Derivative of the Non-macrocyclic Ligand H4OCTAPA for Stable Lanthanide(III) Complexation.

The stability constants of lanthanide complexes with the potentially octadentate ligand CHXOCTAPA4-, which contains a rigid 1,2-diaminocyclohexane scaffold functionalized with two acetate and two picolinate pendant arms, reveal the formation of stable complexes [log KLaL = 17.82(1) and log KYbL = 19.65(1)]. Luminescence studies on the Eu3+ and Tb3+ analogues evidenced rather high emission quantum yields of 3.4 and 11%, respectively. The emission lifetimes recorded in H2O and D2O solutions indicate the presence of a water molecule coordinated to the metal ion. 1H nuclear magnetic relaxation dispersion profiles and 17O NMR chemical shift and relaxation measurements point to a rather low water exchange rate of the coordinated water molecule (kex298 = 1.58 × 106 s-1) and relatively high relaxivities of 5.6 and 4.5 mM-1 s-1 at 20 MHz and 25 and 37 °C, respectively. Density functional theory calculations and analysis of the paramagnetic shifts induced by Yb3+ indicate that the complexes adopt an unprecedented cis geometry with the two picolinate groups situated on the same side of the coordination sphere. Dissociation kinetics experiments were conducted by investigating the exchange reactions of LuL occurring with Cu2+. The results confirmed the beneficial effect of the rigid cyclohexyl group on the inertness of the Lu3+ complex. Complex dissociation occurs following proton- and metal-assisted pathways. The latter is relatively efficient at neutral pH, thanks to the formation of a heterodinuclear hydroxo complex.

Complexes of Bifunctional DO3A-N-(α-amino)propinate Ligands with Mg(II), Ca(II), Cu(II), Zn(II), and Lanthanide(III) Ions: Thermodynamic Stability, Formation and Dissociation Kinetics, and Solution Dynamic NMR Studies.

The thermodynamic, kinetic, and structural properties of Ln3+ complexes with the bifunctional DO3A-ACE4- ligand and its amide derivative DO3A-BACE4- (modelling the case where DO3A-ACE4- ligand binds to vector molecules) have been studied in order to confirm the usefulness of the corresponding Gd3+ complexes as relaxation labels of targeted MRI contrast agents. The stability constants of the Mg2+ and Ca2+ complexes of DO3A-ACE4- and DO3A-BACE4- complexes are lower than for DOTA4- and DO3A3-, while the Zn2+ and Cu2+ complexes have similar and higher stability than for DOTA4- and DO3A3- complexes. The stability constants of the Ln(DO3A-BACE)- complexes increase from Ce3+ to Gd3+ but remain practically constant for the late Ln3+ ions (represented by Yb3+). The stability constants of the Ln(DO3A-ACE)4- and Ln(DO3A-BACE)4- complexes are several orders of magnitude lower than those of the corresponding DOTA4- and DO3A3- complexes. The formation rate of Eu(DO3A-ACE)- is one order of magnitude slower than for Eu(DOTA)-, due to the presence of the protonated amine group, which destabilizes the protonated intermediate complex. This protonated group causes the Ln(DO3A-ACE)- complexes to dissociate several orders of magnitude faster than Ln(DOTA)- and its absence in the Ln(DO3A-BACE)- complexes results in inertness similar to Ln(DOTA)- (as judged by the rate constants of acid assisted dissociation). The 1H NMR spectra of the diamagnetic Y(DO3A-ACE)- and Y(DO3A-BACE)- reflect the slow dynamics at low temperatures of the intramolecular isomerization process between the SA pair of enantiomers, R-Λ(λλλλ) and S-Δ(δδδδ). The conformation of the Cα-substituted pendant arm is different in the two complexes, where the bulky substituent is further away from the macrocyclic ring in Y(DO3A-BACE)- than the amino group in Y(DO3A-ACE)- to minimize steric hindrance. The temperature dependence of the spectra reflects slower ring motions than pendant arms rearrangements in both complexes. Although losing some thermodynamic stability relative to Gd(DOTA)-, Gd(DO3A-BACE)- is still quite inert, indicating the usefulness of the bifunctional DO3A-ACE4- in the design of GBCAs and Ln3+-based tags for protein structural NMR analysis.

