title: Heteronuclear transverse and longitudinal relaxation in AX4 spin systems: application to (15)N relaxations in (15)NH4(+).
abstract: The equations that describe the time-evolution of transverse and longitudinal (15)N magnetisations in tetrahedral ammonium ions, (15)NH4(+), are derived from the Bloch-Wangsness-Redfield density operator relaxation theory. It is assumed that the relaxation of the spin-states is dominated by (1) the intra-molecular (15)N-(1)H and (1)H-(1)H dipole-dipole interactions and (2) interactions of the ammonium protons with remote spins, which also include the contribution to the relaxations that arise from the exchange of the ammonium protons with the bulk solvent. The dipole-dipole cross-correlated relaxation mechanisms between each of the (15)N-(1)H and (1)H-(1)H interactions are explicitly taken into account in the derivations. An application to (15)N-ammonium bound to a 41kDa domain of the protein DnaK is presented, where a comparison between experiments and simulations show that the ammonium ion rotates rapidly within its binding site with a local correlation time shorter than approximately 1ns. The theoretical framework provided here forms the basis for further investigations of dynamics of AX4 spin systems, with ammonium ions in solution and bound to proteins of particular interest.
authors.affiliation: Institute of Structural and Molecular Biology, Division of Biosciences, University College London, London WC1E 6BT, United Kingdom.
authors.forename: Nicolas D
authors.initials: ND
authors.lastname: Werbeck
authors.affiliation: Institute of Structural and Molecular Biology, Division of Biosciences, University College London, London WC1E 6BT, United Kingdom. Electronic address: d.hansen@ucl.ac.uk.
authors.forename: D Flemming
authors.initials: DF
authors.lastname: Hansen
mesh_terms: D000465:Algorithms; D064751:Ammonium Compounds / Q000032:analysis* / Q000737:chemistry*; D003198:Computer Simulation; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008956:Models, Chemical; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013113:Spin Labels
chemical_list: D064751:Ammonium Compounds; D013113:Spin Labels
keywords: AX(4) spin systems; Ammonium; Nuclear spin relaxation