Lanthanide(III) Complexes of Pyridine-N-Oxide Analogues of DOTA in Solution and in the Solid State. A New Kind of Isomerism in Complexes of DOTA-like Ligands

Structural, spectroscopic properties ant prospective application of a new nitropyridine amino N-oxide derivative: 2-[(4-nitropyridine-3-yl)amino]ethan-1-ol N-oxide

A new nitropyridine amino N-oxide derivative 2-[(4-nitropyridine-3-yl)amino]ethan-1-ol N-oxide was synthesized and its structural and optical properties were described. Its structure obtained from the X-ray diffraction (XRD) studies was related to the Infrared spectroscopy (IR), Raman spectroscopy (RS), electron Ultraviolet-Visible spectroscopy (UV-VIS) and emission spectra measurements. The experimental results were analyzed in terms of theoretical data obtained from quantum chemical Density Functional Theory (DFT) and Natural Bond Orbital (NBO) calculations. The 6-311G(2d,2p) basis set with the B3LYP functional was used to discuss its optimized structure and vibrational properties. Femtosecond excitation was used to recognize the depopulation mechanism of the electron excited states in the studied compound. It was found that the intermolecular interactions strongly influence the physicochemical properties and application of the new nitropyridine amino N-oxide derivative.

Rigidified and Hydrophilic DOTA-like Lanthanoid Ligands: Design, Synthesis, and Dynamic Properties

Limiting the dynamics of paramagnetic tags is crucial for the accuracy of the structural information derived from paramagnetic nuclear magnetic resonance (NMR) experiments. A hydrophilic rigid 2,2',2″,2‴-(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid (DOTA)-like lanthanoid complex was designed and synthesized following a strategy that allows the incorporation of two sets of two adjacent substituents. This resulted in a C₂ symmetric hydrophilic and rigid macrocyclic ring, featuring four chiral hydroxyl-methylene substituents. NMR spectroscopy was used to investigate the conformational dynamics of the novel macrocycle upon complexation with europium and compared to DOTA and its derivatives. The twisted square antiprismatic and square antiprismatic conformers coexist, but the former is favored, which is different from DOTA. Two-dimensional ¹H exchange spectroscopy shows that ring flipping of the cyclen-ring is suppressed due to the presence of the four chiral equatorial hydroxyl-methylene substituents at proximate positions. The reorientation of the pendant arms causes conformational exchange between two conformers. The reorientation of the coordination arms is slower when the ring flipping is suppressed. This indicates that these complexes are suitable scaffolds to develop rigid probes for paramagnetic NMR of proteins. Due to their hydrophilic nature, it is anticipated that they are less likely to cause protein precipitation than their more hydrophobic counterparts.

Lanthanide complexes with polyaminopolycarboxylates as prospective NMR/MRI diagnostic probes: peculiarities of molecular structure, dynamics and paramagnetic properties

The paramagnetic lanthanide complexes with polyaminopolycarboxylate (PAPC) ligands attract considerable attention from the standpoint of potential applications thereof as relaxation agents used in medical magnetic resonance imaging (MRI) and in luminescent materials, as well as owing to promising use thereof as paramagnetic labels for studying the properties of biopolymers since they exhibit thermodynamic stability, good solubility in aqueous media and moderate toxicity. For the last decades, the NMR methods have been used to determine the physical and chemical properties of paramagnetic Ln compounds. The studies concerning paramagnetic NMR lanthanide-induced shifts (LISs) in dissolved Ln complexes, as well as the analysis of band shape as a function of temperature make it possible to obtain valuable information on the structure, intra- and intermolecular dynamics and paramagnetic properties thereof. This review is devoted solely to the following features: firstly, the processes of intramolecular dynamics of lanthanide complexes with polyamino-polycarboxylate ligands such as DOTA, EDTA and DTPA and their derivatives studied by NMR; secondly, the LISs of lanthanide complexes with EDTA, DOTA, DTPA and some of their derivatives depending on temperature and pH. Moreover, in this review, for the first time, the dependence of the activation energy of molecular dynamics in complexes with polydentate ligands on the atomic number of the lanthanide cation is analyzed and a monotonic change in energy is detected, which is due to the effect of lanthanide contraction. It should be noted that this phenomenon is quite general and may also appear in the future in many other series of lanthanide complexes with both other multidentate ligands and with bidentate and monodentate ligands. In the future, it is possible to predict the dependence of the properties of certain lanthanide complexes on the ionic radius of the lanthanide cation based on the approaches presented in the review. In this review, we have also presented the dynamic NMR as the main research method widely used to analyze the processes of molecular dynamics, and the structural studies based on the NMR relaxation spectroscopy and LIS analysis.

