|French, Alfred - Al|
Submitted to: Journal of the American Chemical Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2008
Publication Date: 11/14/2008
Citation: Yoneda, Y., Mereiter, K., Jaeger, C., Brecker, L., Kosma, P., Rosenau, T., French, A.D. 2008. Van der Waals vs. Hydrogen-Bonding Forces in a Crystalline Analog of Cellotetraose: cyclohexyl 4'-O-cyclohexyl beta-D-cellobioside-cyclohexane solvate. Journal of the American Chemical Society. 130:16678-16690. Interpretive Summary: Cellulose is the main component of cotton fibers as well as all plant cell walls. Despite many years of study, its structures are not understood in sufficient detail. The present work seeks additional understanding by studying a fragment of the cellulose molecule that has been chemically modified. The paper describes these chemical modifications that added cyclohexane rings and provides details of the new compound with X-ray diffraction and nuclear magnetic resonance experiments. It compares those details with similar compounds, including cellulose, as well as with theoretical quantum mechanics calculations. Several new types of intermolecular relationships were found because of the presence of the cyclohexane. This information is primarily of interest to scientists studying cellulose structures in the areas of textile finishing, in paper manufacture, in plant biology and in biofuels.
Technical Abstract: Hydrogen bonding is important in cellulosic and other carbohydrate structures, but the role of non-polar interactions is less understood. Therefore, we synthesized cyclohexyl 4'-O-cyclohexyl '-Dcellobioside (8), a molecule that has two glucose rings and two non-polar cyclohexyl rings. Key to attaching the 4'-O-cyclohexyl group was making the 4'-O, 6'-O-cyclohexylidene ketal. After peracetylation, the cyclohexylidene ketal ring was opened regioselectively, providing 65% of 8 after final deacetylation. Comparison of the crystal structure of 8, as the cyclohexane solvate, with those of cellulose and its fragments, especially cellotetraose with four glucose rings, revealed extensive effects from the cyclohexyl groups. Three conformationally unique molecules (A, B and C) are in the triclinic unit cell of 8, along with two solvent cyclohexanes. When viewed down the crystal’s a-axis, the array of C, A, and B looks like the letter N, with A inclined so that its cyclohexyl groups can stack with those of the reducing ends of the B and C molecules. The lower left and upper right points of the N are stacks of cyclohexyl rings on the non-reducing ends of B and C, interspersed with solvent cyclohexanes. Whereas cellotetraose has antiparallel (up-down) packing, A and B in 8 are oriented “down” in the unit cell while C is “up.” “Down-down-up” (or, alternatively, “up-up-down”) packing is rare for carbohydrates. Other unusual details include O6 in all three staggered orientations: one is tg, two are gg and three are gt, confirmed with CP/MAS 13C NMR. The tg O6 donates a proton to an intramolecular hydrogen bond to O2', opposite to the major schemes in native cellulose I. A similar but novel O6B-H...O2'B hydrogen bond is based on a slightly distorted gg orientation. The hydrogen bonds between parallel molecules are unique, with linkages between O2'A and O2'B, O3'A and O3'B, and O6A and O6B. Other details, such as the bifurcated O3...O5' and ...O6' hydrogen bonds are similar to those of other cellulosic structures. CH... O hydrogen bonds are extensive along the [1-10] line of quarter-staggering. The unusual features described here expand the range of structural motifs to be considered for as-yet undetermined cellulose structures.