Atomic cranks and levers control sugar ring conformations

Abstract

In this paper we review the conformational analysis of sugar rings placed under tensionduring mechanical manipulations of single polysaccharide molecules with the atomic forcemicroscope and during steered molecular dynamics simulations. We examine the role ofvarious chemical bonds and linkages between sugar rings in inhibiting or promoting theirconformational transitions by means of external forces. Small differences in the orientationof one chemical bond on the sugar ring can produce significantly different mechanicalproperties at the polymer level as exemplified by two polysaccharides: cellulose, composed ofβ--linked D-glucose, and amylose, composed ofα--linked D-glucose. In contrast toβ-glucose rings, which are mechanically stable and produce simple entropic elasticity of the chain,α-glucose rings flip under tension from their chair to a boat-like structure and thesetransitions produce deviations of amylose elasticity from the freely jointed chain model.We also examine the deformation of two mechanically complementary -linked polysaccharides: pustulan, aβ--linked glucan, and dextran, a α--linked glucan. Forced rotations about theC5–C6 bondsgovern the elasticity of pustulan, and complex conformational transitions that involve simultaneousC5–C6 rotations and chair–boat transitions govern the elasticity of dextran. Finally, we discuss thelikelihood of various conformational transitions in sugar rings in biological settings andspeculate on their significance.