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

Authors: KIRUI, ALEX - Louisiana State University; LING, ZHE - Beijing Forestry University; KANG, XUE - Louisiana State University; DICKWELLA WIDANAGE, MALITHA - Louisiana State University; MENTINK-VIGLER, FREDERIC - National High Magnetic Field Laboratory; French, Alfred - Al; WANG, TUO - Louisiana State University

Submitted to: Cellulose
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/21/2018
Publication Date: 2/17/2019
Citation: Kirui, A., Ling, Z., Kang, X., Dickwella Widanage, M.C., Mentink-Vigler, F., French, A.D., Wang, T. 2019. Atomic resolution of cotton cellulose structure enabled by dynamic nuclear polarization solid-state NMR. Cellulose. 26(1):329-339. https://doi.org/10.1007/s10570-018-02230-x.
DOI: https://doi.org/10.1007/s10570-018-02230-x

Interpretive Summary: Cotton fiber is mostly composed of the cellulose molecule, and the physical properties of the cotton fiber are thought to depend in part on the degree to which the cellulose molecule is crystallized. Such physical traits include strength, ability to absorb moisture, and biodegradability. However, the extent of crystallinity is not known with certainty, partly because different experimental approaches measure different properties and depend on underlying assumptions. Furthermore, the structural nature of the non-crystalline (amorphous) is not understood either. The present work involved a new type of nuclear magnetic resonance (NMR) spectroscopy (dynamic nuclear polarization) that can greatly amplify the signals from the carbon atoms and permit higher resolution, enabling the use of more sophisticated two dimensional NMR methods. It was employed on a series of cotton fiber samples that had been subjected to decrystallization by ball milling. Surprisingly, the sample milled for two hours had new, well-resolved peaks that indicated that a specific type of new structure was created, as well as the expected decrease in the peaks arising from the original cotton fiber. These cellulose molecules in ball-milled samples give NMR signals similar to those on surfaces of the considerably smaller crystals in most plants. The results indicate that conventional (much less costly) nuclear magnetic resonance results should be evaluated in a new way.

Technical 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 reinvestigation f 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, with statistical analysis of the observed and literaturereported chemical shifts. The atomic resolution allows us to monitor the loss of Ia and Ib allomorphs and the generation of a novel structure during ball-milling, which reveals the importance of large crystallite size for maintaining the Ia and Ib model structures. Partial order has been identified in the "disordered" domains, as evidenced by a discrete distribution of wellresolved 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.