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

Abstract

In two closely related series of eight-coordinate lanthanide complexes,aswitch 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 Ln 3+ ion is permuted conspire to mask modest changes in NMR paramagnetic shifts, but are evident in Yb EPR and Eu emission spectra.Lanthanide induced shift and paramagnetic relaxation enhancement are subjects of continuous research driven by new applications of lanthanide complexes as tags for biomolecular structure analysis, [1] and PA RASHIFT probes for magnetic resonance imaging (MRI). [2] While Gd 3+ tags are employed in double electron-electron resonance (DEER) distance measurements in biomolecules, [3] and Gd 3+ MRI contrast agents are routinely used, [4] non-Gd tags with pronounced magnetic anisotropy continue to challenge existing theoretical models of paramagnetic shift [5] and relaxation. [6] Many of the conclusions made in the early literature examining NMR behaviour in paramagnetic lanthanide systems,f or example,m ay have low significance because of the limitations of Bleaneyst heory of magnetic anisotropy. [7] Often, unreasonable estimations of ac ontact contribution were invoked in order to allow experimental data to be fitted, and sometimes "distance only" dependent relaxation rate data were used to aid assignment, notwithstanding the fact that the electron-nuclear dipolar interaction has been shown to have as trong directional dependence,b oth in the dipolar and the Curie contributions to paramagnetic relaxation. [6] Thet heory of Bleaney assumed that ligand field (zerofield) splitting of the ground J-multiplet is less than kT and that the ligand-field parameters are invariant as the lanthanide ion is permuted in an isostructural series.N either Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under https://doi.

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

“…The variation of overall ligand polarizability and its directionality, involving the electrostatic interaction between induced dipoles on the ligand and the quadrupole moment on the Ln 3+ ion, is important in determining the “allowedness” of f–f electronic transitions. , Thus, it is the overall ligand molecular polarizability that is important in determining the ligand field. Other examples of switches in the sign of B 0 2 can be identified when complex constitution and local symmetry vary. The difference between the emission spectra of [EuL 8b ] and [EuL 9 ] + (Figure ) is consistent with a change in sign, as symmetry changes from C 1 to C 2 . , Other cases have been reported, including systems involving reversible coordination of a polarizable N atom, which following protonation is replaced by water. − …”

“…The difference between the emission spectra of [EuL 8b ] and [EuL 9 ] + (Figure 2) is consistent with a change in sign, as symmetry changes from C 1 to C 2 . 3,14 Other cases have been reported, including systems involving reversible coordination of a polarizable N atom, which following protonation is replaced by water. 49−51…”

“…Thus, the combined effect of the change in sign and orientation of the ligand field were shown to give rise to similar PCS fields for the t Bu protons, explaining the "hidden" changes in PCS behavior. 3 ■ NMR SHIFT BEHAVIOR OF SYSTEMS WITH SMALL LIGAND FIELD SPLITTINGS Complexes [LnL 1−3 ] adopt tricapped trigonal prismatic structures and possess small ligand field splittings close to kT. Yet, their PCS values do not conform to Bleaney's theory.…”

“…F.ParkerD. Harnden, A. C. Suturina, E. A. Batsanov, A. S. Senanayake, P. K. Fox, M. A. Mason, K. Vonci, M. McInnes, E. J. Chilton, N. F. Parker, D. 20191311039610400 …”

Design and applications of lanthanide chelating tags for pseudocontact shift NMR spectroscopy with biomacromolecules

“…".Using a rigid framework as e.g. DOTA-derived chelators, the lanthanide ion can be conveniently accommodated in the cavity and the tightly chelating ligand ensures chemical stability and inertness towards the environment of the LCT, which is a unique feature that clearly supersedes the stability of open-chain chelator-based LCTs[9].The investigation of the intrinsic anisotropies of lanthanide complexes in combination with their structural properties is subject of current research[52,53,158,159]. Furthermore, Mironov et al reported quantitative relations between ligand atom positions and geometries and the observed anisotropies[144].…”