Barriers to forced transitions in polysaccharides

Recent atomic force microscopy (AFM) experiments have measured the elastic properties of polysaccharides. The results of these experiments suggest that their elastic responses can be understood in terms of the conformational transitions of the monomeric units. In amylose, the unbranched form of starch found in plants, forced extensions by AFM have lead to the conclusion that the basic elastic unit, an alpha -D-glucopyranose ring, extends by a chair-to-boat conformational transition. Forced extensions on cellulose, the main component in plants, Archea (in a modified form), and Eubacteria cell walls, showed no elongation of the monomeric unit, beta -D-glucopyranose. In this study, we used ab initio HF/6-31G* calculations to investigate a series of boat conformations and a twist-boat conformation, which are likely candidates for the elongated structures seen in AFM experiments. Using a linear transit method, we constructed conformational pathways in the form of potential curves which start from the ground-state chair conformation and end at one of these boat or twist-boat structures. For the amylose monomer, we examined three conformational transitions, including two distinct boat forms and a twist-boat form. The results match those reported in the AFM experiments. For the cellulose monomer, we show that only forced compressions are possible whereas extensions are forbidden. The compression has a barrier height of similar to 13 kcal mol(-1). First-passage times are determined for each transition and used to understand the effect of the applied force on the time scales of these ring conformational transitions.