Magnetic-Field-Dependent Lifetimes of Hyperpolarized ¹³C Spins at Cryogenic Temperature

Using a home-built cryogen-free dynamic nuclear polarization (DNP) system with a variable magnetic field capability, ¹³C spin-lattice T₁ relaxation times of hyperpolarized [1-¹³C] carboxylates (sodium acetate, glycine, sodium pyruvate, and pyruvic acid) doped with trityl OX063 free radical were systematically measured for the first time at different field strengths up to 9 T at T = 1.8 K. Our data reveal that the ¹³C T₁ values of these frozen hyperpolarized ¹³C samples vary drastically with the applied magnetic field B according to an apparent empirical power-law dependence (¹³C T₁ ∝ B^α, 2.3 < α < 3.1), with relaxation values ranging from a few hundred seconds at 1 T to over 200,000 s at fields close to 9 T. This low temperature relaxation behavior can be ascribed approximately to a model that accounts for the combined effect of ¹³C-¹H intramolecular dipolar interaction and the relaxation contribution from the paramagnetic impurities present in the DNP sample. Since the lifetime or T₁ storage of the hyperpolarized state is intimately linked to DNP efficiency, these ¹³C relaxation data at cryogenic temperature have important theoretical and experimental implications as the DNP of ¹³C-labeled biomolecules is pushed to higher magnetic fields.