On the role of the counter-ion in defining water structure and dynamics: order, structure and dynamics in hydrophilic and hydrophobic gadolinium salt complexes

LnDOTA puppeteering: removing the water molecule and imposing tetragonal symmetry

Complexes of lanthanide(iii) ions (Ln) with tetraazacyclododecane-N,N',N'',N'''-tetraacetate (DOTA) are a benchmark in the field of magnetism due to their well-investigated and sometimes surprising features. Ab initio calculations suggest that the ninth ligand, an axial water molecule, is key in defining the magnetic properties because it breaks the potential C4 symmetry of the resulting complexes. In this paper, we experimentally isolate the role of the water molecule by excluding it from the metal coordination sphere without altering the chemical structure of the ligand. Our complexes are therefore designed to be geometrically tetragonal and strict crystallographic symmetry is achieved by exploiting a combination of solution ionic strength and solid state packing effects. A thorough multitechnique approach has been used to unravel the electronic structure and magnetic anisotropy of the complexes. Moreover, the geometry enhancement allows us to predict, using only one angle obtained from the crystal structure, the ground state composition of all the studied derivatives (Ln = Tb to Yb). Therefore, these systems also provide an excellent platform to test the validity and limitations of the ab initio methods. Our combined experimental and theoretical investigation proves that the water molecule is indeed key in defining the magnetic anisotropy and the slow relaxation of these complexes.

Toward quantification of hypoxia using fluorinated EuII/III-containing ratiometric probes

Hypoxia is a prognostic biomarker of rapidly growing cancers, where the extent of hypoxia is an indication of tumor progression and prognosis; therefore, hypoxia is also used for staging while performing chemo- and radiotherapeutics for cancer. Contrast-enhanced MRI using EuII-based contrast agents is a noninvasive method that can be used to map hypoxic tumors, but quantification of hypoxia using these agents is challenging due to the dependence of signal on the concentration of both oxygen and EuII. Here, we report a ratiometric method to eliminate concentration dependence of contrast enhancement of hypoxia using fluorinated EuII/III-containing probes. We studied three different EuII/III couples of complexes containing 4, 12, or 24 fluorine atoms to balance fluorine signal-to-noise ratio with aqueous solubility. The ratio between the longitudinal relaxation time (T1) and 19F signal of solutions containing different ratios of EuII- and EuIII-containing complexes was plotted against the percentage of EuII-containing complexes in solution. We denote the slope of the resulting curves as hypoxia indices because they can be used to quantify signal enhancement from Eu, that is related to oxygen concentration, without knowledge of the absolute concentration of Eu. This mapping of hypoxia was demonstrated in vivo in an orthotopic syngeneic tumor model. Our studies significantly contribute toward improving the ability to radiographically map and quantify hypoxia in real time, which is critical to the study of cancer and a wide range of diseases.

Structural and dynamic properties of some aqueous salt solutions

Aqueous salt solutions are utilized and encountered in wide-ranging technological applications and natural settings. Towards improved understanding of the effect of salts on the dynamic properties of such systems, dilute aqueous salt solutions (up to 1 molar concentration) are investigated here, via experiments and molecular simulations. Four salts are considered: sodium chloride, for which published results are readily available for comparison, ammonium acetate, barium acetate and barium nitrate, for which published data are scarce. In the present work, molecular dynamics (MD) simulations are conducted to quantify viscosity and water self-diffusion coefficients, together with rheometry and Pulsed Field Gradient Spin Echo (PFGSE)-NMR experiments for validation. Simulation predictions are consistent with experimental observations in terms of trend and magnitude of salt-specific effects. Combining insights from the approaches considered, an interpretation of the results is proposed whereby the capacity of salts to influence bulk dynamics arises from their molecular interfacial area and strength of interaction with first hydration-shell water molecules. For the concentration range investigated, the interpretation could be useful in formulating aqueous systems for applications including the manufacturing of advanced catalysts.

