Physical Properties of Eu(2+)-Containing Cryptates as Contrast Agents for Ultra-High Field Magnetic Resonance Imaging

Europium(II/III) coordination chemistry toward applications

Europium is an f-block metal with two easily accessible oxidation states (+2 and +3) that have vastly different magnetic and optical properties from each other. These properties are tunable using coordination chemistry and are useful in a variety of applications, including magnetic resonance imaging, luminescence, and catalysis. This review describes important aspects of coordination chemistry of Eu from the Allen Research Group and others, how ligand design has tuned the properties of Eu ions, and how those properties are relevant to specific applications. The review begins with an introduction to the coordination chemistry of divalent and trivalent Eu followed by examples of how the coordination chemistry of Eu has made contributions to magnetic resonance imaging, luminescence, catalysis, and separations. The article concludes with a brief outlook on future opportunities in the field.

X-ray-Sensitive Doped CaF2-Based MRI Contrast Agents for Local Radiation Dose Measurement

Ionizing radiation has become widely used in medicine, with application in diagnostic techniques, such as computed tomography (CT) and radiation therapy (RT), where X-rays are used to diagnose and treat tumors. The X-rays used in CT and, in particular, in RT can have harmful side effects; hence, an accurate determination of the delivered radiation dose is of utmost importance to minimize any damage to healthy tissues. For this, medical specialists mostly rely on theoretical predictions of the delivered dose or external measurements of the dose. To extend the practical use of ionizing radiation-based medical techniques, such as magnetic resonance imaging (MRI)-guided RT, a more precise measurement of the internal radiation dose internally is required. In this work, a novel approach is presented to measure dose in liquids for potential future in vivo applications. The strategy relies on MRI contrast agents (CAs) that provide a dose-sensitive signal. The demonstrated materials are (citrate-capped) CaF2 nanoparticles (NPs) doped with Eu3+ or Fe2+/Fe3+ ions. Free electrons generated by ionizing radiation allow the reduction of Eu3+, which produces a very small contrast in MRI, to Eu2+, which induces a strong contrast. Oxidative species generated by high-energy X-rays can be measured indirectly using Fe2+ because it oxidizes to Fe3+, increasing the contrast in MRI. Notably, in the results, a strong increase in the proton relaxation rates is observed for the Eu3+-doped NPs at 40 kV. At 6 MV, a significant increase in proton relaxation rates is observed using CaF2 NPs doped with Fe2+/Fe3+ after irradiation. The presented concept shows great promise for use in the clinic to measure in vivo local ionizing radiation dose, as these CAs can be intravenously injected in a saline solution.

Longitudinal and transverse NMR relaxivities of Ln(III)-DOTA complexes: A comprehensive investigation

Longitudinal and transverse ¹H nuclear magnetic resonance relaxivities of Ln(III)-DOTA complexes (with Ln = Gd, Tb, Dy, Er; DOTA = 1,4,7,10-tetraazacyclododecane-N,N',N^″,N‴-tetraacetic acid) and Mn(II) aqueous solutions were measured in a wide range of frequencies, 10 kHz to 700 MHz. The experimental data were interpreted by means of models derived from the Solomon-Bloembergen-Morgan theory. The data analysis was performed assuming the orbital angular momentum L = 0 for Gd-DOTA and the aqua ion [Mn(H₂O)₆]²⁺ and L ≠ 0 for Dy-, Tb-, and Er-DOTA. A refined estimation of the zero-field-splitting barrier Δ and of the modulation correlation time τ_v was obtained for [Mn(H₂O)₆]²⁺ by extending the fitting of nuclear magnetic relaxation dispersion profiles to the low-field regime. The Gd-DOTA fitting parameters resulted in good agreement with the literature, and the fit of transverse relaxivity data confirmed the negligibility of the scalar interaction in the nuclear relaxation mechanism. Larger transverse relaxivities of Dy-DOTA and Tb-DOTA (∼10 mM^-1 s^-1) with respect to Er-DOTA (∼1 mM^-1 s^-1) were observed at 16 T. Such higher values are suggested to be due to a shorter residence time τ_m that is possibly linked to the fluctuations of the hyperfine interaction and the different shape of the magnetic anisotropy. The possible employment of Dy-DOTA, Tb-DOTA, and Er-DOTA as negative magnetic resonance imaging contrast agents for high-field applications was envisaged by collecting spin-echo images at 7 T. Particularly in Dy- and Tb-derivatives, the transverse relaxivity at 16 T is of the order of the Gd-one at 1.5 T.

