Lanthanide-induced relaxation anisotropy

Abstract

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.

NMR Spectroscopy and Metal Ions in Life Sciences

“…As already pointed out, the presence of a paramagnetic metal ion in a protein causes nuclear line broadening – as well as longitudinal relaxation time ( T 1 ) shortening. The theory describing this general effect has been formulated in simple forms starting from the fifties, has been developed to include the effects of more and more complicated spin Hamiltonians in the eighties, and has been practically under continuous development and revisiting until now . Irrespectively of the detailed treatments for the various cases, in very general terms nuclear relaxation depends on the stochastic modulation of the averaged squared energy of the electron‐nucleus coupling.…”

“…We have here implicitly assumed that the relaxation is isotropic, which is the case for those metals, such as copper(II), manganese(II) and gadolinium(III) that have limited susceptibility anisotropy and yield large PRE contributions. However, relaxation can also depend not only on the length of the nucleus–metal vector but also its direction, about which we leave the interested reader to references , , , . Lanthanoid(III) ions other than gadolinium that are sizably anisotropic may also yield large PREs (see below), and these PREs would be anisotropic.…”

High‐Field MRI Contrast Agents and their Synergy with Optical Imaging: the Evolution from Single Molecule Probes towards Nano‐architectures

“…In 2018, Kuprov et al. reported a series of isostructural lanthanide(III)‐1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid (DOTA) like complexes revealing the strong angular dependencies in both Curie and dipolar relaxation . They were previously estimated by taking into account only an r −6 dependence on distance from the metal center, but such approximations should be treated with appropriate caution …”

“…[25] In 2018, Kuprov et al reportedaseries of isostructural lanthanide(III)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) like complexes revealing the strong angular dependencies in both Curie and dipolar relaxation. [26] They were previously estimated by takingi nto account only an r À6 dependence on distance from the metal center,b ut such approximations should be treated with appropriate caution. [26] The high surface area of lanthanide nanoparticles (generally oxides and fluorides) is one of the features whichm ake them attractive as ap latform for MRI applications, which results in impressive contrasting.…”

“…[26] They were previously estimated by takingi nto account only an r À6 dependence on distance from the metal center,b ut such approximations should be treated with appropriate caution. [26] The high surface area of lanthanide nanoparticles (generally oxides and fluorides) is one of the features whichm ake them attractive as ap latform for MRI applications, which results in impressive contrasting. [27][28][29] The rigid inorganic structure and organic coating should reduce the toxicity risks associated with the leakageo fl anthanide ions into the body.…”