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

Complexes of paramagnetic metal ions, in particular Gd 3+ , 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 3 ) 3 is suitable for use as an "off-the-shelf" MAS DNP polarising agent, providing promising sensitivity enhancements to 1 H, 13 C, and 15 N NMR signals. Here we expand upon this approach by investigating the impact of the Gd(NO 3 ) 3 concentration and by exploring a larger range of readily available Gd 3+ sources. We found that a Gd(NO 3 ) 3 concentration of 20 mM in the case of 1 H and 13 C, and 40 mM in the case of 15 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 3 , Gd 2 (SO 4 ) 3 , GdBr 3 , and Gd(OAc) 3 ) are also suitable for use as polarising agents and yield 1 H, 13 C, and 15 N 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 Gd 3+ 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.