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

Complexes of paramagnetic metal ions, in particular Gd³⁺, have been demonstrated as efficient polarising agents for magic-angle spinning (MAS) dynamic nuclear polarisation (DNP). We recently demonstrated that commercially available and inexpensive Gd(NO₃)₃ is suitable for use as an "off-the-shelf" MAS DNP polarising agent, providing promising sensitivity enhancements to ¹H, ¹³C, and ¹⁵N NMR signals. Here we expand upon this approach by investigating the impact of the Gd(NO₃)₃ concentration and by exploring a larger range of readily available Gd³⁺ sources. We found that a Gd(NO₃)₃ concentration of 20 mM in the case of ¹H and ¹³C, and 40 mM in the case of ¹⁵N, offers optimum signal enhancements and is rationalised as a trade-off between DNP enhancements, polarisation build-up times, and electron paramagnetic resonance (EPR) spin-spin relaxation times. We determined that a range of different gadolinium compounds (GdCl₃, Gd₂(SO₄)₃, GdBr₃, and Gd(OAc)₃) are also suitable for use as polarising agents and yield ¹H, ¹³C, and ¹⁵N signal enhancements of variable values. Gd(OAc)₃ yields lower signal enhancements, which is proposed to be the result of greater local asymmetry at the Gd³⁺ centre leading to EPR line broadening, and the methyl group in the acetate ion acting as a relaxation sink and limiting the nuclear polarisation available.