Dynamic Nuclear Polarization of ¹H, ¹³C, and ⁵⁹Co in a Tris(ethylenediamine)cobalt(III) Crystalline Lattice Doped with Cr(III)

Abstract

The study of inorganic crystalline materials by solid-state NMR spectroscopy is often complicated by the low sensitivity of heavy nuclei. However, these materials often contain or can be prepared with paramagnetic dopants without significantly affecting the structure of the crystalline host. Dynamic nuclear polarization (DNP) is generally capable of enhancing NMR signals by transferring the magnetization of unpaired electrons to the nuclei. Therefore, the NMR sensitivity in these paramagnetically doped crystals might be increased by DNP. In this paper we demonstrate the possibility of efficient DNP transfer in polycrystalline samples of [Co(en)3Cl3]2·NaCl·6H2O (en = ethylenediamine, C2H8N2) doped with Cr(III) in varying concentrations between 0.1 and 3 mol %. We demonstrate that 1H, 13C, and 59Co can be polarized by irradiation of Cr(III) with 140 GHz microwaves at a magnetic field of 5 T. We further explain our findings on the basis of electron paramagnetic resonance spectroscopy of the Cr(III) site and analysis of its temperature-dependent zero-field splitting, as well as the dependence of the DNP enhancement factor on the external magnetic field and microwave power. This first demonstration of DNP transfer from one paramagnetic metal ion to its diamagnetic host metal ion will pave the way for future applications of DNP in paramagnetically doped materials or metalloproteins.

Studying the Conformation of a Silaffin-Derived Pentalysine Peptide Embedded in Bioinspired Silica using Solution and Dynamic Nuclear Polarization Magic-Angle Spinning NMR

“…DNP MAS NMR was also implemented for 1D and 2D 13 C and 29 Si measurements in unlabeled samples, relieving the need for elaborate and tedious sample labeling [40][41][42][43][44][45] . However, rapid deployment of the technique for extensive structural 2 characterization in biological systems is still hampered by low-resolution spectra [46][47] with some exceptions reported [48][49][50] .…”

Paramagnetic Metal‐Ion Dopants as Polarization Agents for Dynamic Nuclear Polarization NMR Spectroscopy in Inorganic Solids

“…Recently it was shown that paramagnetic metal ion chelates can be used as polarizing agents for DNP in biomolecules and molecular crystals . Similarly, in the early days of DNP, paramagnetic defect sites were used to polarize single crystals at low magnetic fields and extremely low temperatures ,.…”

“…[9] Recently it was shown that paramagnetic metal ion chelates can be used as polarizing agents [10] for DNP in biomolecules [11] and molecular crystals. [12] Similarly, in the early days of DNP, paramagnetic defect sites were used to polarize single crystals at low magnetic fields and extremely low temperatures. [13,14] Here we expand this approach and show that paramagnetic metal ions incorporated as dopants can be used efficiently for sensitivity enhancement in powders of micron sized inorganic solids.…”

Biosilica and bioinspired silica studied by solid-state NMR

“…Among those, dynamic nuclear polarization (DNP), which uses microwave irradiation of EPR transitions to drive the magnetization from unpaired electrons to the nuclear spins, [86,87] provides dramatic enhancements, expanding the applicability of NMR to more and more complex chemical entities. [88][89][90] After the pioneering studies by Griffin and co-workers on lyophilized samples, [91][92][93] and Cross and Opella on oriented samples, [94] also the solid-state NMR characterization of biological macromolecules has flourished significantly, because of improved hardware, pulse sequences and sample preparation approaches. [85] Higher magnetic fields have led to improved sensitivity and resolution, especially when coupled to fast magic angle spinning.…”