Atomic resolution of cotton cellulose structure enabled by dynamic nuclear polarization solid-state NMR

Abstract

The insufficient resolution of conventional methods has long limited the structural elucidation of cellulose and its derivatives, especially for those with relatively low crystallinities or in native cell walls. Recent 2D/3D solid-state NMR studies of 13C uniformly labeled plant biomaterials have initiated a re-investigation of our existing knowledge in cellulose structure and its interactions with matrix polymers but for unlabeled materials, this spectroscopic method becomes impractical due to limitations in sensitivity. Here, we investigate the molecular structure of unlabeled cotton cellulose by combining natural abundance 13C-13C 2D correlation solid-state NMR spectroscopy, as enabled by the sensitivity-enhancing technique of dynamic nuclear polarization (DNP), with statistical analysis of the observed and literature-reported chemical shifts. The atomic resolution allows us to monitor the loss of Iα and Iβ allomorphs and the generation of a novel structure during ball-milling, which reveals the importance of large crystallite size for maintaining the Iα and Iβ model structures. Partial order has been identified in the “disordered” domains, as evidenced by a discrete distribution of well-resolved peaks. This study not only provides heretofore unavailable high-resolution insights into cotton cellulose but also presents a widely applicable strategy for analyzing the structure of cellulose-rich materials without isotope-labeling. This work was part of a multi-technique study of ball-milled cotton described in the previous article in the same issue.

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

“…Recent high-resolution ssNMR studies support that these signals originate from the interior and surface glucan chains in cellulose microfibrils rather than the longitudinally distributed domains of crystalline and disordered domains [ 1 ]. The cellulose microfibrils in intact plant cell walls were found to adapt a unique structure that differs from the model Iα and Iβ allomorphs [ 45 , 46 ], which were later found to only exist in highly crystalline cellulose with large crystallites, for example, the cotton balls [ 23 ]. The surface residues have extensive interactions with water molecules and matrix polymers and adapt a gauche-trans conformation, while the interior chains adapt trans-gauche conformation and exhibit substantially weaker contacts with other molecules [ 46 – 48 ].…”

Molecular orientation distribution of regenerated cellulose fibers investigated with rotor synchronized solid state NMR spectroscopy

“…Additional results are included based on polarized Raman spectroscopy and X-ray scattering on a single fiber published elsewhere (Svenningsson et al 2019 poly(ethylene terephthalate). Further investigations can be made by expanding ROSMAS to 3D experiment with PASS sideband separation (Song et al 1996;Antzutkin et al 1994), enabled by the more resent DNP signal enhancement methods (Takahashi et al 2012;Gutmann et al 2017;Kirui et al 2019). A 13 C T 1q relaxation filtering to remove amorphous signal was also explored, however it was unsuccessful due to extremely short cellulose T 1q relaxation times.…”

Preparation of Cellulose Insulating Paper with Low Dielectric Constant by BTCA Esterification Crosslinking

“…The polar groups (hydroxyl) on the molecular chains of cellulose and the flexibility of the chains in the amorphous region lead to the strong polarity of cellulosic paper. [ 16–18 ] Adopting a chemical treatment to substitute hydroxyl groups is a feasible strategy to decrease the dielectric constant of kraft paper as long as the mechanical properties (tensile strength, elongation at break, and Young’s modulus) are guaranteed. [ 19 ] Hence, a new chemical treatment method that differs from acetylation is in demand.…”