Expanding the Ligand Classes Used for Mn(II) Complexation: Oxa-aza Macrocycles Make the Difference.

We report two macrocyclic ligands based on a 1,7-diaza-12-crown-4 platform functionalized with acetate (tO2DO2A2-) or piperidineacetamide (tO2DO2AMPip) pendant arms and a detailed characterization of the corresponding Mn(II) complexes. The X-ray structure of [Mn(tO2DO2A)(H2O)]·2H2O shows that the metal ion is coordinated by six donor atoms of the macrocyclic ligand and one water molecule, to result in seven-coordination. The Cu(II) analogue presents a distorted octahedral coordination environment. The protonation constants of the ligands and the stability constants of the complexes formed with Mn(II) and other biologically relevant metal ions (Mg(II), Ca(II), Cu(II) and Zn(II)) were determined using potentiometric titrations (I = 0.15 M NaCl, T = 25 °C). The conditional stabilities of Mn(II) complexes at pH 7.4 are comparable to those reported for the cyclen-based tDO2A2- ligand. The dissociation of the Mn(II) chelates were investigated by evaluating the rate constants of metal exchange reactions with Cu(II) under acidic conditions (I = 0.15 M NaCl, T = 25 °C). Dissociation of the [Mn(tO2DO2A)(H2O)] complex occurs through both proton- and metal-assisted pathways, while the [Mn(tO2DO2AMPip)(H2O)] analogue dissociates through spontaneous and proton-assisted mechanisms. The Mn(II) complex of tO2DO2A2- is remarkably inert with respect to its dissociation, while the amide analogue is significantly more labile. The presence of a water molecule coordinated to Mn(II) imparts relatively high relaxivities to the complexes. The parameters determining this key property were investigated using 17O NMR (Nuclear Magnetic Resonance) transverse relaxation rates and 1H nuclear magnetic relaxation dispersion (NMRD) profiles.

Complexation of Mn(II) by Rigid Pyclen Diacetates: Equilibrium, Kinetic, Relaxometric, Density Functional Theory, and Superoxide Dismutase Activity Studies.

We report the Mn(II) complexes with two pyclen-based ligands (pyclen = 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene) functionalized with acetate pendant arms at either positions 3,6 (3,6-PC2A) or 3,9 (3,9-PC2A) of the macrocyclic fragment. The 3,6-PC2A ligand was synthesized in five steps from pyclen oxalate by protecting one of the secondary amine groups of pyclen using Alloc protecting chemistry. The complex with 3,9-PC2A is characterized by a higher thermodynamic stability [log KMnL = 17.09(2)] than the 3,6-PC2A analogue [log KMnL = 15.53(1); 0.15 M NaCl]. Both complexes contain a water molecule coordinated to the metal ion, which results in relatively high 1H relaxivities (r1p = 2.72 and 2.91 mM-1 s-1 for the complexes with 3,6-PC2A and 3,9-PC2A, respectively, at 25 °C and 0.49 T). The coordinated water molecule displays fast exchange kinetics with the bulk in both cases; the rates (kex298) are 140 × 106 and 126 × 106 s-1 for [Mn(3,6-PC2A)(H2O)] and [Mn(3,9-PC2A)(H2O)], respectively. The two complexes were found to be remarkably inert with respect to their dissociation, with half-lives of 63 and 21 h, respectively, at pH = 7.4 in the presence of excess Cu(II). The r1p values recorded in blood serum remain constant at least over a period of 120 h. Cyclic voltammetry experiments show irreversible oxidation features shifted to higher potentials with respect to [Mn(EDTA)(H2O)]2- (H4EDTA = ethylenediaminetetraacetic acid) and [Mn(PhDTA)(H2O)]2- (H4PhDTA = phenylenediamine-N,N,N',N'-tetraacetic acid), indicating that the PC2A complexes reported here have a lower tendency to stabilize Mn(III). The superoxide dismutase activity of the Mn(II) complexes was tested using the xanthine/xanthine oxidase/p-nitro blue tetrazolium chloride assay at pH = 7.8. The Mn(II) complexes of 3,6-PC2A and 3,9-PC2A are capable of assisting decomposition of the superoxide anion radical. The kinetic rate constant of the complex of 3,9-PC2A is smaller by 1 order of magnitude than that of 3,6-PC2A.