The solid-state structures and ligand cavity evaluation of lanthanide(iii) complexes of a DOTA analogue with a (dibenzylamino)methylphosphinate pendant arm

A series of lanthanide(iii) complexes of a monophosphinate analogue of H4dota, 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic-10-methyl[(N,N-dibenzylamino)methyl]phosphinic acid (H4do3apDBAm = H4L1), were prepared and their solid-state structures were studied using single-crystal X-ray diffraction. In all structures, the ligand anion was octadentately coordinated to the Ln(iii) or Sc(iii) ions similarly to other DOTA-like ligands, i.e. forming parallel N4- and O4-planes. The lighter lanthanide(iii) complexes (till dysprosium) were nonacoordinated in the twisted square-antiprismatic (TSA) configuration with the apical coordination of water molecules or oxygen atoms from the neighbouring complex unit. The heavier lanthanide(iii) complexes (from terbium) were found as the "anhydrous" octacoordinated twisted square-antiprismatic (TSA') isomer. For the terbium(iii) ion, both forms were structurally characterized. The structural data of the Ln(iii)-H4L1 complexes and complexes of several related DOTA-like ligands were analysed. It clearly showed that the structural parameters for the square-antiprismatic (SA) isomers were clustered in a small range while those for the TSA/TSA' isomers were significantly more spread. The analysis also gave useful information about the influence of various pendant arms on the structure of the complexes of the DOTA-like ligands. The twist angle (torsion) of the chelate ring containing a larger phosphorus atom was similar to those of the remaining three acetate pendants. It led to a larger separation of the N4O4 planes and to smaller trans-O-Ln-O angles than the parameters found in the complexes of H4dota and its tetraamide derivatives dotam(R). It resulted in a relatively long bond between the metal ion and the coordinated water molecule. It led, together with the negative charge of the oxygen atoms forming the O4-plane, to an extremely fast water exchange rate reported for the Gd(iii)-H4L1 complex and, generally, to a fast water exchange of Gd(iii) complexes with the monophosphorus acid analogues of H4dota, H5do3ap/H4do3apR.

Coordination Properties of GdDO3A-Based Model Compounds of Bioresponsive MRI Contrast Agents

We report a detailed characterization of the thermodynamic stability and dissociation kinetics of Gd³⁺ complexes with DO3A derivatives containing a (methylethylcarbamoylmethylamino)acetic acid (L¹), (methylpropylcarbamoylmethylamino)acetic acid (L²), 2-dimethylamino- N-ethylacetamide (L³), or 2-dimethylamino- N-propylacetamide (L⁴) group attached to the fourth nitrogen atom of the macrocyclic unit. These ligands are model systems of Ca²⁺- and Zn²⁺-responsive contrast agents (CA) for application in magnetic resonance imaging (MRI). The results of the potentiometric studies ( I = 0.15 M NaCl) provide stability constants with log K_GdL values in the range 13.9-14.8. The complex speciation in solution was found to be quite complicated due to the formation of protonated species at low pH, hydroxido complexes at high pH, and stable dinuclear complexes in the case of L^1,2. At neutral pH significant fractions of the complexes are protonated at the amine group of the amide side chain (log K_GdL×H = 7.2-8.1). These ligands form rather weak complexes with Mg²⁺ and Ca²⁺ but very stable complexes with Cu²⁺ (log K_CuL = 20.4-22.3) and Zn²⁺ (log K_ZnL = 15.5-17.6). Structural studies using a combination of ¹H NMR and luminescence spectroscopy show that the amide group of the ligand is coordinated to the metal ion at pH ∼8.5, while protonation of the amine group provokes the decoordination of the amide O atom and a concomitant increase in the hydration number and proton relaxivity. The dissociation of the complexes occurs mainly through a rather efficient proton-assisted pathway, which results in kinetic inertness comparable to that of nonmacrocyclic ligands such as DTPA rather than DOTA-like complexes.

