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

► Very low magnetic field enables fast relaxation in 14N coupled 13C. ► Scalar coupling (II° type) is found responsible for sensible 13C T1 shortening. ► Effective only with relatively coupled 14N/13C nuclei and fast relaxing 14N nuclei. ► Important relaxation for hyperpolarized DNP MR spectroscopy/imaging application. ► Effect minimized if 15N labeling or sustained field during sample transfer is used.