Experimental Measurement and Theoretical Assessment of Fast Lanthanide Electronic Relaxation in Solution with Four Series of Isostructural Complexes

Electron paramagnetic resonance of lanthanides

Many applications of lanthanides exploit their electron spin relaxation properties. Double electron-electron measurements of distances are possible because of the relatively long relaxation times of Gd³⁺. Relaxation enhancement measurements of distance are possible because of the much shorter relaxation times of other lanthanides. Magnetic resonance imaging contrast agents use the long relaxation time of the S-state Gd³⁺ ion, and NMR shift reagents use the fast relaxation of selected other lanthanides. Other than Gd³⁺ and the isoelectronic Eu²⁺ ion, spin relaxation of the lanthanides is so fast that their EPR spectra can be observed only in the liquid helium temperature range. In this chapter the EPR properties of each of the lanthanides is briefly summarized, with an emphasis on electron spin relaxation.

How the Ligand Field in Lanthanide Coordination Complexes Determines Magnetic Susceptibility Anisotropy, Paramagnetic NMR Shift, and Relaxation Behavior

Complexes of lanthanide(III) ions are being actively studied because of their unique ground and excited state properties and the associated optical and magnetic behavior. In particular, they are used as emissive probes in optical spectroscopy and microscopy and as contrast agents in magnetic resonance imaging (MRI). However, the design of new complexes with specific optical and magnetic properties requires a thorough understanding of the correlation between molecular structure and electric and magnetic susceptibilities, as well as their anisotropies. The traditional Judd-Ofelt-Mason theory has failed to offer useful guidelines for systematic design of emissive lanthanide optical probes. Similarly, Bleaney's theory of magnetic anisotropy and its modifications fail to provide accurate detail that permits new paramagnetic shift reagents to be designed rather than discovered.A key determinant of optical and magnetic behavior in f-element compounds is the ligand field, often considered as an electrostatic field at the lanthanide created by the ligands. The resulting energy level splitting is a sensitive function of several factors: the nature and polarizability of the whole ligand and its donor atoms; the geometric details of the coordination polyhedron; the presence and extent of solvent interactions; specific hydrogen bonding effects on donor atoms and the degree of supramolecular order in the system. The relative importance of these factors can vary widely for different lanthanide ions and ligands. For nuclear magnetic properties, it is both the ligand field splitting and the magnetic susceptibility tensor, notably its anisotropy, that determine paramagnetic shifts and nuclear relaxation enhancement.We review the factors that control the ligand field in lanthanide complexes and link these to aspects of their utility in magnetic resonance and optical emission spectroscopy and imaging. We examine recent progress in this area particularly in the theory of paramagnetic chemical shift and relaxation enhancement, where some long-neglected effects of zero-field splitting, magnetic susceptibility anisotropy, and spatial distribution of lanthanide tags have been accommodated in an elegant way.

Fluorous-Soluble Metal Chelate for Sensitive Fluorine-19 Magnetic Resonance Imaging Nanoemulsion Probes

Fluorine-19 MRI is an emerging cellular imaging approach, enabling lucid, quantitative "hot-spot" imaging with no background signal. The utility of ¹⁹F-MRI to detect inflammation and cell therapy products in vivo could be expanded by improving the intrinsic sensitivity of the probe by molecular design. We describe a metal chelate based on a salicylidene-tris(aminomethyl)ethane core, with solubility in perfluorocarbon (PFC) oils, and a potent accelerator of the ¹⁹F longitudinal relaxation time ( T₁). Shortening T₁ can increase the ¹⁹F image sensitivity per time and decrease the minimum number of detectable cells. We used the condensation between the tripodal ligand tris-1,1,1-(aminomethyl)ethane and salicylaldehyde to form the salicylidene-tris(aminomethyl)ethane chelating agent (SALTAME). We purified four isomers of SALTAME, elucidated structures using X-ray scattering and NMR, and identified a single isomer with high PFC solubility. Mn⁴⁺, Fe³⁺, Co³⁺, and Ga³⁺ cations formed stable and separable chelates with SALTAME, but only Fe³⁺ yielded superior T₁ shortening with modest line broadening at 3 and 9.4 T. We mixed Fe³⁺ chelate with perfluorooctyl bromide (PFOB) to formulate a stable paramagnetic nanoemulsion imaging probe and assessed its biocompatibility in macrophages in vitro using proliferation, cytotoxicity, and phenotypic cell assays. Signal-to-noise modeling of paramagnetic PFOB shows that sensitivity enhancement of nearly 4-fold is feasible at clinical magnetic field strengths using a ¹⁹F spin-density-weighted gradient-echo pulse sequence. We demonstrate the utility of this paramagnetic nanoemulsion as an in vivo MRI probe for detecting inflammation macrophages in mice. Overall, these paramagnetic PFC compounds represent a platform for the development of sensitive ¹⁹F probes.