Synthesis and characterisation of bismacrocyclic DO3A-amide derivatives – an approach towards metal-responsive PARACEST agents

Three bismacrocyclic amide derivatives bearing two types of chelating moieties were prepared. The properties of their lanthanide complexes in the absence and presence of calcium, magnesium or zinc ions were studied.

Enantiomeric two-armed lanthanide-binding tags for complementary effects in paramagnetic NMR spectroscopy

A new pair of two-armed lanthanide-binding tags provide distinct sets of structural restraints when attached to the same site of a protein.

Gd(3+)-Based Magnetic Resonance Imaging Contrast Agent Responsive to Zn(2+)

We report the heteroditopic ligand H5L, which contains a DO3A unit for Gd(3+) complexation connected to an NO2A moiety through a N-propylacetamide linker. The synthesis of the ligand followed a convergent route that involved the preparation of 1,4-bis(tert-butoxycarbonylmethyl)-1,4,7-triazacyclononane following the orthoamide strategy. The luminescence lifetimes of the Tb((5)D4) excited state measured for the TbL complex point to the absence of coordinated water molecules. Density functional theory calculations and (1)H NMR studies indicate that the EuL complex presents a square antiprismatic coordination in aqueous solution, where eight coordination is provided by the seven donor atoms of the DO3A unit and the amide oxygen atom of the N-propylacetamide linker. Addition of Zn(2+) to aqueous solutions of the TbL complex provokes a decrease of the emission intensity as the emission lifetime becomes shorter, which is a consequence of the coordination of a water molecule to the Tb(3+) ion upon Zn(2+) binding to the NO2A moiety. The relaxivity of the GdL complex recorded at 7 T (25 °C) increases by almost 150% in the presence of 1 equiv of Zn(2+), while Ca(2+) and Mg(2+) induced very small relaxivity changes. In vitro magnetic resonance imaging experiments confirmed the ability of GdL to provide response to the presence of Zn(2+).

A bis(pyridine N-oxide) analogue of DOTA: relaxometric properties of the Gd(III) complex and efficient sensitization of visible and NIR-emitting lanthanide(III) cations including Pr(III) and Ho(III)

We report the synthesis of a cyclen-based ligand (4,10-bis[(1-oxidopyridin-2-yl)methyl]-1,4,7,10-tetraazacyclododecane-1,7-diacetic acid=L1) containing two acetate and two 2-methylpyridine N-oxide arms anchored on the nitrogen atoms of the cyclen platform, which has been designed for stable complexation of lanthanide(III) ions in aqueous solution. Relaxometric studies suggest that the thermodynamic stability and kinetic inertness of the Gd(III) complex may be sufficient for biological applications. A detailed structural study of the complexes by (1) H NMR spectroscopy and DFT calculations indicates that they adopt an anti-Δ(λλλλ) conformation in aqueous solution, that is, an anti-square antiprismatic (anti-SAP) isomeric form, as demonstrated by analysis of the (1) H NMR paramagnetic shifts induced by Yb(III) . The water-exchange rate of the Gd(III) complex is ${k{{298\hfill \atop {\rm ex}\hfill}}}$=6.7×10(6)  s(-1) , about a quarter of that for the mono-oxidopyridine analogue, but still about 50 % higher than the ${k{{298\hfill \atop {\rm ex}\hfill}}}$ of GdDOTA (DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). The 2-methylpyridine N-oxide chromophores can be used to sensitize a wide range of Ln(III) ions emitting in both the visible (Eu(III) and Tb(III) ) and NIR (Pr(III) , Nd(III) , Ho(III) , Yb(III) ) spectral regions. The emission quantum yield determined for the Yb(III) complex (${Q{{{\rm L}\hfill \atop {\rm Yb}\hfill}}}$=7.3(1)×10(-3) ) is among the highest ever reported for complexes of this metal ion in aqueous solution. The sensitization ability of the ligand, together with the spectroscopic and relaxometric properties of its complexes, constitute a useful step forward on the way to efficient dual probes for optical imaging (OI) and MRI.