Magnetic resonance thermometry using a GdIII-based contrast agent

The complexes described here serve as contrast agents for magnetic resonance imaging thermometry. The complexes differentially enhance contrast between 275 and 325 K. The basis of the temperature response of the fluorinated contrast complex is the modulation of water exchange caused by trifluoromethyl groups that can be chemically controlled.

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.

Electronic Effects of the Substituents on Relaxometric and CEST Behaviour of Ln(III)-DOTA-Tetraanilides

Three different 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetamide (DOTAM) derivatives bearing as amide N-substituents phenyl, p-methoxyphenyl and p-ethylbenzoate groups were synthesized and the 1H and 17O NMR relaxometric behaviour of the Gd(III)-chelates and chemical exchange saturation transfer (CEST) effect of the Eu(III) complexes were evaluated. The electronic properties of the substituents were shown to strongly influence the coordinated water exchange rate (kex), resulting in five times faster kex for the electron donating phenylmethoxy group compared to the electron withdrawing ethylbenzoate group.

Water exchange in lanthanide complexes for MRI applications. Lessons learned over the last 25 years

Coordination chemistry offers convenient strategies to modulate the exchange of coordinated water molecules in lanthanide-based contrast agents.

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.

Selective functionalization of mesoporous silica nanoparticles with ibuprofen and Gd(III) chelates: a new probe for potential theranostic applications

Organo-modified mesoporous silica nanoparticles, loaded with ibuprofen into the pores and functionalized on the external surface with a stable Gd(iii)-DOTA-monoamide chelate, were prepared and explored as potential theranostic probes.

Water-exchange rates of lanthanide ions in an ionic liquid

The first ligand-exchange rate measurements of lanthanide ions in an ionic liquid are reported here. The trend of water-exchange rates in the ionic liquid is the opposite of the trend in water.

The effect of regioisomerism on the coordination chemistry and CEST properties of lanthanide(III) NB-DOTA-tetraamide chelates

Chemical exchange saturation transfer (CEST) offers many advantages as a method of generating contrast in magnetic resonance images. However, many of the exogenous agents currently under investigation suffer from detection limits that are still somewhat short of what can be achieved with more traditional Gd(3+) agents. To remedy this limitation we have undertaken an investigation of Ln(3+) DOTA-tetraamide chelates (where DOTA is 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) that have unusually rigid ligand structures: the nitrobenzyl derivatives of DOTA-tetraamides with (2-phenylethyl)amide substituents. In this report we examine the effect of incorporating hydrophobic amide substituents on water exchange and CEST. The ligand systems chosen afforded a total of three CEST-active isomeric square antiprismatic chelates; each of these chelates was found to have different water exchange and CEST characteristics. The position of a nitrobenzyl substituent on the macrocyclic ring strongly influenced the way in which the chelate and Ln(3+) coordination cage distorted. These differential distortions were found to affect the rate of water proton exchange in the chelates. But, by far the greatest effect arose from altering the position of the hydrophobic amide substituent, which, when forced upwards around the water binding site, caused a substantial reduction in the rate of water proton exchange. Such slow water proton exchange afforded a chelate that was 4.5 times more effective as a CEST agent than its isomeric counterparts in dry acetonitrile and at low temperatures and very low presaturation powers.

Modulating water-exchange rates of lanthanide(III)-containing polyaminopolycarboxylate-type complexes using polyethylene glycol

We have synthesized a series of Ln(III)-containing polyethylene glycol conjugates and studied the structural and electronic properties of these complexes. These studies demonstrate that polyethylene glycol can be used to fine-tune water-exchange rates of Ln(III)-containing polyaminopolycarboxylate-type complexes; this control is desirable in developing Ln(III)-containing contrast agents for magnetic resonance imaging.