Periodic trends and hidden dynamics of magnetic properties in three series of triazacyclononane lanthanide complexes

In three structurally related series of nine-coordinate lanthanide(iii) complexes, solution NMR studies and DFT/CASSCF calculations have provided key information on the magnetic susceptibility anisotropy.

Lanthanide-induced relaxation anisotropy

Lanthanide ions accelerate nuclear spin relaxation by two primary mechanisms: dipolar and Curie. Both are commonly assumed to depend on the length of the lanthanide-nucleus vector, but not on its direction. Here we show experimentally that this is wrong - careful proton relaxation data analysis in a series of isostructural lanthanide complexes (Ln = Tb, Dy, Ho, Er, Tm, Yb) reveals angular dependence in both Curie and dipolar relaxation. The reasons are: (a) that magnetic susceptibility anisotropy can be of the same order of magnitude as the isotropic part (contradicting the unstated assumption in Guéron's theory of the Curie relaxation process), and (b) that zero-field splitting can be much stronger than the electron Zeeman interaction (Bloembergen's original theory of the lanthanide-induced dipolar relaxation process makes the opposite assumption). These factors go beyond the well researched cross-correlation effects; they alter the relaxation theory treatment and make strong angular dependencies appear in the nuclear spin relaxation rates. Those dependencies are impossible to ignore - this is now demonstrated both theoretically and experimentally, and suggests that a major revision is needed of the way lanthanide-induced relaxation data are used in structural biology.

Transient versus Static Electron Spin Relaxation in Mn(2+) Complexes Relevant as MRI Contrast Agents

The zero-field splitting (ZFS) parameters of the [Mn(EDTA)(H2O)](2-)·2H2O and [Mn(MeNO2A)(H2O)]·2H2O systems were estimated by using DFT and ab initio CASSCF/NEVPT2 calculations (EDTA = 2,2',2″,2‴-(ethane-1,2-diylbis(azanetriyl))tetraacetate; MeNO2A = 2,2'-(7-methyl-1,4,7-triazonane-1,4-diyl)diacetate). Subsequent molecular dynamics calculations performed within the atom-centered density matrix propagation (ADMP) approach provided access to the transient and static ZFS parameters, as well as to the correlation time of the transient ZFS. The calculated ZFS parameters present a reasonable agreement with the experimental values obtained from the analysis of (1)H relaxation data. The correlation times calculated for the two systems investigated turned out to be very short (τc ∼ 0.02-0.05 ps), which shows that the transient ZFS is modulated by molecular vibrations. On the contrary, the static ZFS is modulated by the rotation of the complexes in solution, which for the small complexes investigated here is characterized by rotational correlation times of τR ∼ 35-60 ps. As a result, electron spin relaxation in small Mn(2+) complexes is dominated by the static ZFS.

Paramagnetic fluorinated nanoemulsions for sensitive cellular fluorine-19 magnetic resonance imaging

Fluorine-19 magnetic resonance imaging ((19)F MRI) probes enable quantitative in vivo detection of cell therapies and inflammatory cells. Here, we describe the formulation of perfluorocarbon-based nanoemulsions with improved sensitivity for cellular MRI. Reduction of the (19)F spin-lattice relaxation time (T1) enables rapid imaging and an improved signal-to-noise ratio, thereby improving cell detection sensitivity. We synthesized metal-binding β-diketones conjugated to linear perfluoropolyether (PFPE), formulated these fluorinated ligands as aqueous nanoemulsions, and then metallated them with various transition and lanthanide ions in the fluorous phase. Iron(III) tris-β-diketonate ('FETRIS') nanoemulsions with PFPE have low cytotoxicity (<20%) and superior MRI properties. Moreover, the (19)F T1 can readily be reduced by an order of magnitude and tuned by stoichiometric modulation of the iron concentration. The resulting (19)F MRI detection sensitivity is enhanced by three- to fivefold over previously used tracers at 11.7 T, and is predicted to increase by at least eightfold at the clinical field strength of 3 T.

Fluoride Binding and Crystal-Field Analysis of Lanthanide Complexes of Tetrapicolyl-Appended Cyclen

Lanthanide complexes of tetrapicolyl cyclen displayed remarkably high affinities for fluoride (log K≈5) in water, and were shown to form 1:1 complexes. The behaviour of these systems can be rationalised by changes to the magnitude of the crystal-field parameter, B20 . However, such changes are not invariably accompanied by a change in sign of this parameter: for early lanthanides, the N8 donor set with a coordinated axial water molecule ensures that the magnetic anisotropy has the opposite sense to that observed in the analogous dehydrated lanthanide complexes.

Multifunctional β-NaGdF4:Ln3+ (Ln = Yb, Er, Dy) nanoparticles with NIR to visible upconversion and high transverse relaxivity: a potential bimodal contrast agent for high-field MRI and optical imaging

Developed nano β-NaGdF₄ doped with Yb³⁺/Er³⁺ in the core and Yb³⁺/Dy³⁺ in the shell with favorable properties for combined magnetic resonance and optical imaging.

