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

Abstract

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.

Carbohydrate-aromatic interface and molecular architecture of lignocellulose

“…DNP analysis has been applied to various carbohydrate and plant systems using different protocols 75 – 77 . Recently, we have demonstrated that the radical can effectively and homogeneously polarize all molecules in cell wall materials 78 . Lignin had a slightly shorter DNP buildup time (1.8 s) compared with polysaccharides (2.2 s).…”

Solid State NMR a Powerful Technique for Investigating Sustainable/Renewable Cellulose-Based Materials

“…At first, this 2D experiment was considered impossible to achieve, because there is a low natural abundance of 13 C (1.1%) and a lower probability of detecting a cross peak between two carbons. However, now this experiment can be completed using DNP system within 35 h [77]. In cellulose, eight types of glucose units appear: (a) a-e for the glucan chains found in the core of the microfibrils, and (b) f-h for those on the surface.…”

“…MAS-DNP is efficient for the detection of ordered molecules, such as cellulose carbons and part of xylan backbones, but does not detect arabinose sidechains. This is a result of the wide distribution of conformations of these mobile molecules, resulting in a broadening of their signals [77].…”

Metabolomics and health: from nutritional crops and plant-based pharmaceuticals to profiling of human biofluids

“…Metabolomics is also of great interest for food quantification, including molecular based traceability and nutritional value [20,[52][53][54][55][56][57][58]. It was applied for the generation of metabolic markers between two species used as sources of goji berries, such as Lycium barbarum L. and Lycium chinense Mill., which are very similar red ovoid fruits and difficult to discriminate with morphological or molecular markers [20].…”

“…The multi-dimensional solid-state NMR metabolomics has been applied to lignin, cellulose, and hemicellulose quantification through direct measurement without chemical or enzymatic pre-treatment [ 57 ]. Using solid-state NMR spectroscopy revealed increased biosynthesis of cellulose and xylan in O. sativa mutants compared to the wild type [ 58 ].…”