This does not affect in any other way either the contents of the remaining material in the paper’s main text or in its Appendix. “
“The transverse and longitudinal nuclear spin-relaxation rates, which can be obtained from NMR spectra, are accurate reporters on the interactions
and dynamics of molecules ranging from small organic molecules and ions , ,  and  to large Akt inhibitor macromolecular complexes , ,  and . The observed relaxation rates can be modulated when the nuclei in question exchange between different magnetic environments, which has stimulated the development of theory  and solution-state NMR pulse sequences ,  and  to probe chemical exchange from nuclear relaxation rates and also methods to separate the contributions from exchange and internal dynamics  and . Under physiological conditions, the chemical exchange of the 15NH4+ protons with the bulk solvent is so fast that these protons are barely observed in even simple one-dimensional 1H NMR spectra. Moreover, the exchange rate of the ammonium protons with the bulk solvent is often much faster than the 15N–1H scalar coupling  thus hindering the acquisition of two-dimensional 15N–1H correlation spectra. However, Selleckchem Gefitinib under certain conditions, including acidic
aqueous solutions and when the ammonium ion is bound to proteins  or nucleic acid complexes ,  and , the exchange rate of the ammonium protons becomes sufficiently slow to allow for both detection of the ammonium protons and acquisition Ribonucleotide reductase of 15N–1H correlation spectra. The feasibility of obtaining such 15N–1H correlation maps provides a promising tool for characterising the dynamics of the ammonium ion and for correlating the dynamics with the environments. The ionic radius of the ammonium ion (1.44 Å)
is similar to the radius of the potassium ion (1.33 Å), so that ammonium can be used as a proxy for potassium to probe potassium binding sites , ,  and  in proteins and nucleic acids. As was shown recently , 15NH4+ can be observed even when bound to proteins with molecular weights in excess of 40 kDa, but it is currently not clear whether it is fast reorientation of the ammonium ion within the binding site or favourable cross-correlated relaxation mechanisms that allow for such measurements. Given the development of techniques to probe ammonium ions in proteins and nucleic acids and also considering the interest in probing the regulations of enzymes by monovalent cations in general, it is of interest to derive equations that describe the transverse and longitudinal relaxations of ammonium ions under various conditions. A derivation of the 15N relaxation rates of ammonium ions is presented here, which is based on Bloch-Wangsness-Redfield relaxation theory as well as group theory.