Lanthanide complexes of DOTA-nitroxide conjugates for redox imaging: spectroelectrochemistry, CEST, relaxivity, and cytotoxicity

The lanthanide(iii) complexes (Gd, Eu, Dy, and Yb) of DOTA tris(amide) and bis(amide) derivatives (L1 and L2) featuring one redox active TEMPO arm were prepared. Ligand L2 harbours an alkyne fragment for further functionalization. The X-ray crystal structure of ligand L2 in complexation with Na+ was solved. The complexes showed in their CV one oxidation wave (0.26-0. 34 V vs. Fc+/Fc) due to an oxoammonium/nitroxide redox couple and a broad reduction corresponding to the nitroxide/hydroxylamine system. The Eu complexes demonstrated the presence of one water molecule in their coordination sphere. The nitroxide complexes were characterized by EPR spectroscopy, showing the typical 3-line pattern in the high temperature regime, which is quenched upon the addition of ascorbate (reduction into hydroxylamine). In their nitroxide form, the complexes show essentially no CEST peak. Conversely, the reduced complexes demonstrate a 12% CEST peak at 51 ppm, corresponding to the metal bound water molecule. Fast exchange precluded the CEST activity for the amide protons. All the complexes proved to be essentially non-toxic for M21 cells at concentrations up to 50 μM.

Rational Design of High-Relaxivity EuII -Based Contrast Agents for Magnetic Resonance Imaging of Low-Oxygen Environments

Metal-based contrast agents for magnetic resonance imaging present a promising avenue to image hypoxia. EuII -based contrast agents have a unique biologically relevant redox couple, EuII/III , that distinguishes this metal for use in hypoxia imaging. To that end, we investigated a strategy to enhance the contrast-enhancing capabilities of EuII -based cryptates in magnetic resonance imaging by controlling the rotational dynamics. Two dimetallic, EuII -containing cryptates were synthesized to test the efficacy of rigid versus flexible coupling strategies. A flexible strategy to dimerization led to a modest (114 %) increase in contrast enhancement per Eu ion (60 MHz, 298 K), but a rigid linking strategy led to an excellent (186 %) increase in contrast enhancement despite this compound's having the smaller molecular mass of the two dimetallic complexes. We envision the rigid linking strategy to be useful in the future design of potent EuII -based contrast agents for magnetic resonance imaging.

Spectroscopic and Electrochemical Trends in Divalent Lanthanides through Modulation of Coordination Environment

Due to the importance of both visible-light luminescence and lanthanides in modern society, the influence of the ligand environment on complexes of Yb^II were studied and compared with analogous complexes of Eu^II. Four ligands with systematically varied electronic and steric characteristics were used to probe the coordination environment and electronic and redox properties of the corresponding Yb^II-containing complexes. Strong-field nitrogenous donors gave rise to bathochromic shifts, leading to visible-light absorption by Yb^II. Trends in properties across the series of Yb^II-containing complexes were compared to trends reported for the analogous Eu^II-containing complexes, revealing the translatability of coordination environment effects across the divalent lanthanide series. These studies provide valuable information regarding the behavior of small and medium-sized divalent lanthanides outside of the solid state.