Concentration-Dependent Interactions of Amphiphilic PiB Derivative Metal Complexes with Amyloid Peptides Aß and Amylin*.

Invited for the cover of this issue are Jean-François Morfin and Éva Tóth at the CNRS in Orléans, and their collaborators from University of Debrecen, University of Coimbra and Université de Toulouse. The image depicts that when an amphiphilic compound is intravenously injected, monomer, pre-micellar and micellar forms can co-exist in the blood and have different affinities for amyloid peptides. Read the full text of the article at 10.1002/chem.202004000.

Concentration-Dependent Interactions of Amphiphilic PiB Derivative Metal Complexes with Amyloid Peptides Aß and Amylin*.

Metal chelates targeted to amyloid peptides are widely explored as diagnostic tools or therapeutic agents. The attachment of a metal complex to amyloid recognition units typically leads to a decrease in peptide affinity. We show here that by separating a macrocyclic GdL chelate and a PiB targeting unit with a long hydrophobic C10 linker, it is possible to attain nanomolar affinities for both Aß1-40 (Kd =4.4 nm) and amylin (Kd =4.5 nm), implicated, respectively in Alzheimer's disease and diabetes. The Scatchard analysis of surface plasmon resonance data obtained for a series of amphiphilic, PiB derivative GdL complexes indicate that their Aß1-40 or amylin binding affinity varies with their concentration, thus micellar aggregation state. The GdL chelates also affect peptide aggregation kinetics, as probed by thioflavin-T fluorescence assays. A 2D NMR study allowed identifying that the hydrophilic region of Aß1-40 is involved in the interaction between the monomer peptide and the Gd3+ complex. Finally, ex vivo biodistribution experiments were conducted in healthy mice by using 111 In labeled analogues. Their pancreatic uptake, ∼3 %ID g-1 , is promising to envisage amylin imaging in diabetic animals.

Unexpected Trends in the Stability and Dissociation Kinetics of Lanthanide(III) Complexes with Cyclen-Based Ligands across the Lanthanide Series.

We report a detailed study of the thermodynamic stability and dissociation kinetics of lanthanide complexes with two ligands containing a cyclen unit, a methyl group, a picolinate arm, and two acetate pendant arms linked to two nitrogen atoms of the macrocycle in either cis (1,4-H3DO2APA) or trans (1,7-H3DO2APA) positions. The stability constants of the Gd3+ complexes with these two ligands are very similar, with log KGdL values of 16.98 and 16.33 for the complexes of 1,4-H3DO2APA and 1,7-H3DO2APA, respectively. The stability constants of complexes with 1,4-H3DO2APA follow the usual trend, increasing from log KLaL = 15.96 to log KLuL = 19.21. However, the stability of [Ln(1,7-DO2APA)] complexes decreases from log K = 16.33 for Gd3+ to 14.24 for Lu3+. The acid-catalyzed dissociation rates of the Gd3+ complexes differ by a factor of ∼15, with rate constants (k1) of 1.42 and 23.5 M-1 s-1 for [Gd(1,4-DO2APA)] and [Gd(1,7-DO2APA)], respectively. This difference is magnified across the lanthanide series to reach a 5 orders of magnitude higher k1 for [Yb(1,7-DO2APA)] (1475 M-1 s-1) than for [Yb(1,4-DO2APA)] (5.79 × 10-3 M-1 s-1). The acid-catalyzed mechanism involves the protonation of a carboxylate group, followed by a cascade of proton-transfer events that result in the protonation of a nitrogen atom of the cyclen unit. Density functional theory calculations suggest a correlation between the strength of the Ln-Ocarboxylate bonds and the kinetic inertness of the complex, with stronger bonds providing more inert complexes. The 1H NMR resonance of the coordinated water molecule in the [Yb(1,7-DO2APA)] complex at 176 ppm provides a sizable chemical exchange saturation transfer effect thanks to a slow water exchange rate of (15.9 ± 1.6) × 103 s-1.