Naphthyridine-based lanthanide complexes worked as magnetic resonance imaging contrast for guanosine 5'-monophosphate in vivo

New lanthanide complex Gd-ANAMD containing 2-amino-7-methyl-1,8-naphthyridine was achieved for selective magnetic resonance imaging towards guanosine 5'-monophosphate over other ribonucleotide polyphosphates in aqueous media and in vivo. The formation of strong multi-hydrogen bonds between naphthyridine and guanosine made the phosphate in guanosine 5'-monophosphate positioned on a suitable site to coordinate with the lanthanide ion. The substitution of the coordination naphthyridine by the phosphate oxygen atoms caused obvious relaxivity decrease. The negligible cytotoxicity and appropriate blood circulation time of Gd-ANAMD allow potential application of Magnetic Resonance Imaging in vivo. (1)H NMR confirmed that the selectivity of these lanthanide complexes towards guanosine was attributed to the formation of hydrogen bonds between the guanine moeity and the naphthyridine. The fluorescence detection and lifetime measurement of Tb-ANAMD and Eu-ANAMD suggested that the decrease of the relaxivity is not attributed to the change of the q value, but caused by the prolonging of the residence lifetime of inner-sphere water.

Towards the rational design of MRI contrast agents: δ-substitution of lanthanide(III) NB-DOTA-tetraamide chelates influences but does not control coordination geometry

LnDOTA-tetraamide chelates (DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) have received considerable recent attention as a result of their potential to act as PARACEST contrast agents for magnetic resonance imaging (MRI). Although PARACEST agents afford several advantages over conventional contrast agents they suffer from substantially higher detection limits; thus, improving the effectiveness of LnDOTA-tetraamide chelates is an important goal. In this study we investigate the potential to extend conformational control of LnDOTA-type ligands to those applicable to PARACEST. Furthermore, the question of whether δ- rather than α-substitution of the pendant arms could be used to control the chelate coordination geometry is addressed. Although δ-substitution does influence coordination geometry it does not afford control. However, it can play an important role in governing the conformation of the amide substituent relative to the chelate in such as way that suggests a PARACEST agent could be designed that has detection limits at least as low as a conventional MRI contrast agent.

Investigations into whole water, prototropic and amide proton exchange in lanthanide(III) DOTA-tetraamide chelates

Lanthanide(III) chelates of DOTA-tetraamide ligands have been an area of particular interest since the discovery that water exchange kinetics are dramatically affected by the switch from acetate to amide side-chain donors. More recently these chelates have attracted interest as potential PARACEST agents for use in MRI. In this paper we report the results of studies using chemical exchange saturation transfer (CEST) and some more recently reported chelates to re-examine the exchange processes in this class of chelate. We find that the conclusions of Parker and Aime are, for the most part, solid; water exchange is slow and a substantial amount of prototropic exchange occurs in aqueous solution. The extent of prototropic exchange increases as the pH increases above 8, leading to higher relaxivities at high pH. However, amide protons are found to contribute only a small amount to the relaxivity at high pH.

Probing the structure, conformation, and stereochemical exchange in a family of lanthanide complexes derived from tetrapyridyl-appended cyclen

A series of lanthanide complexes have been synthesized from 1,4,7,10-tetrakis(2-pyridylmethyl)-1,4,7,10-tetraazacyclododecane. Crystallographic studies indicate that, in the solid phase, all of the lanthanide ions are 9-coordinate and are bound to eight N atoms from the donor ligand, with the ninth site being filled by a counterion or solvent molecule. In solution, time-resolved luminescence studies indicate that the luminescence exhibits contributions from two species corresponding to the nonhydrated and hydrated forms. The NMR spectra in protic media show the presence of two dominant isomers on the NMR time scale; furthermore, the spectra are very different from those obtained for 1,4,7,10-tetraazacyclododecane-N',N'',N''',N''''-tetraacetic acid (DOTA) and its derivatives. The different forms of the complex undergo slow conformational and enantiomeric exchange in solution, which has been measured by NMR. The exchange path has been mapped out by density functional theory calculations and shows multiple metastable conformations (with respect to the dihedral angles of the cyclen ring). This contrasts with the established NMR behavior of DOTA complexes, which has been described by a two-state solution equilibrium.

