One should point out that low-frequency peaks of N nuclei that are coupled to the protons were not visible in the ULF spectra of 15 N-MP. This is not surprising because, due to low gyromagnetic ratio of N nuclei, the signal from coupled N-spin precession in the ULF regime is expected to be approximately two orders of magnitude smaller than the signals from protons. To remedy that, parahydrogen-based hyperpolarization techniques can be employed to study N nuclei under the ZULF regime even using unlabeled compounds (natural abundance of 15 N = 0.36%) as it was recently demonstrated .