Polysaccharide elasticity governed by chair-boat transitions of the glucopyranose ring

Many common, biologically important polysaccharides contain pyranose rings made of five carbon atoms and one oxygen atom. They occur in a variety of c ellular structures, where they are often subjected to considerable tensile stress(1-6). The polysaccharides are thought to respond to this stress by e lastic deformation, but the underlying molecular rearrangements allowing su ch a response remain poorly understood. It is typically assumed, however, t hat the pyranose ring structure is inelastic and locked into a chair-like c onformation. Here we describe single-molecule force measurements(7-12) on i ndividual polysaccharides that identify the pyranose rings as the structura l unit controlling the molecule's elasticity. In particular, we find that t he enthalpic component of the polymer elasticity(10,11,13,14) Of amylose, d extran and pullulan is eliminated once their pyranose rings are cleaved. We interpret these observations as indicating that the elasticity of the thre e polysaccharides results from a force-induced elongation of the ring struc ture and a final transition from a chair-like to a boat-like conformation. We expect that the force-induced deformation of pyranose rings reported her e plays an important role in accommodating mechanical stresses and modulati ng ligand binding in biological systems.