Measurement of the Dissociation of EuII-Containing Cryptates Using Murexide

The dissociation rates of five Eu^II-containing cryptates in water were measured using UV-visible spectroscopy and murexide at pH 6.5, 7, 7.5, 8, and 9. Murexide was used as a coordinating dye for Eu^II. The results for a known cryptate were within experimental error of the value obtained using other methods and enabled the measurement of other cryptates. This validation of the use of murexide to study the dissociation of Eu^II-containing cryptates enables its use with other complexes of Eu^II.

Self-assembled fluorescent Ce(Ⅲ) coordination polymer as ratiometric probe for HIV antigen detection

The viral capsid protein p24 of human immunodeficiency virus is expressed at different level during viral invasion. Detection of p24 is of great importance in acquired immunodeficiency syndrome monitoring and therapy. A ratiometric probe that is easily-synthesized was constructed based on self-assembled fluorescent Ce(Ⅲ) and fluorescein. Fluorescein was used as reference. Hydrogen peroxide quenches the fluorescence of the Ce(III) easily but does not quench the fluorescence of fluorescein. The mechanism of reaction was discussed. Benefiting from the sensitive response to hydrogen peroxide, this probe was applied for p24 detection in enzyme linked immunoassay. The fluorescence ratio was in a good linear relationship with the concentration of p24, and the detection limit was 1.1 pg mL^-1. This proposed method has shown potential in virus detection with easy operation.

Synthesis and Structure of Copper Complexes of a N6O4 Macrocyclic Ligand and Catalytic Application in Alcohol Oxidation

Reactions between N6O4 macrocyclic 1,4,19,22,25,40-hexaaza-10,13,31,34-tetraoxa-6,14,27,35(1,4)-tetrabenzenacyclopentacontane (L) and several copper salts (viz. trifuoromethane and toluene sulfonates, nitrate, perchlorate, benzoate, and acetate) led to the formation of dinuclear compounds [Cu2(OSO2CF3)2(DMF)2L](SO3CF3)2 (1), [Cu2(p-OSO2C6H4Me)2L(DMF)2](SO3C6H4Me)2 (2), [Cu2(ONO2)2L(DMF)2](NO3)2 (3), [Cu2(OClO3)2(DMF)2L](ClO4)2 (4), [Cu2(OOCPh)2L(H2O)2](O2CPh)2 (5), and [Cu2(OOCMe)4L] (6), which were characterized by IR, elemental analysis and TG-DTA (thermogravimetric-differential thermal analysis), as well as by single-crystal X-ray diffraction, EPR (electron paramagnetic resonance) spectroscopy, and electrochemical techniques (cyclic voltammetry and controlled potential electrolysis). The molecular structures of compounds 1–6 reveal a considerable conformational flexibility of the ligand L, which allowed its readjustment for the formation of the metal compounds and confirmed the presence of dinuclear endo macrocyclic species. In every case, the L ligand coordinates to each copper cation via three nitrogen atoms, with the remaining coordination positions of the metal square pyramid environment being accomplished by neutral or anionic ligands. The macrocyclic cavities appear to be adequate for the enclosure of a neutral species as proved by compound 6 with 1,4-dioxane. The compounds, in combination with the TEMPO (2,2,6,6-tetramethyl-piperidinyloxyl) radical and in alkaline aqueous solution, act as efficient catalysts in the aerobic oxidation of different alcohols to the corresponding aldehydes (yields up to 99% and TON up 232) after 20 h at 70 °C. In addition, the microwave-assisted solvent-free peroxidative oxidation (by tert-butylhydroperoxide, TBHP) of 1-phenylethanol led to acetophenone yields up to 99% and TOF of 1.1 × 103 after 0.5 h, without any additive.

