Location: Cotton Structure and Quality ResearchTitle: Electron (charge) density studies of cellulose models) Author
Submitted to: Cellulose
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/31/2013
Publication Date: 9/1/2013
Citation: French, A.D., Concha, M.C., Dowd, M.K., Stevens, E.D. 2013. Electron (charge) density studies of cellulose models. Cellulose. 21:1051-1063 DOI 10.1007/s10570-013-0042-0. Interpretive Summary: Cotton fibers are composed of many small crystalline particles of cellulose, the carbohydrate molecule that is part of all plant cell walls. These crystallites provide major clues for understanding the behavior of cellulose, such as its insolubility in water or relatively low reactivity to enzymes. However, the forces that hold the crystal together are not well understood. The present work studies small subsets of the crystals, with just two molecules, as a well-structured cloud of electron density. This cloud, furnished by a quantum mechanics calculation, can be analyzed to show which atoms on one molecule are interacting with weak forces on atoms on the other molecule, including hydrogen bonds and van der Waals forces. Indicators of the strength of these weak forces (as well as the normal chemical bonds) are furnished by the theoretical method of charge density analysis. This paper catalogs these interactions and their strength indicators. The effects of modeling the molecules in water were also reported. This information is primarily of interest to scientists trying to understand issues of cellulose structure, including biologists, chemists and various engineers.
Technical Abstract: Introductory material first describes electron density approaches and demonstrates visualization of electron lone pairs and bonding as concentrations of electron density. Then it focuses on the application of Bader’s Quantum Theory of Atoms-in-Molecules (AIM) to cellulose models. The purpose of the work is to identify the various interactions that stabilize cellulose structure. AIM analysis aids study of non-covalent interactions, especially those for which geometric criteria are not well established. The models were in the form of pairs of cellotriose molecules, methylated at the O1 and O4 ends. Based on the unit cell of cellulose I', there were corner-corner, and center-center pairs that correspond to (200) sheets, and corner-center pairings that corresponded to (1-10) and (110) stacks. AIM analysis (or charge-density topology analysis) was applied before and after minimization in vacuum and in continuum solvation. Besides the conventional O-H'O hydrogen bonds, all of which were known from geometric criteria, C-H'O hydrogen bonds (some previously reported), and some O'O and H'H interactions were found. Non-covalent bonds in the (200) sheets were retained after the minimizations with the exception of a weak, bifurcated O6-H'O2'' bond that was found only in the center-center pair model. Also, the O6'O4 interactions on the reducing ends of the triosides did not survive minimization. There were 12 non-covalent bonds in both the pairs of molecules along the (110) and (1-10) planes, but the AIM parameters indicated the bonds between the pairs in the (110) plane were weaker. Intra-molecular O-H'O hydrogen bonds survived in these minimized pairs, but the relative chain alignments usually did not.