Dual-frequency calcium-responsive MRI agents

Responsive or smart magnetic resonance imaging (MRI) contrast agents are molecular sensors that alter the MRI signal upon changes in a particular parameter in their microenvironment. Consequently, they could be exploited for visualization of various biochemical events that take place at molecular and cellular levels. In this study, a set of dual-frequency calcium-responsive MRI agents are reported. These are paramagnetic, fluorine-containing complexes that produce remarkably high MRI signal changes at the (1)H and (19)F frequencies at varying Ca(2+) concentrations. The nature of the processes triggered by Ca(2+) was revealed, allowing a better understanding of these complex systems and their further improvement. The findings indicate that these double-frequency tracers hold great promise for development of novel functional MRI methods.

A pH-sensitive, colorful, lanthanide-chelating paramagnetic NMR probe

Paramagnetic lanthanides ions are broadly used in NMR spectroscopy. The effects of unpaired electrons on NMR spectral parameters provide a powerful tool for the characterization of macromolecular structures and dynamics. Here, a new lanthanide-chelating NMR probe, Caged Lanthanide NMR Probe-7 (CLaNP-7), is presented. It can be attached to protein surfaces via two disulfide bridges, yielding a probe that is rigid relative to the protein backbone. CLaNP-7 extends the application range of available probes. It has a yellow color, which is helpful for sample preparation. Its effects are comparable to those of CLaNP-5, but its charge is two units lower (+1) than that of CLaNP-5 (+3), reducing the change in surface potential after probe attachment. It also has a different magnetic susceptibility tensor, so by using both tags, two sets of structural restraints can be obtained per engineered cysteine pair. Moreover, it was found that the orientation of the magnetic susceptibility tensor is pH dependent (pK(a) ≈ 7) when a histidine residue is located in the neighborhood of the probe attachment site. The results show that the His imidazole group interacts with the CLaNP-7 tag. It is proposed that the histidine residue forms a hydrogen bond to a water/hydroxyl molecule that occupies the ninth coordination position on the lanthanide, thus breaking the two-fold symmetry of the CLaNP tag in a pH-dependent way.

The effect of ring size variation on the structure and stability of lanthanide(III) complexes with crown ethers containing picolinate pendants