Chemistry of MRI Contrast Agents: Current Challenges and New Frontiers

Tens of millions of contrast-enhanced magnetic resonance imaging (MRI) exams are performed annually around the world. The contrast agents, which improve diagnostic accuracy, are almost exclusively small, hydrophilic gadolinium(III) based chelates. In recent years concerns have arisen surrounding the long-term safety of these compounds, and this has spurred research into alternatives. There has also been a push to develop new molecularly targeted contrast agents or agents that can sense pathological changes in the local environment. This comprehensive review describes the state of the art of clinically approved contrast agents, their mechanism of action, and factors influencing their safety. From there we describe different mechanisms of generating MR image contrast such as relaxation, chemical exchange saturation transfer, and direct detection and the types of molecules that are effective for these purposes. Next we describe efforts to make safer contrast agents either by increasing relaxivity, increasing resistance to metal ion release, or by moving to gadolinium(III)-free alternatives. Finally we survey approaches to make contrast agents more specific for pathology either by direct biochemical targeting or by the design of responsive or activatable contrast agents.

Screening of ligands for redox-active europium using magnetic resonance imaging

We report a screening procedure to predict ligand coordination to Eu^II and Eu^III using magnetic resonance imaging in which bright images indicate complexation and dark images indicate no complexation. Here, paramagnetic Gd^III is used as a surrogate for Eu^III in the screening procedure to enable detection with magnetic resonance imaging. The screening procedure was tested using a set of eight ligands with known coordination to Eu^II and Eu^III, and results were found to be consistent with expected binding. Validation of the screening procedure with known coordination chemistry enables use with new ligands in the future.

Study on the Photoluminescent and Thermal Properties of Zinc Complexes with a N₆O₄ Macrocyclic Ligand

Reactions between a N₆O₄ macrocyclic ligand (L¹) and several Zn(II) salts (trifluoromethane sulfonate, p-toluenesulfonate, acetate, benzoate, o-, m- or p-hydroxybenzoate) led to the formation of seven complexes, [Zn₂L¹ (DMSO)₄](OSO₂CF₃)₄ (1), [Zn₂(p-OSO₂PhCH₃)₄L¹] (2), [Zn₂(OCOCH₃)₄L¹] (3), [Zn₂(OCOPh)₄L¹] (4), [Zn₂(o-OCOPhOH)₄L¹] (5), [Zn₂(m-OCOPhOH)₄ L¹] (6) and [Zn₂(p-OCOPhOH)₄ L¹] (7), which were characterized by elemental analysis, ¹H-NMR, ¹³C-NMR, IR, fluorescence spectroscopies and single crystal X-ray diffraction. In 1, the Zn atom is pentacoordinated with a N₃O₂ irregular trigonal bipyramidal coordination environment, like the geometries in compounds 3–7, whereas in structure 2 the metal atom is envisaged as possessing a distorted N₃O₃ octahedronal environment. All the compounds show interesting photoluminescent properties in solid states and solutions in DMF and DMSO, which are reported along with their TG-DTA thermal decomposition processes, UV-vis absorption spectroscopy and fluorescence quantum yields in DMF and DMSO.

Synthesis, Characterization, and Handling of EuII-Containing Complexes for Molecular Imaging Applications

Considerable research effort has focused on the in vivo use of responsive imaging probes that change imaging properties upon reacting with oxygen because hypoxia is relevant to diagnosing, treating, and monitoring diseases. One promising class of compounds for oxygen-responsive imaging is Eu^II-containing complexes because the Eu^II/III redox couple enables imaging with multiple modalities including magnetic resonance and photoacoustic imaging. The use of Eu^II requires care in handling to avoid unintended oxidation during synthesis and characterization. This review describes recent advances in the field of imaging agents based on discrete Eu^II-containing complexes with specific focus on the synthesis, characterization, and handling of aqueous Eu^II-containing complexes.