Mn(II)-Based MRI Contrast Agent Candidate for Vascular Imaging.

Toxicity concerns related to Gd(III)-based magnetic resonance imaging (MRI) agents prompted an intensive research toward their replacement by complexes of essential metal ions, like Mn(II). Here, we report a macrocyclic chelate, [Mn(PC2A-BP)], which possesses high thermodynamic stability (log KMnL = 14.86 and pMn=8.35) and kinetic inertness (t1/2pH=7.4 = 286.2 h) as well as as remarkable relaxivity (r1p = 23.5 mM-1 s-1, 0.49 T, 37 °C) in the presence of human serum albumin, allowing a significant MRI signal intensity increase in the vasculature even at low dose (25 µmol/kg) of the complex.

Comparison of the equilibrium, kinetic and water exchange properties of some metal ion-DOTA and DOTA-bis(amide) complexes.

The 1,7-diacetate-4,10-diacetamide substituted 1,4,7,10-tetraazacyclododecane structural unit is common to several responsive Magnetic Resonance Imaging (MRI) contrast agents (CAs). While some of these complexes (agents capable of sensing fluctuations in Zn2+, Ca2+ etc. ions) have already been tested in vivo, the detailed physico-chemical characterization of such ligands have not been fully studied. To fill this gap, we synthesized a representative member of this ligand family possessing two acetate and two n-butylacetamide pendant side-arms (DO2A2MnBu = 1,4,7,10-tetraazacyclodoecane-1,7-di(acetic acid)-4,10-di(N-butylacetamide)), and studied its complexation properties with some essential metal and a few lanthanide(III) (Ln(III)) ions. Our studies revealed that the ligand basicity, the stability of metal ion complexes, the trend of stability constants along the Ln(III) series, the formation rates of the Ln(III) complexes and the exchange rate of the bound water molecule in the Gd(III) complex fell between those of Ln(DOTA)- and Ln(DOTA-tetra(amide))3+ complexes (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, DOTAM = 1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane). The only exception is the stability of Cu(DO2A2MnBu) which was found to be only slightly lower than that of Cu(DOTA)2- (log KCuL = 19.85 vs. 21.98). This is likely reflects exclusive coordination of the negatively charged acetate donor atoms to the Cu2+ ion forming an octahedral complex with the amides remaining uncoordinated. The only anomaly observed during the study was the rates of acid assisted dissociation of the Ln(III) complexes, which occur at a rate similar to those observed for the Ln(DOTA)- complexes. These data indicate that even though the Ln(DO2A2MnBu)+ complexes have lower thermodynamic stabilities, their kinetic inertness should be sufficient for in vivo use.

Effect of the Nature of Donor Atoms on the Thermodynamic, Kinetic and Relaxation Properties of Mn(II) Complexes Formed With Some Trisubstituted 12-Membered Macrocyclic Ligands.

