The Macromolecular Organization of Cellulose and Chitin

Abstract

In this chapter, we will review what is currently known about the physical structures of cellulose and chitin and discuss the implications of these structures for studies of the biosynthesis of these two polymers. Cellulose and chitin are both fiber-forming polymers, and as fibers they serve as the load-bearing components of major groups of natural composite tissues—cellulose in many plant-cell walls and chitin in the skeletal materials of many lower animals. Their chemical structures are very similar: cellulose is poly-β(1→4)-d-glucose and chitin is its 2-acetamido derivative (see Fig. 1); not surprisingly, this leads to similar structures and morphologies. The β(1→4) glucan chain is inherently stiff and extended and must be judged ideal for the formation of fibrous structures. Both polymers are naturally crystalline, and their crystal structures have been the subject of X-ray crystallographic studies for over 60 years. From early studies, it was known that both cellulose and chitin could adopt a number of different polymorphic structures. These have different unit cells, but in all cases the chains have extended ribbonlike conformations, with two pyranose residues repeating in approximately 10.3 Å. The actual packing of these chains and their intermolecular hydrogen bonding proved more difficult to determine, and it has only been in the last decade that these problems have yielded to X-ray investigations. The central question for cellulose was the polarity of adjacent chains: the unit cells contain sections of two chains that could have the same (parallel) or opposite (antiparallel) sense.