Fluorinated EuII-based multimodal contrast agent for temperature- and redox-responsive magnetic resonance imaging

Magnetic resonance imaging (MRI) using redox-active, EuII-containing complexes is one of the most promising techniques for noninvasively imaging hypoxia in vivo. In this technique, positive (T1-weighted) contrast enhancement persists in areas of relatively low oxidizing ability, such as hypoxic tissue. Herein, we describe a fluorinated, EuII-containing complex in which the redox-active metal is caged by intramolecular interactions. The position of the fluorine atoms enables temperature-responsive contrast enhancement in the reduced form of the contrast agent and detection of the oxidized contrast agent via MRI in vivo. Positive contrast is observed in 1H-MRI with Eu in the +2 oxidation state, and chemical exchange saturation transfer and 19F-MRI signal are observed with Eu in the +3 oxidation state. Contrast enhancement is controlled by the redox state of Eu, and modulated by the fluorous interactions that cage a bound water molecule reduce relaxivity in a temperature-dependent fashion. Together, these advancements constitute the first report of in vivo, redox-responsive imaging using 19F-MRI.

Biological effects of MRI contrast agents: gadolinium retention, potential mechanisms and a role for phosphorus

No discussion of challenges for chemistry in molecular imaging would be complete without addressing the elephant in the room-which is that the purest of chemical compounds needs to interact with a biological system in a manner that does not perturb normal biology while still providing efficacious feedback to assist in diagnosis of disease. In the past decade, magnetic resonance imaging (MRI) agents long considered inert have produced adverse effects in certain patient populations under certain treatment regimens. More recently, inert blood pool agents have been found to deposit in the brain. Release of free metal is often suspected as the culprit but that hypothesis has yet to be validated. In addition, even innocuous agents can cause painful side effects during injection in some patients. In this brief review, we summarize known biological effects for gadolinium- and iron-based MRI contrast agents, and discuss some of the potential mechanisms for the observed biological effects, including the potential role of phosphorus imbalance, related to kidney disease or cancer, in destabilizing gadolinium-based chelates and precipitating free gadolinium.This article is part of the themed issue 'Challenges for chemistry in molecular imaging'.

Structural Features of Europium(II)-Containing Cryptates That Influence Relaxivity

Eu^II -containing complexes were studied with respect to properties relevant to their use as contrast agents for magnetic resonance imaging. The influences of molecular parameters and field strength on relaxivity were studied for a series of Eu^II -containing cryptates and their adducts with β-cyclodextrins, poly-β-cyclodextrins, and human serum albumin. Solid- and solution-phase characterization of Eu^II -containing complexes is presented that demonstrates the presence of inner-sphere molecules of water. Additionally, relaxivity, water-exchange rate, rotational correlation time, and electronic relaxation times were determined using variable-temperature ¹⁷ O NMR, nuclear magnetic relaxation dispersion, and electron paramagnetic resonance spectroscopic techniques. These results are expected to be instrumental in the design of future Eu^II -based contrast agents.

Oxidation-Responsive, EuII/III-Based, Multimodal Contrast Agent for Magnetic Resonance and Photoacoustic Imaging

We report, for the first time, a multimodal, oxidation-responsive contrast agent for magnetic resonance imaging and photoacoustic imaging that uses the differences in the properties between Eu in the +2 and +3 oxidation states. The enhancement of contrast in T1-weighted magnetic resonance and photoacoustic imaging was observed in the +2 but not in the +3 oxidation state, and the complex is a known chemical exchange saturation transfer agent for magnetic resonance imaging in the +3 oxidation state.

Aqueous Lanthanide Chemistry in Asymmetric Catalysis and Magnetic Resonance Imaging

This account describes lanthanide coordination chemistry with a focus on the similarities between lanthanide complexes used in catalysis and those used as contrast agents in magnetic resonance imaging.

