Low-field thermal mixing in [1-13C] pyruvic acid for brute-force hyperpolarization

We detail the process of low-field thermal mixing (LFTM) between H-1 and C-13 nuclei in neat [1-C-13] pyruvic acid at cryogenic temperatures (4-15 K). Using fast-field-cycling NMR, H-1 nuclei in the molecule were polarized at modest high field (2 T) and then equilibrated with C-13 nuclei by fast cycling (similar to 300-400 ms) to a low field (0-300 G) that activates thermal mixing. The C-13 NMR spectrum was recorded after fast cycling back to 2 T. The C-13 signal derives from 1H polarization via LFTM, in which the polarized ('cold') proton bath contacts the unpolarised ('hot') C-13 bath at a field so low that Zeeman and dipolar interactions are similarsized and fluctuations in the latter drive H-1-C-13 equilibration. By varying mixing time (t(mix)) and field (B-mix), we determined field-dependent rates of polarization transfer (1/tau) and decay (1/T-1m) during mixing. This defines conditions for effective mixing, as utilized in 'brute-force' hyperpolarization of low-gamma nuclei like C-13 using Boltzmann polarization from nearby protons. For neat pyruvic acid, near-optimum mixing occurs for t(mix) similar to 100-300 ms and B-mix similar to 30-60 G. Three forms of frozen neat pyruvic acid were tested: two glassy samples, (one well-deoxygenated, the other O-2-exposed) and one sample pre-treated by annealing (also well-deoxygenated). Both annealing and the presence of O-2 are known to dramatically alter high-field longitudinal relaxation (T-1) of H-1 and C-13 (up to 10(2)-10(3)-fold effects). Here, we found smaller, but still critical factors of similar to (2-5) x on both tau and T-1m. Annealed, well-deoxygenated samples exhibit the longest time constants, e.g., tau similar to 30-70 ms and T-1m similar to 1-20 s, each growing vs. B-mix. Mixing 'turns off' for B-mix > similar to 100 G. That T-1m >> tau is consistent with earlier success with polarization transfer from H-1 to C-13 by LFTM.