Force-induced Conformational Transitions in Single Polysaccharide Molecules by AFM

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Force-induced conformational transitions in single polysaccharide molecules by AFMPyranose ring-based molecules are of extraordinary importance to biological systems (e.g.glucose). Many biologically important polysaccharides are composed of pyranose rings and theyare placed under tensile stress in a wide variety of cellular structures such as the cell wall ofplants or the extracellular matrix (ECM) in animal tissues. Mechanical stress is thought toregulate assembly and physiological properties of these complex elastic systems. Thepolysaccharides respond to that stress by mechanical rearrangements of their components butthe underlying mechanism is not well understood. The simplest view assumes thatpolysaccharides are entropic springs and the pyranose ring structure is typically portrayed asinelastic and locked into a stable conformation. The development of single molecule AFMtechniques allowed for critically examining these views. AFM instruments can stretchmechanically single molecules and have superb length and force resolution. Stretching ofpolysaccharides by AFM revealed that they do not behave as simple entropic springs but thatthey display yielding phenomena. The origin of these elastic deviations was pinpointed to thepyranose ring that was found to undergo, upon stretching, conformational transitions such aschair-boat or chair inversion. These forced transitions change, in a step-wise fashion, theseparation of the glycosidic oxygen atoms, and therefore affect the contour length of thepolysaccharide chain and its elasticity. Axial glycosidic bonds were found to drive thosetransitions by acting as atomic levers. The long-term objective of this proposal is to understand,at the atomic level, the mechanism of these force-induced conformational transitions inpolysaccharides. During the second grant period single molecule AFM techniques will becombined with the tools of computational chemistry to examine in detail force-inducedconformational transitions in the pyranose ring. The mechanical properties of the ring and itsmechanical conformational transitions will be probed by pulling on the ring from variousdirections and by different attachment points. These studies will determine the contribution of each type of the monomer to thecomplex elasticity of a mixed chain and will be valuable in interpreting the molecular elasticity ofmany native polysaccharides. The experimental and theoretical findings of this proposal will beintegrated to develop an AFM-based methodology for identifying individual polysaccharidemolecules in solution from their unique force-extension spectra. Such a methodology will be animportant addition to the arsenal of analytical tools for carbohydrate research.