Polysaccharide elasticity governed by chair-boat transition of the glucopyranose ring
Many common, biologically important polysaccharides contain pyranose rings made of ®ve carbon atoms and one oxygen atom. They occur in a variety of cellular structures, where they are often subjected to considerable tensile stress1±6. The polysaccharides are thought to respond to this stress by elastic deformation, but the underlying molecular rearrangements allowing such a response remain poorly understood. It is typically assumed, however, that the pyranose ring structure is inelastic and locked into a chair-like conformation. Here we describe single-molecule force measurements 7±12 on individual polysaccharides that identify the pyranose rings as the structural unit controlling the molecule's elasticity. In particular, we ®nd that the enthalpic component of the polymer elasticity10,11,13,14 of amylose, dextran and pullulan is eliminated once their pyranose rings are cleaved.We interpret these observations as indicating that the elasticity of the three polysaccharides results from a force-induced elongation of the ring structure and a ®nal transition from a chair-like to a boat-like conformation. We expect that the force-induced deformation of pyranose rings reported here plays an important role in accommodating mechanical stresses and modulating ligand binding in biological systems.