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

Complexes of paramagnetic metal ions, in particular Gd, 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(NO) is suitable for use as an "off-the-shelf" MAS DNP polarising agent, providing promising sensitivity enhancements to H, C, and N NMR signals. Here we expand upon this approach by investigating the impact of the Gd(NO) concentration and by exploring a larger range of readily available Gd sources. We found that a Gd(NO) concentration of 20 mM in the case of H and C, and 40 mM in the case of N, 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 (GdCl, Gd(SO), GdBr, and Gd(OAc)) are also suitable for use as polarising agents and yield H, C, and N signal enhancements of variable values. Gd(OAc) yields lower signal enhancements, which is proposed to be the result of greater local asymmetry at the Gd 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.