During the past few years increasing attention has been devoted to Mn(II) complexes as possible substitutes for Gd(III) complexes as contrast agents in MRI. Equilibrium (log KMnL or pMn value), kinetic parameters (rates and half-lives of dissociation) and relaxivity of the Mn(II) complexes formed with 12-membered macrocyclic ligands were studied. The ligands were selected in a way to gain information on how the ligand rigidity, the nature of the donor atoms in the macrocycle (pyridine N, amine N, and etheric O atom), the nature of the pendant arms (carboxylates, phosphonates, primary, secondary and tertiary amides) affect the physicochemical parameters of the Mn(II) complexes. As expected, decreasing the denticity of DOTA (to afford DO3A) resulted in a drop in the stability and inertness of [Mn(DO3A)]- compared to [Mn(DOTA)]2-. This decrease can be compensated partially by incorporating the fourth nitrogen atom into a pyridine ring (e.g., PCTA) or by replacement with an etheric oxygen atom (ODO3A). Moreover, the substitution of primary amides for acetates resulted in a noticeable drop in the stability constant (PC3AMH), but it increased as the primary amides (PC3AMH) were replaced by secondary (PC3AMGly) or tertiary amide (PC3AMPip) pendants. The inertness of the Mn(II) complexes behaved alike as the rates of acid catalyzed dissociation increased going from DOTA (k1 = 0.040 M-1s-1) to DO3A (k1 = 0.45 M-1s-1). However, the rates of acid catalyzed dissociation decreased from 0.112 M-1s-1 observed for the anionic Mn(II) complex of PCTA to 0.0107 M-1s-1 and 0.00458 M-1s-1 for the cationic Mn(II) complexes of PC3AMH and PC3AMPip ligands, respectively. In spite of its lower denticity (as compared to DOTA) the sterically more hindered amide complex ([Mn(PC3AMPip)]2+) displays surprisingly high conditional stability (pMn = 8.86 vs. pMn = 9.74 for [Mn(PCTA)]-) and excellent kinetic inertness. The substitution of phosphonates for the acetate pendant arms (DOTP and DO3P), however, resulted in a noticeable drop in the conditional stability as well as dissociation kinetic parameters of the corresponding Mn(II) complexes ([Mn(DOTP)]6- and [Mn(DO3P)]4-) underlining that the phosphonate pedant should not be considered as a suitable building block for further ligand design while the tertiary amide moiety will likely have some implications in this respect in the future.

Physico-chemical properties of MnII complexes formed with cis- and trans-DO2A: thermodynamic, electrochemical and kinetic studies.

SYNOPSIS: MnII complexes formed with cis- and trans-DO2A (DO2A=1,4,7,10-tetraazacyclododecane-1,4 (or 1,7) -diacetic acid) chelators were investigated by pH-potentiometry, 1H relaxometry, UV-vis spectrophotometry and cyclic voltammetry. The physico-chemical characteristics of MnII complexes of these structure isomers do not differ dramatically, however the cis-DO2A platform has better potential for further development. Manganese (MnII) is a promising alternative to gadolinium (GdIII) as a magnetic resonance imaging (MRI) agent. Unlike gadolinium, this biogenic metal might be better tolerated by the body, reducing the risk of toxicity associated with dissociation of the complex. Herein we report detailed equilibrium and kinetic studies performed with MnII complexes of 1,4,7,10-tetraazacyclododecane-1,4-diacetic acid (1,4-DO2A or cis-DO2A) and 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (1,7-DO2A or trans-DO2A). The protonation constants of the ligands as well as stability constants of their MnII complexes have been determined by pH-potentiometry. The stability constants of [Mn(cis-DO2A)] are slightly higher than those of [Mn(trans-DO2A)] (log KMnL=15.68 and 15.22, respectively). Cyclic voltammetric (CV) experiments performed on [Mn(cis-DO2A)] and [Mn(trans-DO2A)] revealed quasireversible systems with a half-wave potential of +636 and +705mV versus Ag/AgCl, respectively. These values indicate that the MnII ion in these complexes is more stabilized against the oxidation than in [Mn(EDTA)]2-. The kinetic inertness of the complexes has been studied in transmetallation reactions with CuII or ZnII ions. Kinetic measurements indicate that both MnII complexes primarily undergo acid catalyzed dissociation and positions of the acetate pendant arms do not influence kinetic inertness. The inertness of these complexes is comparable to that of [Mn(NOTA)]- (NOTA=1,4,7-triazacyclononane-1,4,7-triacetic acid) and about twenty times lower than that of [Mn(DOTA)]2- (DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). In conclusion, [Mn(cis-DO2A)] displays some very interesting features (thermodynamic and redox stability as well as kinetic inertness) which makes this complex a promising platform for the development of more efficient MnII complexes as alternatives to Gd-based MRI agents.