Activation of a PARACEST agent for MRI through selective outersphere interactions with phosphate diesters

Ln(S-THP)(3+) complexes are paramagnetic chemical exchange saturation transfer (PARACEST) agents for magnetic resonance imaging (MRI; S-THP = (1S,4S,7S,10S)-1,4,7,10-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane, Ln(III) = Ce(III), Eu(III), Yb(III)). CEST spectra at 11.7 T show that the PARACEST effect of these complexes is enhanced at neutral pH in buffered solutions containing 100 mM NaCl upon the addition of 1-2 equiv of diethylphosphate (DEP). CEST images of phantoms at 4.7 T confirm that DEP enhances the properties of Yb(S-THP)(3+) as a PARACEST MRI agent in buffered solutions at neutral pH and 100 mM NaCl. Studies using (1)H NMR, direct excitation Eu(III) luminescence spectroscopy, and UV-visible spectroscopy show that DEP is an outersphere ligand. Dissociation constants for [Ln(S-THP)(OH(2))](DEP) are 1.9 mM and 2.8 mM for Ln(III) = Yb(III) at pH 7.0 and Eu(III) at pH 7.4. Related ligands including phosphorothioic acid, O,O-diethylester, ethyl methylphosphonate, O-(4-nitrophenylphosphoryl)choline, and cyclic 3,5-adenosine monophosphate do not activate PARACEST. BNPP (bis(4-nitrophenyl phosphate) activates PARACEST of Ln(S-THP)(3+) (Ln(III) = Eu(III), Yb(III)), albeit less effectively than does DEP. These data show that binding through second coordination sphere interactions is selective for phosphate diesters with two terminal oxygens and two identical ester groups. A crystal structure of [Eu(S-THP)(OH(2))]((O(2)NPhO)(2)PO(2))(2)(CF(3)SO(3)) x 2 H(2)O x iPrOH has two outersphere BNPP anions that form hydrogen bonds to the alcohol groups of the macrocycle and the bound water ligand. This structure supports (1)H NMR spectroscopy studies showing that outersphere interactions of the phosphate diester with the alcohol protons modulate the rate of alcohol proton exchange to influence the PARACEST properties of the complex. Further, DEP interacts only with the nonionized form of the complex, Ln(S-THP)(OH(2))(3+) contributing to the pH dependence of the PARACEST effect.

Application of the Ugi four-component reaction to the synthesis of ditopic bifunctional chelating agents

The Ugi four-component reaction (Ugi 4CR) was exploited for the first time to obtain in a single synthetic step bifunctional ditopic chelators by using DOTA monoamide (DOTAMA) derivatives as amino and acid components. A number of ditopic systems in which the two DOTAMA units are connected by a central alpha-acylaminoamide group were synthesized by reacting different aldehydes, isocyanides and two DOTAMA chelates containing amino and acid functionalities. Variation of the components allows the insertion of another functional group into the alpha-acylaminoamide skeleton for further conjugation to biomolecules. The optimal reaction conditions were found by using methanol as solvent and ultrasound irradiation at a power of 60 W (20 kHz) for 3 h. The Gd(III) complexes of the dimeric ligands L1 and L2 (bearing a cyclohexyl ring and an octadecyl chain on the central alpha-acylaminoamide moiety, respectively) were fully characterized in aqueous media by relaxometric techniques with varying temperature and magnetic field strength. The relaxivity of Gd(2)L1 and Gd(2)L2 (in the aggregated form), at 20 MHz and 310 K, are 5.6 and 20.0 mM(-1) s(-1), respectively. The enhanced value found for Gd(2)L2 indicates that this lipophilic complex forms micelles at concentrations <0.1 mM. Finally, the binding of Gd(2)L2 to human serum albumin (HSA) was investigated by proton relaxometry, and the affinity constant of the complex and the relaxivity of the macromolecular adduct (r(1p)(b) = 38.1 mM(-1) s(-1); 20 MHz and 310 K) derived.

