Optimisation of dynamic nuclear polarisation using "off-the-shelf" Gd(III)-based polarising agents

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) Spectroscopy
• Dynamic Nuclear Polarization (DNP)
• Paramagnetic Relaxation Agents

Background:

• Magic-angle spinning (MAS) dynamic nuclear polarization (DNP) utilizes paramagnetic metal ions to enhance NMR signal sensitivity.
• Gadolinium(III) nitrate (Gd(NO3)3) has been previously shown as a cost-effective "off-the-shelf" DNP polarizing agent.
• Further investigation is needed to optimize Gd3+ concentration and explore alternative gadolinium sources for MAS DNP.

Purpose of the Study:

• To determine the optimal concentration of Gd(NO3)3 for MAS DNP.
• To evaluate the efficacy of various readily available gadolinium compounds as MAS DNP polarizing agents.
• To understand the factors influencing DNP performance with different Gd3+ sources.

Main Methods:

• Systematic variation of Gd(NO3)3 concentration in DNP experiments.
• Testing of different gadolinium salts (GdCl3, Gd2(SO4)3, GdBr3, Gd(OAc)3) as polarizing agents.
• Analysis of NMR signal enhancements, polarization build-up times, and electron paramagnetic resonance (EPR) relaxation times.

Main Results:

• Optimal Gd(NO3)3 concentrations were found to be 20 mM for 1H and 13C, and 40 mM for 15N NMR.
• Various gadolinium compounds (GdCl3, Gd2(SO4)3, GdBr3) effectively serve as DNP polarizing agents, yielding variable signal enhancements.
• Gd(OAc)3 showed reduced performance, potentially due to local asymmetry and relaxation effects from the acetate ligand.

Conclusions:

• Gd(NO3)3 concentration significantly impacts DNP performance, with specific optimal values identified for different nuclei.
• A range of common gadolinium salts are viable alternatives to Gd(NO3)3 for MAS DNP applications.
• The chemical environment and properties of the gadolinium complex influence its effectiveness as a DNP polarizing agent.