Earth's magnetic field enabled scalar coupling relaxation of 13C nuclei bound to fast-relaxing quadrupolar 14N in amide groups

Scalar coupling relaxation, which is usually only associated with closely resonant nuclei (e.g., Br-79-C-13), can be a very effective relaxation mechanism. While working on hyperpolarized [5-C-13]glutamine, fast liquid-state polarization decay during transfer to the MRI scanner was observed. This behavior could hypothetically be explained by substantial T-1 shortening due to a scalar coupling contribution (type II) to the relaxation caused by the fast-relaxing quadrupolar N-14 adjacent to the C-13 nucleus in the amide group. This contribution is only effective in low magnetic fields (i.e., less than 800 mu T) and prevents the use of molecules bearing the C-13-amide group as hyperpolarized MRS/MRI probes. In the present work, this hypothesis is explored both theoretically and experimentally. The results show that high hyperpolarization levels can be retained using either a N-15-labeled amide or by applying a magnetic field during transfer of the sample from the polarizer to the MRI scanner. (C) 2012 Elsevier Inc. All rights reserved.