Dipolar Order Parameters in Large Systems With Fast Spinning

Order parameters are a useful tool for quantifying amplitudes of molecular motions. Here we measure dipolar order parameters by recoupling heteronuclear dipole-dipole couplings under fast spinning. We apply symmetry based recoupling methods to samples spinning under magic angle at 60 kHz by employing a variable flip angle compound inversion pulse. We validate the methods by measuring site-specific 15N-1H order parameters of a microcrystalline protein over a small temperature range and the same protein in a large, precipitated complex with antibody. The measurements of the order parameters in the complex are consistent with the observed protein undergoing overall motion within the assembly.

Boosting Dissolution-Dynamic Nuclear Polarization by Multiple-Step Dipolar Order Mediated 1H->13C Cross-Polarization

<p>Dissolution-dynamic nuclear polarization can be boosted by employing multiplecontact cross-polarization techniques to transfer polarization from 1H to 13C spins. The method is efficient and significantly reduces polarization build-up times, however, it involves high-power radiofrequency pulses in a superfluid helium environment which limit its implementation and applicability and prevent a significant scaling-up of the sample size.</p> <p>We propose to overcome this limitation by a stepwise transfer of polarization using a lowenergy and low-peak power radiofrequency pulse sequence where the 1H®13C polarization transfer is mediated by a dipolar spin order reservoir. An experimental demonstration is presented for [1-13C]sodium acetate. A solid-state 13C polarization of ~43.5% was achieved using this method with a build-up time constant of ~5.1 minutes, leading to a ~28.5% 13C polarization in the liquidstate after sample dissolution. The low-power multiple-step polarization transfer efficiency with respect to the most advanced and highest-power multiple-contact cross-polarization approach was found to be ~0.69.</p>

Boosting Dissolution-Dynamic Nuclear Polarization by Multiple-Step Dipolar Order Mediated 1H->13C Cross-Polarization

<p>Dissolution-dynamic nuclear polarization can be boosted by employing multiplecontact cross-polarization techniques to transfer polarization from 1H to 13C spins. The method is efficient and significantly reduces polarization build-up times, however, it involves high-power radiofrequency pulses in a superfluid helium environment which limit its implementation and applicability and prevent a significant scaling-up of the sample size.</p> <p>We propose to overcome this limitation by a stepwise transfer of polarization using a lowenergy and low-peak power radiofrequency pulse sequence where the 1H®13C polarization transfer is mediated by a dipolar spin order reservoir. An experimental demonstration is presented for [1-13C]sodium acetate. A solid-state 13C polarization of ~43.5% was achieved using this method with a build-up time constant of ~5.1 minutes, leading to a ~28.5% 13C polarization in the liquidstate after sample dissolution. The low-power multiple-step polarization transfer efficiency with respect to the most advanced and highest-power multiple-contact cross-polarization approach was found to be ~0.69.</p>

Solid-state NMR of unlabeled plant cell walls: high-resolution structural analysis without isotopic enrichment

Background Multidimensional solid-state nuclear magnetic resonance (ssNMR) spectroscopy has emerged as an indispensable technique for resolving polymer structure and intermolecular packing in primary and secondary plant cell walls. Isotope (13C) enrichment provides feasible sensitivity for measuring 2D/3D correlation spectra, but this time-consuming procedure and its associated expenses have restricted the application of ssNMR in lignocellulose analysis. Results Here, we present a method that relies on the sensitivity-enhancing technique Dynamic Nuclear Polarization (DNP) to eliminate the need for 13C-labeling. With a 26-fold sensitivity enhancement, a series of 2D 13C–13C correlation spectra were successfully collected using the unlabeled stems of wild-type Oryza sativa (rice). The atomic resolution allows us to observe a large number of intramolecular cross peaks for fully revealing the polymorphic structure of cellulose and xylan. NMR relaxation and dipolar order parameters further suggest a sophisticated change of molecular motions in a ctl1 ctl2 double mutant: both cellulose and xylan have become more dynamic on the nanosecond and microsecond timescale, but the motional amplitudes are uniformly small for both polysaccharides. Conclusions By skipping isotopic labeling, the DNP strategy demonstrated here is universally extendable to all lignocellulose materials. This time-efficient method has landed the technical foundation for understanding polysaccharide structure and cell wall assembly in a large variety of plant tissues and species.

Pulse sequence and sample formulation optimization for dipolar order mediated 1H→13C cross-polarization

Optimization of a dipolar cross-polarization (dCP) sequence increases the efficiency of 1H polarization transfers to 13C spins in a diverse range of molecular candidates with 13C polarizations surpassing 30%.

Emergent helical texture of electric dipoles

Long-range ordering of magnetic dipoles in bulk materials gives rise to a broad range of magnetic structures, from simple collinear ferromagnets and antiferromagnets, to complex magnetic helicoidal textures stabilized by competing exchange interactions. In contrast, dipolar order in dielectric crystals is typically limited to parallel (ferroelectric) and antiparallel (antiferroelectric) collinear alignments of electric dipoles. Here, we report an observation of incommensurate helical ordering of electric dipoles by light hole doping of the quadruple perovskite BiMn7O12. In analogy with magnetism, the electric dipole helicoidal texture is stabilized by competing instabilities. Specifically, orbital ordering and lone electron pair stereochemical activity compete, giving rise to phase transitions from a nonchiral cubic structure to an incommensurate electric dipole and orbital helix via an intermediate density wave.

Dipolar order mediated ¹H → ¹³C cross-polarization for dissolution-dynamic nuclear polarization

Magnetic resonance imaging and spectroscopy often suffer from a low intrinsic sensitivity, which can in some cases be circumvented by the use of hyperpolarization techniques. Dissolution-dynamic nuclear polarization offers a way of hyperpolarizing 13C spins in small molecules, enhancing their sensitivity by up to 4 orders of magnitude. This is usually performed by direct 13C polarization, which is straightforward but often takes more than an hour. Alternatively, indirect 1H polarization followed by 1H→13C polarization transfer can be implemented, which is more efficient and faster but is technically very challenging and hardly implemented in practice. Here we propose to remove the main roadblocks of the 1H→13C polarization transfer process by using alternative schemes with the following: (i) less rf (radiofrequency) power; (ii) less overall rf energy; (iii) simple rf-pulse shapes; and (iv) no synchronized 1H and 13C rf irradiation. An experimental demonstration of such a simple 1H→13C polarization transfer technique is presented for the case of [1-13C]sodium acetate, and is compared with the most sophisticated cross-polarization schemes. A polarization transfer efficiency of ∼0.43 with respect to cross-polarization was realized, which resulted in a 13C polarization of ∼8.7 % after ∼10 min of microwave irradiation and a single polarization transfer step.