Approaching the Kinetic Inertness of Macrocyclic Gadolinium(III)-Based MRI Contrast Agents with Highly Rigid Open-Chain Derivatives.

A highly rigid open-chain octadentate ligand (H4 cddadpa) containing a diaminocylohexane unit to replace the ethylenediamine bridge of 6,6'-[(ethane-1,2 diylbis{(carboxymethyl)azanediyl})bis(methylene)]dipicolinic acid (H4 octapa) was synthesized. This structural modification improves the thermodynamic stability of the Gd(3+) complex slightly (log KGdL =20.68 vs. 20.23 for [Gd(octapa)](-) ) while other MRI-relevant parameters remain unaffected (one coordinated water molecule; relaxivity r1 =5.73 mm(-1) s(-1) at 20 MHz and 295 K). Kinetic inertness is improved by the rigidifying effect of the diaminocylohexane unit in the ligand skeleton (half-life of dissociation for physiological conditions is 6 orders of magnitude higher for [Gd(cddadpa)](-) (t1/2 =1.49×10(5)  h) than for [Gd(octapa)](-) . The kinetic inertness of this novel chelate is superior by 2-3 orders of magnitude compared to non-macrocyclic MRI contrast agents approved for clinical use.

Stable Mn(2+), Cu(2+) and Ln(3+) complexes with cyclen-based ligands functionalized with picolinate pendant arms.

In this study we present the results of the equilibrium, dissociation kinetics, DFT and X-ray crystallographic studies performed on the complexes of metal ions of biomedical importance (Mn(2+), Cu(2+) and Gd(3+)) formed with octadentate ligands based on a cyclen platform incorporating two picolinate pendant arms (dodpa(2-) and Medodpa(2-)). The stability constants of the complexes were accessed by multiple methods (pH-potentiometry, direct and competition UV-vis spectrophotometry and (1)H-relaxometry). The stability constants of the complexes formed with dodpa(2-) and Medodpa(2-) do not differ significantly (e.g. log K[Mn(dodpa)] = 17.40 vs. log K[Mn(Medodpa)] = 17.46, log K[Cu(dodpa)] = 24.34-25.17 vs. log K[Cu(Medodpa)] = 24.74 and log K[Gd(dodpa)](+) = 17.27 vs. log K[Gd(Medodpa)](+) = 17.59), which indicates that the steric hindrance brought by the methyl groups has no significant effect on the stability of the complexes. The stability constants of the Mn(2+) complexes formed with the cyclen dipicolinates were found to be ca. 3 log K units higher than those determined for the complex of the cyclen monopicolinate (dompa(-)), which indicates that the second picolinate moiety attached to the backbone of the macrocycle is very likely coordinated to the Mn(2+) ion. However, the stability of the [Cu(dodpa)] and [Cu(Medodpa)] complexes agrees well with the stability constant of [Cu(dompa)](+), in line with the hexadentate coordination around the metal ion observed in the X-ray structure of [Cu(Medodpa)]. The [Gd(dodpa)](+) and [Gd(Medodpa)](+) complexes display a fairly high kinetic inertness, as the rate constants of acid catalysed dissociation (k1 = 2.5(4) × 10(-3) and 8.3(4) × 10(-4) M(-1) s(-1) for [Gd(dodpa)](+) and [Gd(Medodpa)](+), respectively) are smaller than the value reported for [Gd(do3a)] (k1 = 2.5 × 10(-2) M(-1) s(-1)). The [Mn(dodpa)] complex was found to be more inert than [Mn(Medodpa)]. The results of the diffusion-ordered NMR spectroscopy (DOSY) and DFT calculations of diamagnetic [La(dodpa)](+) and [Lu(dodpa)](+) complexes indicate the formation of a trinuclear entity of the La complex in aqueous solution.

Lanthanide(III) complexes with a reinforced cyclam ligand show unprecedented kinetic inertness.

