Cellodextrins: structure and dissolution

Cellulose, an essential constituent in the assembly of plant cell walls, is the most abundant and also the (economically) most important natural polymer. Especially the processes of swelling and dissolution of cellulose are fundamental and of high industrial relevance, as for instance in the dissolution of cellulose for chemical derivatization or fiber-making. But despite their importance, these processes were still only marginally understood at a molecular level. With the help of low-molecular weight compounds that mimic the chemical structure of cellulose and based on the isotopic labelling technique, many aspects of the detailed molecular processes during swelling and dissolution of cellodextrins and cellulose were clarified. Now we are much closer to a comprehensive understanding of cellulose processing and of the special nature of cellulose solvents, and we are even on the way to designing cellulose solvents de novo. Because of its superb nature-made properties, cellulose has found use in a large variety of applications, first of all in the pulp and paper and textile industries, but also in food stuffs, pharmaceuticals as drug-delivery systems, cosmetics, and many nanomaterial-based applications. Many structural parameters of the polymer cellulose are not obtained directly, but are derived from or correlated with structures of low-molecular weight model compounds (cellodextrin derivatives) that serve as short-chain cellulose fragment analogues. The elucidation of the structural properties of cellulose is fundamental to understanding its functionality in applications. Within the project, cellodextrin derivatives from disaccharides to tetrasaccharides were synthesized and crystallized, some of them in isotopically perlabeled form (13C). The analogues carried either methyl groups as truncated cellulose chains or cyclohexyl groups as hydrophobic caps. In addition, cellulose was chemically synthesized in isotopically perlabeled form (13-C) according to a cationic ring-opening polymerization approach. In combination with isotopically labeled (15-N, 2-H) cellulose solvents that were synthesized as well, these model compounds allowed for the first time a direct study of the dynamic changes upon swelling and dissolution with a special focus on the hydrogen bond network. The studies showed that swelling is a reversible process of 3-4 stages, connected with cleavage of hydrogen bonds mainly to/from OH-6 and OH-2. Dissolution, by contrast, is irreversible and involves in addition H-bonds to/from OH-3. Cellodextrin / cellulose solvents can be distinguished by (1) the order in which specific H-bonds are broken, (2) the number of distinguishable swelling stages, (3) the number of solvent molecules per anhydroglucose unit (AGU), and (4) the distance of the solvent to the different AGU carbons. Cellulose solvents form non-conventional C-H hydrogen bonds which appeared to be a prerequisite to cellulose dissolution, in addition to common O-H hydrogen bonds, and they surround the cellodextrin / cellulose molecule with a non-exchangeable solvent shell.