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.

Paramagnetic probes for chemical shift imaging are described, in which a reporting tert-butyl resonance is integrated and placed about 6 to 6.5 Å from a lanthanide ion. At this distance, the resonance is shifted to a region of the ¹H NMR spectral window far away from the usual 0–12 ppm range, permitting selective observation. Furthermore, judicious selection of the lanthanide ion, according to the applied magnetic field strength, leads to relaxation rate enhancements of the order of 100–200, permitting more rapid data acquisition per unit time in both spectroscopy and imaging experiments. The enhanced sensitivity allows the detection of these complexes in mice within a few minutes using shift imaging, following tail vein injection of doses of the order of 0.1 mmol kg⁻¹.

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.

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 Ln³⁺ ion and with reduced flexibility compared to existing methods.