Lanthanide(III) complexes of a cross-bridged cyclam derivative containing two picolinate pendant arms are kinetically inert in very harsh conditions such as 2 M HCl, with no dissociation being observed for at least 5 months. Importantly, the [Ln(dota)](-) complexes, which are recognized to be extremely inert, dissociate under these conditions with lifetimes in the range ca. 1 min to 12 h depending upon the Ln(3+) ion. X-ray diffraction studies reveal octadentate binding of the ligand to the metal ion in the [Eu(cb-tedpa)](+) complex, while (1)H and (13)C NMR experiments in D2O point to the presence of a single diastereoisomer in solution with a very rigid structure. The structure of the complexes in the solid state is retained in solution, as demonstrated by the analysis of the Yb(3+)-induced paramagnetic shifts.

Lanthanide(III) complexes of some natural siderophores: a thermodynamic, kinetic and relaxometric study.

Stability constants of the complexes formed between the natural trihydroxamic acids desferrioxamine B (DFB) and desferricoprogen (DFC) with Nd(III), Gd(III) and Yb(III) ions were determined using pH-potentiometry. The equilibrium in these systems can be described by models containing mononuclear protonated (Ln(HL), Ln(H2L) and Ln(H3L)), deprotonated (LnL) and ternary hydroxo Ln(H-1L) complexes, but for both ligands dinuclear complexes of low stability were also detected. The stability constants for the Ln(HDFB)(+) complexes are 11.95 (Nd(III)), 13.16 (Gd(III)) and 14.67 (Yb(III)), while these values of the Ln(DFC) complexes are considerably higher (14.42 (Nd(III)), 15.14 (Gd(III)) and 16.49 (Yb(III))). The stability constants of the complexes of DFB and DFC are much lower than those of the Ln(L)3 complexes formed with some aromatic hydroxamic acids indicating that the relatively long spacer between the hydroxamic acid moieties in DFB and DFC is unfavorable for Ln(III) complexation. The relaxometric study conducted for the Gd(HDFB)(+) species revealed an interesting pH dependence of the relaxivity associated with a large hydration number (bishydrated complex) and fast water exchange (kex=(29.9±0.4)×10(6)s(-1)), which would be favorable for CA use. However the dissociation of Gd(HDFB)(+) is fairly fast (<2ms) under all conditions employed in the present work thus the kinetically labile Gd(HDFB)(+) is not suitable for in vivo CA applications. Some low stability ternary complexes were also detected with K(Gd(HDFB)(HCO3))=17.5±1.9 and K(Gd(HDFB)(Lactate))=8.4±3.2 but in the presence of citrate and phosphate ions the Gd(HDFB)(+) complex was found to dissociate.

Properties, solution state behavior, and crystal structures of chelates of DOTMA.

The chemistry of polyamino carboxylates and their use as ligands for Ln(3+) ions is of considerable interest from the point of view of the development of new imaging agents. Of particular interest is the chemistry of the macrocyclic ligand 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and its derivatives. Herein we report that the tetramethylated DOTA derivative, DOTMA, possess several properties that, from an imaging agent development point of view, are more advantageous than those of the parent DOTA. In particular, the Ln(3+) chelates of DOTMA exhibit a marked preference for the monocapped twisted square antiprismatic coordination isomer which imparts more rapid water exchange kinetics on the chelates; τ(M)(298) was determined to be 85 ns for GdDOTMA. Differential analysis of the (17)O R(2ρ) temperature profiles of both GdDOTA and GdDOTMA afforded the τ(M)(298) values for the square (SAP) and twisted square antiprismatic (TSAP) isomers of each chelate that were almost identical: 365 ns (SAP) and 52 ns (TSAP). The origin of this accelerated water exchange in the TSAP isomer appears to be the slightly longer Gd-OH(2) bond distance (2.50 Å) that is observed in the crystal structure of GdDOTMA which crystallizes in the P(2) space group as a TSAP isomer. The Ln(3+) chelates of DOTMA also exhibit high thermodynamic stabilities ranging from log K(ML) = 20.5 for CeDOTMA, 23.5 for EuDOTMA and YbDOTMA comparable to, but a shade lower than, those of DOTA.