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Quantitative solid-state NMR distance measurements in strongly coupled spin systems are often complicated due to the simultaneous presence of multiple noncommuting spin interactions. In the case of zeroth-order homonuclear dipolar recoupling experiments, the recoupled dipolar interaction between distant spins is attenuated by the presence of stronger couplings to nearby spins, an effect known as dipolar truncation. In this article, we quantitatively investigate the effect of dipolar truncation on the polarization-transfer efficiency of various homonuclear recoupling experiments with analytical theory, numerical simulations, and experiments. In particular, using selectively 13C-labeled tripeptides, we compare the extent of dipolar truncation in model three-spin systems encountered in protein samples produced with uniform and alternating labeling. Our observations indicate that while the extent of dipolar truncation decreases in the absence of directly bonded nuclei, two-bond dipolar couplings can generate significant dipolar truncation of small, long-range couplings. Therefore, while alternating labeling alleviates the effects of dipolar truncation, and thus facilitates the application of recoupling experiments to large spin systems, it does not represent a complete solution to this outstanding problem. [ABSTRACT FROM AUTHOR]

Abstract: Partial deuteration is a powerful tool to increase coherence life times and spectral resolution in proton solid-state NMR. The J coupling to deuterium needs, however, to be decoupled to maintain the good resolution in the (usually indirect) 13C dimension(s). We present a simple and reversible way to expand a commercial 1.3mm HCN MAS probe with a 2H channel with sufficient field strength for J-decoupling of deuterium, namely 2–3kHz. The coil is placed at the outside of the stator and requires no significant modifications to the probe. The performance and the realizable gains in sensitivity and resolution are demonstrated using perdeuterated ubiquitin, with selectively CHD2-labeled methyl groups. [Copyright &y& Elsevier]

Abstract: We introduce a second-order dipolar recoupling condition for magic-angle spinning NMR where the isotropic chemical-shift difference of two homonuclear S spins is matched by the rf-irradiation amplitude, or, alternatively, phase-modulation frequency, on a heteronuclear spin. The experiment can be looked at as a mixed rotational and rotary-resonance condition (MIRROR). The effective MIRROR Hamiltonian promotes homonuclear zero-quantum polarization transfer and heteronuclear zero-quantum and double-quantum polarization transfer. The MIRROR recoupling condition is applied to obtain enhanced polarization transfer through spin diffusion (MIRROR-SD) and to establish through-space neighborhood of spins at high MAS frequencies. [Copyright &y& Elsevier]

Despite its central importance for understanding the molecular basis of Alzheimer's disease (AD), high-resolution structural information on amyloid β-peptide (Aβ) fibrils, which are intimately linked with AD, is scarce. We report an atomic-resolution fibril structure of the Aβ1-40 peptide with the Osaka mutation (E22Δ), associated with early-onset AD. The structure, which differs substantially from all previously proposed models, is based on a large number of unambiguous intra- and intermolecular solid-state NMR distance restraints. [ABSTRACT FROM AUTHOR]