Lyndon Emsley

Through-space contributions to two-dimensional double-quantum J correlation NMR spectra of magic-angle-spinning solids

A routinely used assumption when interpreting two-dimensional NMR spectra obtained with a commonly used double-quantum (DQ) magic-angle-spining (MAS) pulse sequence referred to as the refocused incredible natural abundance double-quantum transfer experiment (INADEQUATE) [A. Lesage, M. Bardet, and L. Emsley, J. Am. Chem. Soc. 121, 10987 (1999)] has been that correlation peaks are only observed for pairs of nuclei with a through-bond connectivity. The validity of this assumption is addressed here by theory, experiment, and computer simulations. If the isotropic chemical shifts of the two nuclei are different and the MAS frequency is far from rotational resonance, the theoretical description demonstrates that DQ correlation peaks are indeed indicative of a J coupling. However, if the isotropic chemical shifts are the same, it is shown that DQ peaks can appear for pairs of nuclei even in the absence of a through-bond J coupling. These peaks appear in the specific case of a pair of nuclei with a nonzero through-space dipole-dipole coupling and chemical shift anisotropy tensors having different principal magnitudes or orientations, provided that the MAS frequency is comparable to or smaller than the chemical shift anisotropies. Experimental 31P spectra recorded on a sample of TiP2O7 and computer simulations show that the magnitude of these anomalous peaks increases with increasing B0 magnetic field and that they decrease with increasing MAS frequency. This behavior is explained theoretically.

Atomic-Resolution Structural Dynamics in Crystalline Proteins from NMR and Molecular Simulation

Solid-state NMR can provide atomic-resolution information about protein motions occurring on a vast range of time scales under similar conditions to those of X-ray diffraction studies and therefore offers a highly complementary approach to characterizing the dynamic fluctuations occurring in the crystal. We compare experimentally determined dynamic parameters, spin relaxation, chemical shifts, and dipolar couplings, to values calculated from a 200 ns MD simulation of protein GB1 in its crystalline form, providing insight into the nature of structural dynamics occurring within the crystalline lattice. This simulation allows us to test the accuracy of commonly applied procedures for the interpretation of experimental solid-state relaxation data in terms of dynamic modes and time scales. We discover that the potential complexity of relaxation-active motion can lead to significant under- or overestimation of dynamic amplitudes if different components are not taken into consideration.

Broadband inversion for MAS NMR with single-sideband-selective adiabatic pulses.

We explain how and under which conditions it is possible to obtain an efficient inversion of an entire sideband family of several hundred kHz using low-power, sideband-selective adiabatic pulses, and we illustrate with some experimental results how this framework opens new avenues in solid-state NMR for manipulating spin systems with wide spinning-sideband (SSB) manifolds. This is achieved through the definition of the criteria of phase and amplitude modulation for designing an adiabatic inversion pulse for rotating solids. In turn, this is based on a framework for representing the Hamiltonian of the spin system in an NMR experiment under magic angle spinning (MAS). Following earlier ideas from Caravatti et al. [J. Magn. Reson. 55, 88 (1983)], the so-called jolting frame is used, which is the interaction frame of the anisotropic interaction giving rise to the SSB manifold. In the jolting frame, the shift modulation affecting the nuclear spin is removed, while the Hamiltonian corresponding to the RF field is frequency modulated and acquires a spinning-sideband pattern, specific for each crystallite orientation.

Single crystal nuclear magnetic resonance in spinning powders.

We present a method for selectively exciting nuclear magnetic resonances (NMRs) from well-defined subsets of crystallites from a powdered sample under magic angle spinning. Magic angle spinning induces a time dependence in the anisotropic interactions, which results in a time variation of the resonance frequencies which is different for different crystallite orientations. The proposed method exploits this by applying selective pulses, which we refer to as XS (for crystallite-selective) pulses, that follow the resonance frequencies of nuclear species within particular crystallites, resulting in the induced flip angle being orientation dependent. By selecting the radiofrequency field to deliver a 180° pulse for the target orientation and employing a train of such pulses combined with cogwheel phase cycling, we obtain a high degree of orientational selectivity with the resulting spectrum containing only contributions from orientations close to the target. Typically, this leads to the selection of between 0.1% and 10% of the crystallites, and in extreme cases to the excitation of a single orientation resulting in single crystal spectra of spinning powders. Two formulations of this method are described and demonstrated with experimental examples on [1-(13)C]-alanine and the paramagnetic compound Sm(2)Sn(2)O(7).