Force-induced Conformational Transitions in Single Polysaccharide Molecules by AFM

Duke University, Durham NC

Investigators

Abstract

Force-induced conformational transitions in single polysaccharide molecules by AFM Pyranose ring-based molecules are of extraordinary importance to biological systems (e.g. glucose). Many biologically important polysaccharides are composed of pyranose rings and they are placed under tensile stress in a wide variety of cellular structures such as the cell wall of plants or the extracellular matrix (ECM) in animal tissues. Mechanical stress is thought to regulate assembly and physiological properties of these complex elastic systems. The polysaccharides respond to that stress by mechanical rearrangements of their components but the underlying mechanism is not well understood. The simplest view assumes that polysaccharides are entropic springs and the pyranose ring structure is typically portrayed as inelastic and locked into a stable conformation. The development of single molecule AFM techniques allowed for critically examining these views. AFM instruments can stretch mechanically single molecules and have superb length and force resolution. Stretching of polysaccharides by AFM revealed that they do not behave as simple entropic springs but that they display yielding phenomena. The origin of these elastic deviations was pinpointed to the pyranose ring that was found to undergo, upon stretching, conformational transitions such as chair-boat or chair inversion. These forced transitions change, in a step-wise fashion, the separation of the glycosidic oxygen atoms, and therefore affect the contour length of the polysaccharide chain and its elasticity. Axial glycosidic bonds were found to drive those transitions 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 in polysaccharides. During the second grant period single molecule AFM techniques will be combined with the tools of computational chemistry to examine in detail force-induced conformational transitions in the pyranose ring. The mechanical properties of the ring and its mechanical conformational transitions will be probed by pulling on the ring from various directions and by different attachment points. These studies will determine the contribution of each type of the monomer to the complex elasticity of a mixed chain and will be valuable in interpreting the molecular elasticity of many native polysaccharides. The experimental and theoretical findings of this proposal will be integrated to develop an AFM-based methodology for identifying individual polysaccharide molecules in solution from their unique force-extension spectra. Such a methodology will be an important addition to the arsenal of analytical tools for carbohydrate research.