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

David T Peat,Matthew L Hirsch,John R Owers-Bradley,David G Gadian,Anthony J Horsewill,James G Kempf

doi:10.1039/c6cp02853e

Abstract

We detail the process of low-field thermal mixing (LFTM) between (1)H and (13)C nuclei in neat [1-(13)C] pyruvic acid at cryogenic temperatures (4-15 K). Using fast-field-cycling NMR, (1)H nuclei in the molecule were polarized at modest high field (2 T) and then equilibrated with (13)C nuclei by fast cycling (∼300-400 ms) to a low field (0-300 G) that activates thermal mixing. The (13)C NMR spectrum was recorded after fast cycling back to 2 T. The (13)C signal derives from (1)H polarization via LFTM, in which the polarized ('cold') proton bath contacts the unpolarised ('hot') (13)C bath at a field so low that Zeeman and dipolar interactions are similar-sized and fluctuations in the latter drive (1)H-(13)C equilibration. By varying mixing time (tmix) and field (Bmix), we determined field-dependent rates of polarization transfer (1/τ) and decay (1/T1m) during mixing. This defines conditions for effective mixing, as utilized in 'brute-force' hyperpolarization of low-γ nuclei like (13)C using Boltzmann polarization from nearby protons. For neat pyruvic acid, near-optimum mixing occurs for tmix∼ 100-300 ms and Bmix∼ 30-60 G. Three forms of frozen neat pyruvic acid were tested: two glassy samples, (one well-deoxygenated, the other O2-exposed) and one sample pre-treated by annealing (also well-deoxygenated). Both annealing and the presence of O2 are known to dramatically alter high-field longitudinal relaxation (T1) of (1)H and (13)C (up to 10(2)-10(3)-fold effects). Here, we found smaller, but still critical factors of ∼(2-5)× on both τ and T1m. Annealed, well-deoxygenated samples exhibit the longest time constants, e.g., τ∼ 30-70 ms and T1m∼ 1-20 s, each growing vs. Bmix. Mixing 'turns off' for Bmix > ∼100 G. That T1m≫τ is consistent with earlier success with polarization transfer from (1)H to (13)C by LFTM.

Highlights

Low-field thermal mixing (LFTM) is the process by which dissimilar spins in a solid sample are brought to mutual equilibrium by exposure to a magnetic field small enough that their magnetic resonance lineshapes come into overlap.[1,2,3] In this way, mutual spin flips can occur with conservation of energy, allowing the noted equilibration, e.g., among heteronuclei in NMR
We detail the process of low-field thermal mixing (LFTM) between 1H and 13C nuclei in neat [1-13C] pyruvic acid at cryogenic temperatures (4–15 K)
In addition to detailing LFTM of 1H and 13C in such samples, we explore the importance of certain sample-handling protocols

Summary

Introduction

Low-field thermal mixing (LFTM) is the process by which dissimilar spins in a solid sample are brought to mutual equilibrium by exposure to a magnetic field small enough that their magnetic resonance lineshapes come into overlap.[1,2,3] In this way, mutual spin flips can occur with conservation of energy, allowing the noted equilibration, e.g., among heteronuclei in NMR (nuclear magnetic resonance). This phenomenon is of special recent interest as a way to hyperpolarize low-g nuclear spins (g = gyromagnetic ratio) such as 13C, 15N or 31P, using spin order originally established in high-g nuclei like 1H.4,5. For mixing 1H with low-g nuclei, the protons have much larger specific heat (proportional to g2), and dominate in establishing the final ‘spin temperature’.1–3 The result is a cold (highly polarized) set of low-g spins

Methods

Results

Discussion

Conclusion