Unravelling the Complexities of Pseudocontact Shift Analysis in Lanthanide Coordination Complexes of Differing Symmetry.

In two closely related series of eight-coordinate lanthanide complexes, a switch in the sign of the dominant ligand field parameter and striking variations in the sign, amplitude and orientation of the main component of the magnetic susceptibility tensor as the Ln3+ ion is permuted conspire to mask modest changes in NMR paramagnetic shifts, but are evident in Yb EPR and Eu emission spectra.

Simultaneous Triple Imaging with Two PARASHIFT Probes: Encoding Anatomical, pH and Temperature Information using Magnetic Resonance Shift Imaging.

The chemical shift of paramagnetically shifted resonances in lanthanide(III) complexes encodes information about temperature and has also been made to report pH in parallel, through introduction of a single phosphonate group adjacent to the reporter tert-butyl resonance. The enhanced sensitivity of this new probe has allowed the simultaneous triple imaging of the water signal and the shifted tert-butyl signals of thulium and dysprosium complexes of a common ligand, separated by over 160 ppm. In parallel spectral imaging experiments, the temperature and pH dependence of the frequency of the Tm and Dy signals has been deconvoluted, allowing the pH and temperature in the liver, kidney and bladder to be measured.

A new paramagnetically shifted imaging probe for MRI.

PURPOSE: To develop and characterize a new paramagnetic contrast agent for molecular imaging by MRI. METHODS: A contrast agent was developed for direct MRI detection through the paramagnetically shifted proton magnetic resonances of two chemically equivalent tert-butyl reporter groups within a dysprosium(III) complex. The complex was characterized in phantoms and imaged in physiologically intact mice at 7 Tesla (T) using three-dimensional (3D) gradient echo and spectroscopic imaging (MRSI) sequences to measure spatial distribution and signal frequency. RESULTS: The reporter protons reside ∼6.5 Å from the paramagnetic center, resulting in fast T1 relaxation (T1 = 8 ms) and a large paramagnetic frequency shift exceeding 60 ppm. Fast relaxation allowed short scan repetition times with high excitation flip angle, resulting in high sensitivity. The large dipolar shift allowed direct frequency selective excitation and acquisition of the dysprosium(III) complex, independent of the tissue water signal. The biokinetics of the complex were followed in vivo with a temporal resolution of 62 s following a single, low-dose intravenous injection. The lower concentration limit for detection was ∼23 µM. Through MRSI, the temperature dependence of the paramagnetic shift (0.28 ppm.K-1 ) was exploited to examine tissue temperature variation. CONCLUSIONS: These data demonstrate a new MRI agent with the potential for physiological monitoring by MRI. Magn Reson Med 77:1307-1317, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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

Measurements of the relaxation rate behaviour of two series of dysprosium complexes have been performed in solution, over the field range 1.0 to 16.5 Tesla. The field dependence has been modelled using Bloch-Redfield-Wangsness theory, allowing estimates of the electronic relaxation time, T1e, and the size of the magnetic susceptibility, µeff, to be made. Changes in relaxation rate of the order of 50% at higher fields were measured, following variation of the para-substituent in the single pyridine donor. The magnetic susceptibilities deviated unexpectedly from the free-ion values for certain derivatives in each series examined, in a manner that was independent of the electron-releasing/withdrawing ability of the pyridine substituent, suggesting that the polarisability of just one pyridine donor in octadenate ligands can play a significant role in defining the magnetic susceptibility anisotropy.

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

Measurements of the proton NMR paramagnetic relaxation rates for several series of isostructural lanthanide(III) complexes have been performed in aqueous solution over the field range 1.0 to 16.5 Tesla. The field dependence has been modeled using Bloch-Redfield-Wangsness theory, allowing values for the electronic relaxation time, Tle and the magnetic susceptibility, µeff, to be estimated. Anomalous relaxation rate profiles were obtained, notably for erbium and thulium complexes of low symmetry 8-coordinate aza-phosphinate complexes. Such behaviour challenges accepted theory and can be interpreted in terms of changes in Tle values that are a function of the transient ligand field induced by solvent collision and vary considerably between Ln(3+) ions, along with magnetic susceptibilities that deviate significantly from free-ion values.

Characterisation and evaluation of paramagnetic fluorine labelled glycol chitosan conjugates for (19)F and (1)H magnetic resonance imaging.

Medium molecular weight glycol chitosan conjugates have been prepared, linked by an amide bond to paramagnetic Gd(III), Ho(III) and Dy(III) macrocyclic complexes in which a trifluoromethyl reporter group is located 6.5 Å from the paramagnetic centre. The faster relaxation of the observed nucleus allows modified pulse sequences to be used with shorter acquisition times. The polydisperse materials have been characterised by gel permeation chromatography, revealing an average molecular weight on the order of 13,800 (Gd), 14,600 (Dy) and 16,200 (Ho), consistent with the presence of 8.5, 9.5 and 13 complexes, respectively. The gadolinium conjugate was prepared for both a q = 1 monoamide tricarboxylate conjugate (r1p 11.2 mM(-1) s(-1), 310 K, 1.4 T) and a q = 0 triphosphinate system, and conventional contrast-enhanced proton MRI studies at 7 T were undertaken in mice bearing an HT-29 or an HCT-116 colorectal tumour xenograft (17 µmol/kg). Enhanced contrast was observed following injection in the tail vein in tumour tissue, with uptake also evident in the liver and kidney with a tumour-to-liver ratio of 2:1 at 13 min, and large amounts in the kidney and bladder consistent with predominant renal clearance. Parallel experiments observing the (19)F resonance in the holmium conjugate complex using a surface coil did not succeed owing to its high R2 value (750 Hz, 7 T). However, the fluorine signal in the dysprosium triphosphinate chitosan conjugate [R1/R2 = 0.6 and R1 = 145 Hz (7 T)] was sharper and could be observed in vivo at -65.7 ppm, following intravenous tail vein injection of a dose of 34 µmol/kg.

19F NMR based pH probes: lanthanide(III) complexes with pH-sensitive chemical shifts.

Experimental measurements and theoretical analysis of magnetic properties, structural dynamics and acid-base equilibria for several lanthanide(III) complexes with tetraazacyclododecane derivatives as 19F NMR chemical shift pH probes are presented; pKa values vary between 6.9 and 7.7, with 18 to 40 ppm chemical shift differences between the acidic and basic forms for Ho(III) complexes possessing T1 values of 10 to 30 ms (4.7-9.4 T, 295 K).

Responsive fluorinated lanthanide probes for 19F magnetic resonance spectroscopy.

The introduction of CF(3) reporter groups close to the paramagnetic centre in macrocyclic lanthanide(iii) complexes allows faster acquisition of (19)F magnetic resonance data, and amplifies chemical shift non-equivalence, as exemplified by the definition of ratiometric chemical shift probes for pH and, in principle, enzyme activity.