Spectroscopic Characterization of the 3+ and 2+ Oxidation States of Europium in a Macrocyclic Tetraglycinate Complex

The 3+ and 2+ oxidation states of europium have drastically different magnetic and spectroscopic properties. Electrochemical measurements are often used to probe Eu^III/II oxidation state changes, but a full suite of spectroscopic characterization is necessary to demonstrate conversion between these two oxidation states in solution. Here, we report the facile conversion of an europium(III) tetraglycinate complex into its Eu^II analogue. We present electrochemical, luminescence, electron paramagnetic resonance, UV-visible, and NMR spectroscopic data demonstrating complete reversibility from the reduction and oxidation of the 3+ and 2+ oxidation states, respectively. The Eu^II-containing analogue has kinetic stability within the range of clinically approved Gd^III-containing complexes using an acid-catalyzed dissociation experiment. Additionally, we demonstrate that the 3+ and 2+ oxidation states provide redox-responsive behavior through chemical-exchange saturation transfer or proton relaxation, respectively. These results will be applicable to a wide range of redox-responsive contrast agents and Eu-containing complexes.

Evaluation of Eu(II) -based positive contrast enhancement after intravenous, intraperitoneal, and subcutaneous injections

Eu(II) -based contrast agents offer physiologically relevant, metal-based redox sensing that is unachievable with Gd(III) -based contrast agents. To evaluate the in vivo contrast enhancement of Eu(II) as a function of injection type, we performed intravenous, intraperitoneal, and subcutaneous injections in mice. Our data reveal a correlation between reported oxygen content and expected rates of diffusion with the persistence of Eu(II) -based contrast enhancement. Biodistribution studies revealed europium clearance through the liver and kidneys for intravenous and intraperitoneal injections, but no contrast enhancement was observed in organs associated with clearance. These data represent a step toward understanding the behavior of Eu(II) -based complexes in vivo. Copyright © 2016 John Wiley & Sons, Ltd.

A Eu(II)-Containing Cryptate as a Redox Sensor in Magnetic Resonance Imaging of Living Tissue

The Eu(II) ion rivals Gd(III) in its ability to enhance contrast in magnetic resonance imaging. However, all reported Eu(II)-based complexes have been studied in vitro largely because the tendency of Eu(II) to oxidize to Eu(III) has been viewed as a major obstacle to in vivo imaging. Herein, we present solid- and solution-phase characterization of a Eu(II)-containing cryptate and the first in vivo use of Eu(II) to provide contrast enhancement. The results indicate that between one and two water molecules are coordinated to the Eu(II) core upon dissolution. We also demonstrate that Eu(II)-based contrast enhancement can be observed for hours in a mouse.

Enhancing magnetic resonance imaging with contrast agents for ultra-high field strengths

Contrast agents are diagnostic tools that often complement magnetic resonance imaging. At ultra-high field strengths (≥7 T), magnetic resonance imaging is capable of generating desirable high signal-to-noise ratios, but clinically available contrast agents are less effective at ultra-high field strengths relative to lower fields. This gap in effectiveness demands the development of contrast agents for ultra-high field strengths. In this minireview, we summarize contrast agents reported during the last three years that focused on ultra-high field strengths.

Aqueous Eu(II)-Containing Complex with Bright Yellow Luminescence

Eu(II)-containing materials have unique luminescence, redox, and magnetic properties that have potential applications in optoelectronics, sensors, and imaging. Here, we report the synthesis and characterization of Eu(II)-containing aza-222 cryptate that displays yellow luminescence and a quantum yield of 26% in aqueous media. The crystal structure reveals a staggered hula-hoop geometry. Both solid-state and solution-phase data are presented that indicate that the high quantum yield is a result of the absence of OH oscillators in the inner sphere of the complex. We expect that Eu(II)-containing aza-222 cryptate is a step toward Eu(II)-containing luminescent materials that can be used in a variety of applications including biological imaging.

Overcoming the concentration-dependence of responsive probes for magnetic resonance imaging

In magnetic resonance imaging, contrast agents are molecules that increase the contrast-to-noise ratio of non-invasively acquired images. The information gained from magnetic resonance imaging can be increased using responsive contrast agents that undergo chemical changes, and consequently changes to contrast enhancement, for example in response to specific biomarkers that are indicative of diseases. A major limitation with modern responsive contrast agents is concentration-dependence that requires the concentration of contrast agent to be known: an extremely challenging task in vivo. Here, we review advances in several strategies aimed at overcoming the concentration-dependent nature of responsive contrast agents.

