Synthesis of lanthanide tag and experimental studies on paramagnetically induced residual dipolar couplings

Abstract

Nuclear Magnetic Resonance (NMR) spectroscopy is an indispensable technique for the structure elucidation of molecules and determination of their characteristic interactions. Residual Dipolar Coupling (RDC) is an NMR parameter that provides global orientation information of molecules but necessitates the use of an anisotropic orientation medium for the partial alignment of the target molecule with respect to the magnetic field. Importantly, anisotropic paramagnetic tags have been successful as orienting media in biomolecular NMR applications but their use in small organic molecules remains imperfect due to challenges in designing functional lanthanide complexes with varying degrees of bonding in the Ln(III) inner coordination sphere. In this study, we propose a strategy for the synthesis of the lanthanide tag 4-mercaptomethylpyridine-2,6-dicarboxylic acid, 4-MMDPA and the measurement of RDCs in a target molecule using several paramagnetic lanthanide complexes.

Keywords: NMR spectroscopy; Paramagnetic lanthanide tag; Residual dipolar coupling; Spin quantum mechanics; Sulfur chemistry.

Fig. 1

Residual Dipolar Couplings in NMR Spectroscopy. a The behavior of a molecule with the magnetic field B₀ depends on physical properties. In its crystal form, the dipolar couplings are too larger, in an isotropic solution the dipolar interaction average to zero due to tumbling and no coupling is observed, whereas in an anisotropic solution with a partially oriented medium, small residual dipolar couplings are visible. b 2D ¹H-¹⁵ N HSQC spectra of a protein where each peak corresponds to the N–H resonance of an amino acid of the backbone. The spectra in green and red correspond to an isotropic solution decoupled in both ¹H and ¹⁵ N dimensions or only ¹⁵ N, respectively. The latter spectrum shows the same ¹⁵ N-¹H splitting for every amino acid peak and represents ¹⁵ N-¹H one-bond scalar coupling. The blue spectrum corresponds to the anisotropic solution decoupled in ¹⁵ N Dimension. The partial orientation allows observation of ¹⁵ N-¹H splitting and represents ¹⁵ N-¹H one-bond scalar coupling in addition to the residual dipolar coupling RDC, the total coupling

Fig. 2

Strategy for the synthesis of 4-MMDPA. a synthesis of 4-MMDPA (b) 1D ¹H NMR spectra showing the aromatic region of the intermediates generated along the pathway of 4-MMDPA synthesis. The full spectra of all intermediates is reported in Additional file 1: Figure S1. The color labeling of atoms highlights the substitution of certain functional groups in every step

Fig. 3

Synthesis of the unsymmetrical disulfide between 4-MMDPA and naphthalene-2-thiol using DTNB chemistry. The color labeling of atoms highlights the substitution of certain functional groups in every step

Fig. 4

RDC Measurements in Lanthanide tagged 4-MMDPA naphthalene-2-thiol. a 1D ¹H NMR and (b) 2D ¹H-¹³C HSQC spectra of the free tag, and in complex with the diamagnetic lanthanide LaCl₃, or the paramagnetic lanthanides Eu(FOD)₃, Eu₂(SO₄)₃, Tb(NO₃)₃. The J coupling constants and total coupling constants T for all complexes are shown on the 2D spectra. The error ranges in RDC measurements are evaluated from the contour levels and the peak intensity. H7 of Naphthalene thiol is reported in the spectra