Heteronuclear transverse and longitudinal relaxation in AX4 spin systems: application to (15)N relaxations in (15)NH4(+).
Bottom Line: 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.
Affiliation: Institute of Structural and Molecular Biology, Division of Biosciences, University College London, London WC1E 6BT, United Kingdom.Show MeSH
Mentions: Evolving anti-phase coherences of AXn spin systems lead to coupling patterns and multiplet structures of the A-spin NMR spectrum that can be intuitively derived from a modified Pascal’s triangle. In the modified Pascal’s triangle presented here, each X spin that is scalar coupled to A and whose spin-state is described with the identity operator splits the NMR line into two lines with equal intensity, while each X spin whose state is described by the longitudinal density element, Xz, splits the NMR line into two lines with opposite intensity (Fig. 3). For the 2N+Hz coherence considered above, the NMR line is therefore first split into two lines with opposite intensity by one Xz operator and subsequently split by three identity operators, which leads to the 1:2:0:−2:−1 multiplet structure. The Appendix A gives a detailed description of using the modified Pascal’s triangle to describe the 15N antiphase spectra of 15NH4+ and Table 5gives a complete list of expected relative intensities for the possible evolutions and detections of antiphase coherences.
Affiliation: Institute of Structural and Molecular Biology, Division of Biosciences, University College London, London WC1E 6BT, United Kingdom.