Influence of Dy³⁺ and Tb³⁺ doping on ¹³C dynamic nuclear polarization

Dynamic nuclear polarization (DNP) is a technique that uses a microwave-driven transfer of high spin alignment from electrons to nuclear spins. This is most effective at low temperature and high magnetic field, and with the invention of the dissolution method, the amplified nuclear magnetic resonance (NMR) signals in the frozen state in DNP can be harnessed in the liquid-state at physiologically acceptable temperature for in vitro and in vivo metabolic studies. A current optimization practice in dissolution DNP is to dope the sample with trace amounts of lanthanides such as Gd³⁺ or Ho³⁺, which further improves the polarization. While Gd³⁺ and Ho³⁺ have been optimized for use in dissolution DNP, other lanthanides have not been exhaustively studied for use in C13 DNP applications. In this work, two additional lanthanides with relatively high magnetic moments, Dy³⁺ and Tb³⁺, were extensively optimized and tested as doping additives for C13 DNP at 3.35 T and 1.2 K. We have found that both of these lanthanides are also beneficial additives, to a varying degree, for C13 DNP. The optimal concentrations of Dy³⁺ (1.5 mM) and Tb³⁺ (0.25 mM) for C13 DNP were found to be less than that of Gd³⁺ (2 mM). W-band electron paramagnetic resonance shows that these enhancements due to Dy³⁺ and Tb³⁺ doping are accompanied by shortening of electron T₁ of trityl OX063 free radical. Furthermore, when dissolution was employed, Tb³⁺-doped samples were found to have similar liquid-state C13 NMR signal enhancements compared to samples doped with Gd³⁺, and both Tb³⁺ and Dy³⁺ had a negligible liquid-state nuclear T₁ shortening effect which contrasts with the significant reduction in T₁ when using Gd³⁺. Our results show that Dy³⁺ doping and Tb³⁺ doping have a beneficial impact on C13 DNP both in the solid and liquid states, and that Tb³⁺ in particular could be used as a potential alternative to Gd³⁺ in C13 dissolution DNP experiments.