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

Using a home-built cryogen-free dynamic nuclear polarization (DNP) system with a variable magnetic field capability, 13 C spin-lattice T1 relaxation times of hyperpolarized [1-13 C] 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 13 C T1 values of these frozen hyperpolarized 13 C samples vary drastically with the applied magnetic field B according to an apparent empirical power-law dependence (13 C T1 ∝ Bα , 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 13 C-1 H 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 13 C relaxation data at cryogenic temperature have important theoretical and experimental implications as the DNP of 13 C-labeled biomolecules is pushed to higher magnetic fields.