Electromagnetic susceptibility anisotropy and its importance for paramagnetic NMR and optical spectroscopy in lanthanide coordination chemistry

Electromagnetic susceptibility anisotropy can explain the spectroscopy and magnetism of lanthanide containing systems, but current theories have limitations.

Another challenge to paramagnetic relaxation theory: a study of paramagnetic proton NMR relaxation in closely related series of pyridine-derivatised dysprosium complexes

The surprising sensitivity of the relaxation behaviour of twelve dysprosium complexes in solution highlights limitations in paramagnetic relaxation theory.

Axial fluoride binding by lanthanide DTMA complexes alters the local crystal field, resulting in dramatic spectroscopic changes

Addition of fluoride to aqueous solutions of lanthanide complexes of DTMA results in the formation of ternary complexes of the form [F·Ln·DTMA](2+) in which an axial solvent molecule is displaced by fluoride. [F·Ln·DTMA](2+) and [H2O·Ln·DTMA](3+) are in exchange on a timescale of around 1 s. Dramatic changes are observed in both the NMR and luminescence spectra of the complexes: these are consistent with a change in the nature of the magnetic anisotropy at the paramagnetic lanthanide centre, itself arising from a change in the local crystal field. Study of paramagnetic lanthanide complexes with anisotropic electronic distributions reveals that, upon replacing water with fluoride, there is an inversion of the sign, and a significant reduction in the magnitude, of the crystal field term that defines the nature of the pseudocontact shift.

MRI CEST at 1T with large µeff Ln(3+) complexes T m(3+)-HPDO3A: An efficient MRI pH reporter

PURPOSE

Chemical exchange saturation transfer (CEST) sensitivity relies on the prototropic exchange rate kex between the agent and the "bulk" water protons. To exploit large kex, a large frequency separation (Δω) between the pools of exchanging protons is necessary. For this reason, high magnetic fields are preferred. Herein it is shown that the use of paramagnetic CEST agents based on lanthanide (III) ions with large effective magnetic moments allows the carrying out of CEST experiments at the relatively low field strength of 1 tesla (T).

METHODS

Measurements were performed on a 1T MR-scanner using continuous wave (cw)-presaturation with a spin echo sequence. ParaCEST complexes have been synthetized by mixing the ligand and Ln(III)Cl3 in a stoichiometric ratio at room temperature and pH 7.

RESULTS

Different lanthanide chelates were investigated (Tm-, Dy-, Yb-, Eu-HPDO3A, and Eu-DOTAMGly). Ratiometric (Tm-HPDO3A) and selective detection (Eu-DOTAMGly and Tm-HPDO3A) experiments have been proven feasible in vivo.

CONCLUSION

In vitro experiments demonstrated the feasibility of the CEST methodology at 1T for nearly every paraCEST candidate under investigation, except for Eu-HPDO3A. Among the studied compounds, Tm-HPDO3A proved suitable for the application of a ratiometric method for assessing pH both in vitro and in vivo.

Challenging lanthanide relaxation theory: erbium and thulium complexes that show NMR relaxation rates faster than dysprosium and terbium analogues

The unususal relaxation behaviour of lanthanide complexes in solution exposes the limitations of paramagnetic relaxation theory.

Luminescent lanthanide coordination polymers for photonic applications

Luminescent lanthanide coordination polymers composed of lanthanide ions and organic joint ligands exhibit characteristic photophysical and thermostable properties that are different from typical organic dyes, luminescent metal complexes, and semiconductor nanoparticles.

Critical analysis of the limitations of Bleaney's theory of magnetic anisotropy in paramagnetic lanthanide coordination complexes

The origins of the breakdown of Bleaney's theory of magnetic anisotropy are described, based on an analysis of eleven different complexes of the second half of the 4f elements that form isostructural series.

The origins of the breakdown of Bleaney's theory of magnetic anisotropy are described, based on an analysis of eleven different complexes of the second half of the 4f elements that form isostructural series. An examination of the chemical shift and relaxation rate behaviour of resonances located at least four bonds away from the paramagnetic centre was undertaken, and correlated to theoretical predictions. The key limitations relate to comparability of ligand field splitting with spin–orbit coupling, variation in the position of the principal magnetic axis between Ln complexes and the importance of multipolar terms in describing lanthanide ligand field interactions.

Synthesis, stereocontrol and structural studies of highly luminescent chiral tris-amidepyridyl-triazacyclononane lanthanide complexes

S at C gives the Δ complex; extending the chromophore gives an emission brightness of 20 mM⁻¹ cm⁻¹.

Direct and selective tagging of cysteine residues in peptides and proteins with 4-nitropyridyl lanthanide complexes

A cysteine-selective tagging method in water is reported, based on rapid displacement of a pyridyl nitro-substituent in simple pyridines and lanthanide complexes. The conjugation reaction creates a short link between the tag and peptide, holding the peptide closer to the Ln(3+) ion and with reduced flexibility compared to existing methods.