Atomic levers control pyranose ring conformations

Abstract

Atomic force microscope manipulations of single polysaccharide molecules have recently expanded conformational chemistry to include force-driven transitions between the chair and boat conformers of the pyranose ring structure. We now expand these observations to include chair inversion, a common phenomenon in the conformational chemistry of six-membered ring molecules. We demonstrate that by stretching single pectin molecules (1 3 4-linked ␣-D-galactouronic acid polymer), we could change the pyranose ring conformation from a chair to a boat and then to an inverted chair in a clearly resolved two-step conversion: 4 C 1 i boat i 1 C 4 . The two-step extension of the distance between the glycosidic oxygen atoms O 1 and O 4 determined by atomic force microscope manipulations is corroborated by ab initio calculations of the increase in length of the residue vector O 1 O 4 on chair inversion. We postulate that this conformational change results from the torque generated by the glycosidic bonds when a force is applied to the pectin molecule. Hence, the glycosidic bonds act as mechanical levers, driving the conformational transitions of the pyranose ring. When the glycosidic bonds are equatorial (e), the torque is zero, causing no conformational change. However, when the glycosidic bond is axial (a), torque is generated, causing a rotation around COC bonds and a conformational change. This hypothesis readily predicts the number of transitions observed in pyranose monomers with 1a-4a linkages (two), 1a-4e (one), and 1e-4e (none). Our results demonstrate single-molecule mechanochemistry with the capability of resolving complex conformational transitions.Atomic force microscope (AFM) manipulations of single polysaccharide molecules have recently expanded conformational chemistry (1) to include force-driven transitions between the chair and boat conformers of the pyranose ring structure (2). The application of a force to a single molecule will deform it elastically and also induce conformational transitions. Although it is easy to understand the origin of an elastic deformation, the mechanics of the conformational transition is less clear.Pyranose-based sugars have two distinct chair conformations, 4 C 1 and 1 C 4 (3), separated by an energy barrier of Ϸ11 kcal͞mol (4). In addition to the chair conformers, pyranoses have intermediate conformers corresponding to the boat conformation, whose energy is Ϸ5-8 kcal͞mol above the energy of the 4 C 1 chair (5). Thermally driven transitions do occur between these conformers. However, in the absence of an applied force, the most stable conformation of a pyranose is that of the 4 C 1 chair (4-9). Application of a force of Ϸ200 pN to polymers of ␣-D-glucopyranose such as amylose drives a conformational change in the pyranose ring that is evident as a sudden elongation of the molecule, marking a prominent enthalpic component of the elasticity of the molecule (2, 10). This enthalpic component results from an increase in the distance between glycosidic oxygen atoms caused by a for...

Nanoscale visualization and characterization of Myxococcus xanthus cells with atomic force microscopy

“…These in vivo systems are complex and very different from the idealized systems presented in previous polysaccharide forcespectroscopy studies (52,53). In an artificial system, the concentration of polysaccharide on a surface can be precisely controlled so that only one polysaccharide filament can be pulled for each force-spectroscopy measurement.…”

Chair-boat transitions in single polysaccharide molecules observed with force-ramp AFM

“…In the most typical atomic force microscopy (AFM) experiment, a single molecule adsorbed between a substrate and the cantilever tip is extended vertically at a constant rate while the resulting force is measured (2)(3)(4)(5)(6)(7)(8)(9)(10). Although it may seem that these experiments can be completed equally by controlling either the extension or the force applied to the molecule, these two modes can yield quite different results (e.g., see ref.…”

“…Our experiments enhance the ability of single-molecule force spectroscopy to make high-resolution measurements of the conformations of single polysaccharide molecules under a stretching force, making an important addition to polysaccharide spectroscopy. S ingle molecule force spectroscopy has become an important tool to examine the conformations of proteins and polysaccharide molecules under a stretching force (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12). Recent experiments have demonstrated that a force field can trigger conformational changes in these molecules that cannot be observed by traditional NMR or x-ray crystallographic techniques.…”

“…For example, force-extension relationships of polysaccharides such as amylose and dextran (13,14) display a characteristic plateau (2,4) that marks forced transitions of the glucose monomers from their chair conformation to the boat-like conformation (4). Similarly, two distinct plateaus in the force-extension curve of pectin (13,14) report a transition of the galactose rings from the chair conformation to the boat conformation and a subsequent flip of the boat to the inverted chair conformation (5). These observations provide an interesting perspective on the behavior of polysaccharides under mechanical tension (15) and suggest that fingerprints of forced conformational transitions can be used as a means for identifying single polysaccharides molecules in solution (9).…”

“…Recent experiments have demonstrated that a force field can trigger conformational changes in these molecules that cannot be observed by traditional NMR or x-ray crystallographic techniques. Force spectroscopy involves measuring the force required to extend a molecule by a certain amount (2)(3)(4)(5)(6)(7)(8)(9)(10). The data are compiled into a forceextension relationship that gives a characteristic fingerprint for the conformational transitions of the molecule under study.…”