1H‐decoupling and Isotopic Labeling for the Measurement of the Longitudinal Relaxation Time of Hyperpolarized 13C‐Methylenes in Choline Analogs

The T₁ of a hyperpolarized site in solution is a key parameter that determines the time‐window in which its NMR signals are observable. For ¹³C sites adjacent to protons, ¹H‐decoupling has been shown to increase the hyperpolarized signal resolution and SNR. Additionally, polarization transfer to protons has shown utility in increasing the sensitivity of detection. However, ¹H‐decoupling could lead to a change in the decay rate of a hyperpolarized ¹³C site. Here we tested this possible effect in a case where the protons are directly bound to an sp³ hyperpolarized ¹³C site (using 1,2‐¹³C₂]choline) and ¹H‐decoupling was applied continuously throughout the hyperpolarized decay measurement. We found that ¹H‐decoupling did not lead to any significant changes in the ¹³C polarization decay time but did result in the expected collapse of J‐coupling and produced sharper signals. This result suggested that ¹H‐decoupling did not affect the decay rate of hyperpolarized sp^{3 13}C sites. The deuterium‐substitution approach (using 1,1,2,2‐D₄,1‐¹³C]choline) showed a dramatic prolongation of T₁. Upper bounds on the T₁ of all investigated sites were calculated.