Composite Dipolar Recoupling: Anisotropy Compensated Coherence Transfer in Solid-State NMR

The efficiency of dipole-dipole coupling driven coherence transfer experiments in solid-state NMR spectroscopy of powder samples is limited by dispersion of the orientation of the internuclear vectors relative to the external magnetic field. Here we introduce general design principles and resulting pulse sequences that approach full polarization transfer efficiency for all crystallite orientations in a powder in magic-angle-spinning experiments. The methods compensate for the defocusing of coherence due to orientation dependent dipolar coupling interactions and inhomogeneous radio-frequency fields. The compensation scheme is very simple to implement as a scaffold (comb) of compensating pulses in which the pulse sequence to be improved may be inserted. The degree of compensation can be adjusted and should be balanced as a compromise between efficiency and length of the overall pulse sequence. We show by numerical and experimental data that the presented compensation protocol significantly improves the efficiency of known dipolar recoupling solid-state NMR experiment

Resolution-Enhanced Solid-State NMR ¹³C−¹³C Correlation Spectroscopy by Optimal Control Dipolar-Driven Spin-State-Selective Coherence Transfer

Using optimal control, we have designed spin-state-selective coherence transfer experiments for biological solid-state NMR based on transfer via dipole−dipole coupling interactions. This enables combined coherence transfer and spin-state-selective excitation using very short pulse sequences compared to previous <i>J</i>_CC coupling-based methods, which have not so far been developed for transfer of coherence between spins but only for spin state selection on the origin spin. Furthermore, coherence transfer through the much larger dipole−dipole couplings renders the experiments more forgiving with respect to the demand of very intense proton decoupling during the long excitation periods of <i>J</i>_CC-based methods. The optimal control dipolar-driven spin-state-selective coherende transfer (^OCDDS₃CT) experiment doubles the resolution in the detection dimension of 2D CACO and 3D NCACO experiments, as demonstrated experimentally using uniformly ¹³C,¹⁵N-labeled amino acids, ubiquitin, and fibrils of the SNNFGAILSS fibrillating core of human islet amyloid polypeptide with the FGAIL part labeled with ¹³C and ¹⁵N

Solid-State ¹³C NMR Delineates the Architectural Design of Biopolymers in Native and Genetically Altered Tomato Fruit Cuticles

Plant cuticles on outer fruit and leaf surfaces are natural macromolecular composites of waxes and polyesters that ensure mechanical integrity and mitigate environmental challenges. They also provide renewable raw materials for cosmetics, packaging, and coatings. To delineate the structural framework and flexibility underlying the versatile functions of cutin biopolymers associated with polysaccharide-rich cell-wall matrices, solid-state NMR spectra and spin relaxation times were measured in a tomato fruit model system, including different developmental stages and surface phenotypes. The hydrophilic–hydrophobic balance of the cutin ensures compatibility with the underlying polysaccharide cell walls; the hydroxy fatty acid structures of outer epidermal cutin also support deposition of hydrophobic waxes and aromatic moieties while promoting the formation of cell-wall cross-links that rigidify and strengthen the cuticle composite during fruit development. Fruit cutin-deficient tomato mutants with compromised microbial resistance exhibit less efficient local and collective biopolymer motions, stiffening their cuticular surfaces and increasing their susceptibility to fracture