Research into europium complexes as magnetic resonance imaging contrast agents (Review)

Europium (Eu) is a paramagnetic lanthanide element that possesses an outstanding luminescent property. Eu complexes are ideal fluorescence imaging (FI) agents. Eu²⁺ has satisfactory relaxivity and optical properties, and can realize magnetic resonance (MRI)-FI dual imaging applications when used with appropriate cryptands that render it oxidatively stable. By contrast, based on the chemical exchange saturation transfer (CEST) mechanism, Eu³⁺ complexes can provide enhanced MRI sensitivity when used with optimal cryptands, incorporated into polymeric CEST agents or blended with Gd³⁺. Eu complexes are promising in MRI-FI dual imaging applications and have a bright future.

Oxidation-responsive Eu(2+/3+)-liposomal contrast agent for dual-mode magnetic resonance imaging

An oxidation-responsive contrast agent for magnetic resonance imaging was synthesized using Eu(2+) and liposomes. Positive contrast enhancement was observed with Eu(2+), and chemical exchange saturation transfer was observed before and after oxidation of Eu(2+). Orthogonal detection modes render the concentration of Eu inconsequential to molecular information provided through imaging.

Redox- and hypoxia-responsive MRI contrast agents

The development of responsive or "smart" magnetic resonance imaging (MRI) contrast agents that can report specific biomarker or biological events has been the focus of MRI contrast agent research over the past 20 years. Among various biological hallmarks of interest, tissue redox and hypoxia are particularly important owing to their roles in disease states and metabolic consequences. Herein we review the development of redox-/hypoxia-sensitive T1 shortening and paramagnetic chemical exchange saturation transfer (PARACEST) MRI contrast agents. Traditionally, the relaxivity of redox-sensitive Gd(3+) -based complexes is modulated through changes in the ligand structure or molecular rotation, while PARACEST sensors exploit the sensitivity of the metal-bound water exchange rate to electronic effects of the ligand-pendant arms and alterations in the coordination geometry. Newer designs involve complexes of redox-active metal ions in which the oxidation states have different magnetic properties. The challenges of translating redox- and hypoxia-sensitive agents in vivo are also addressed.

Interaction of Biphenyl-Functionalized Eu(2+)-Containing Cryptate with Albumin: Implications to Contrast Agents in Magnetic Resonance Imaging

The influence of albumin on the efficacy of a Eu(2+)-containing complex capable of interacting with human serum albumin (HSA) was investigated at different field strengths (1.4, 3, 7, 9.4, and 11.7 T). Relaxometric measurements indicated that the presence of albumin at higher field strengths (>3 T) did not result in an increase in the relaxivity of the Eu(2+) complex, but a relaxation enhancement of 171 ± 11% was observed at 1.4 T. Titration experiments using different percentages (2, 4.5, 6, 10, 15, and 25% w/v) of HSA and variable-temperature (17)O NMR measurements were performed to understand the effect of albumin on the molecular properties of the biphenyl-functionalized Eu(2+) complex that are relevant to magnetic resonance imaging.

Developments in the Coordination Chemistry of Europium(II)

Recent advances in the coordination chemistry of Eu(2+) are reviewed. Common synthetic routes for generating discrete Eu(2+)-containing complexes reported since 2000 are summarized, followed by a description of the reactivity of these complexes and their applications in reduction chemistry, polymerization, luminescence, and as contrast agents for magnetic resonance imaging. Rapid development of the coordination chemistry of Eu(2+) has led to an upsurge in the utilization of Eu(2+)-containing complexes in synthetic chemistry, materials science, and medicine.