Submitted to: Cellulose
Publication Type: Book / Chapter
Publication Acceptance Date: March 1, 2006
Publication Date: June 1, 2006
Citation: French, A.D., Johnson, G.P. Cellulose shapes. In R.M. Brown, I.M. Saxena editors. Cellulose: Molecular and Structural Biology. Springer, Dordrecht. pp. 257-284. Interpretive Summary: The main molecule in cotton and other plant cell walls is cellulose, a long chain of glucose molecules that have been connected. Physical, chemical and biochemical properties of cellulose and other molecules depend on their shapes, and cellulose has been observed in several different shapes. For many years the shapes of cellulose have not been completely understood. This chapter has previously unpublished results that compare the shapes found for the common cellulose forms in recent, reasonably accurate studies. It also predicts as-yet unobserved shapes based on the shapes of related molecules that have been accurately determined, as well as shapes of related molecules that are associated with proteins. A third approach is to use molecular modeling, and agreement among these three approaches is observed. Two major issues regard whether the chain can have an untwisted shape, and when twisted, whether the twist is right- or left-handed. Only very slight right-handed twists seem likely, but untwisted and left-handed twisted forms are both probable.
Technical Abstract: Recent high resolution fiber diffraction studies of four forms of crystalline cellulose are reviewed. All have two-fold screw-axis symmetry, There is little difference among the various chain shapes in the crystal environment, except for the O6 position and the hydrogen bonding schemes. The parallel-antiparallel conversion between cellulose I and mercerized cellulose II is explained. This conversion with retention of fiber form only occurs when the cellulose is inside of primary cell walls. Otherwise, chain-folding occurs, and a model of a fold is discussed. Under other conditions, the cellulose chain is likely to take other shapes as shown with three approaches. In one, a review of the structures of cellulose derivatives and complexes shows shapes ranging between two and three residues per turn. Secondly, these shapes are similar to those that would be extrapolated from shapes found in small relatives of cellulose such as cellobiose and cellotetraose, regardless of whether they are in self-crystals or in complexes with proteins. A final approach shows that this range of shapes has low energy as well. Based on these studies, a left-handed molecular shape with 2.4 glucose residues per turn is proposed as the ideal shape for an isolated molecule.