Gadolinium(III) complexes as MRI contrast agents: ligand design and properties of the complexes

Magnetic resonance imaging is a commonly used diagnostic method in medicinal practice as well as in biological and preclinical research. Contrast agents (CAs), which are often applied are mostly based on Gd(III) complexes. In this paper, the ligand types and structures of their complexes on one side and a set of the physico-chemical parameters governing properties of the CAs on the other side are discussed. The solid-state structures of lanthanide(III) complexes of open-chain and macrocyclic ligands and their structural features are compared. Examples of tuning of ligand structures to alter the relaxometric properties of gadolinium(III) complexes as a number of coordinated water molecules, their residence time (exchange rate) or reorientation time of the complexes are given. Influence of the structural changes of the ligands on thermodynamic stability and kinetic inertness/lability of their lanthanide(III) complexes is discussed.

Novel 15-crown-5 ether or beta-diketone incorporated gadolinium complexes for the detection of potassium ions or magnesium and calcium ions

Novel gadolinium complexes (KMR-series: KMR-K and KMR-Mg), which have a bis-15-crown-5 ether or a charged beta-diketone structure as a recognition site, have been designed, synthesized and applied for the detection of K(+) or of Mg(2+) and Ca(2+) using MRI or NMR techniques. The measurements are based on the modulation of the longitudinal relaxation time (T(1)) of water protons in proximity of the gadolinium complexes. Relaxivity measurements of KMR-K1 in aqueous solution showed that the initial longitudinal relaxivity value (r(1)) of 5.05 mM(-1) s(-1) is monotonously decreasing with increasing K(+) concentrations, reaching a final value of 4.78 mM(-1) s(-1). This decrease is attributed to a change in the second sphere of hydration of the gadolinium (Gd(3+)) complex (KMR-K), resulting in a K(+) concentration-dependent contrast in MR images. From stoichiometric analysis using mass spectrometry and UV/VIS spectrometry, a 1 : 1 complex formation between KMR-K1 and K(+) in a sandwich-type manner with a log K of 3.20 was confirmed. In the case of KMR-Mg, the initial r(1) value of 4.98 mM(-1) s(-1) is monotonously decreasing with increasing Mg(2+) or Ca(2+) concentrations, reaching a final value of 3.95 or 4.16 mM(-1) s(-1), respectively, resulting in Mg(2+) and Ca(2+) concentration-dependent contrast in MR images. The formation of a 1:1 complex with a log K of 2.33 for Mg(2+) and 1.91 for Ca(2+) was confirmed. KMR-K1 and KMR-Mg are the first ion-selective or ion-sensitive gadolinium complexes for K(+) or Mg(2+) and Ca(2+), respectively.

Novel functionalized pyridine-containing DTPA-like ligand. Synthesis, computational studies and characterization of the corresponding GdIII complex

A novel pyridine-containing DTPA-like ligand, carrying additional hydroxymethyl groups on the pyridine side-arms, was synthesized in 5 steps. The corresponding Gd(III) complex, potentially useful as an MRI contrast agent, was prepared and characterized in detail by relaxometric methods and its structure modeled by computational methods.

Synthesis and characterisation of dimeric eight-coordinate lanthanide(iii) complexes of a macrocyclic tribenzylphosphinate ligand

The macrocyclic ligand 1,4,7,10-tetraazacyclododecane-1,4,7-triyl(methylenebenzyl-phosphinic acid) H3L3, has been prepared and its complexes with Eu, Gd and Tb(III) studied by NMR, relaxometry, luminescence and single crystal X-ray crystallography. In solution and in the crystal, the complexes have eight-coordinate metal centres with bridging phosphinate groups linking the two twisted square antiprismatic coordination polyhedra. A single stereoisomer crystallises from solution with an RRR and SSS configuration at the P centres in each sub-unit. The relaxivity of [GdL3]2 is low (1.9 mM-1 s-1, 298 K, 20 MHz), consistent with the absence of any proximate water molecules. The terbium dimer possesses a relatively long excited state lifetime (2.47 ms, 298 K).