The coordination properties of the macrocyclic receptor N,N'-bis[(6-carboxy-2-pyridyl)methylene]-1,10-diaza-15-crown-5 (H(2)bp15c5) towards the lanthanide ions are reported. Thermodynamic stability constants were determined by pH-potentiometric titration at 25 °C in 0.1 M KCl. A smooth decrease in complex stability is observed upon decreasing the ionic radius of the Ln(III) ion from La [log K(LaL) = 12.52(2)] to Lu [log K(LuL) = 10.03(6)]. Luminescence lifetime measurements recorded on solutions of the Eu(III) and Tb(III) complexes confirm the absence of inner-sphere water molecules in these complexes. (1)H and (13)C NMR spectra of the complexes formed with the diamagnetic La(III) metal ion were obtained in D(2)O solution and assigned with the aid of HSQC and HMBC 2D heteronuclear experiments, as well as standard 2D homonuclear COSY and NOESY spectra. The (1)H NMR spectra of the paramagnetic Ce(III), Eu(III) and Yb(III) complex suggest nonadentate binding of the ligand to the metal ion. The syn conformation of the ligand in [Ln(bp15c5)](+) complexes implies the occurrence of two helicities, one associated with the layout of the picolinate pendant arms (absolute configuration Δ or Λ), and the other to the five five-membered chelate rings formed by the binding of the crown moiety (absolute configuration δ or λ). A detailed conformational analysis performed with the aid of DFT calculations (B3LYP model) indicates that the complexes adopt a Λ(λδ)(δδλ) [or Δ(δλ)(λλδ)] conformation in aqueous solution. Our calculations show that the interaction between the Ln(III) ion and several donor atoms of the crown moiety is weakened as the ionic radius of the metal ion decreases, in line with the decrease of complex stability observed on proceeding to the right across the lanthanide series.

Molecular recognition of sialic acid by lanthanide(III) complexes through cooperative two-site binding

Herein we report two new ligands, 1,4,7-tris(carboxymethyl)-10-[2-(dihydroxyboranyl)benzyl]-1,4,7,10-tetraazacyclododecane (L(1)) and 1,4,7-tris(carboxymethyl)-10-[3-(dihydroxyboranyl)benzyl]-1,4,7,10-tetraazacyclododecane (L(2)), which contain a phenylboronic acid (PBA) function and a 1,4,7,10-tetraazacyclododecane-1,4,7-triacetate cage for complexation of lanthanide ions in an aqueous solution. The pK(a) of the PBA function amounts to 4.6 in [Gd(L(1))] and 8.9 in [Gd(L(2))], with the value of the L(2) analogue being very similar to that of PBA (8.8). These results are explained by the coordination of the PBA function of L(1) to the Gd(III) ion, which results in a dramatic lowering of its pK(a). As a consequence, [Gd(L(1))] does not bind to saccharides at physiological pH. The nuclear magnetic relaxation dispersion profiles recorded for [Gd(L(1))] and [Gd(L(2))] confirm that the phenylboronate function is coordinated to the metal ion in the L(1) derivative, which results in a q = 0 complex. The interaction of the [Gd(L(2))] complex with 5-acetylneuraminic acid (Neu5Ac) and 2-alpha-O-methyl-5-acetylneuraminic acid (MeNeu5Ac) has been investigated by means of spectrophotometric titrations in an aqueous solution (pH 7.4, 0.1 M 3-(N-morpholino)propanesulfonic acid buffer). Furthermore, we have also investigated the binding of these receptors with competing monosaccharides such as D-(+)-glucose, D-fructose, D-mannose, D-galactose, methyl alpha-D-galactoside, and methyl alpha-D-mannoside. The binding constants obtained indicate an important selectivity of [Gd(L(2))] for Neu5Ac (K(eq) = 151) over D-(+)-glucose (K(eq) = 12.3), D-mannose (K(eq) = 21.9), and D-galactose (K(eq) = 24.5). Furthermore, a very weak binding affinity was observed in the case of methyl alpha-D-galactoside and methyl alpha-D-mannoside. An 8-fold increase of the binding constant of [Gd(L(2))] with Neu5Ac is observed when compared to that of PBA determined under the same conditions (K(eq) = 19). (13)C NMR spectroscopy and density functional theory calculations performed at the B3LYP/6-31G(d) level show that this is due to a cooperative two-site binding of Neu5Ac through (1) ester formation by interaction on the PBA function of the receptor and (2) coordination of the carboxylate group of Neu5Ac to the Gd(III) ion. The emission lifetime of the (5)D(4) level of Tb(III) in [Tb(L(2))] increases upon Neu5Ac binding, in line with the displacement of inner-sphere water molecules due to coordination of Neu5Ac to the metal ion.