Atomic levers control pyranose ring conformations

Atomic force microscope manipulations of single polysaccharide molecules ha ve recently expanded conformational chemistry to include force-driven trans itions between the chair and boat conformers of the pyranose ring structure . We now expand these observations to include chair inversion, a common phe nomenon in the conformational chemistry of six-membered ring molecules. We demonstrate that by stretching single pectin molecules (1 --> 4-linked alph a-D-galactouronic acid polymer), we could change the pyranose ring conforma tion from a chair to a boat and then to an inverted chair in a clearly reso lved two-step conversion: C-4(1) reversible arrow boat reversible arrow C-1 (4). The two-step extension of the distance between the glycosidic oxygen a toms O-1 and O-4 determined by atomic force microscope manipulations is cor roborated by ab initio calculations of the increase in length of the residu e vector O1O4 On chair inversion. We postulate that this conformational cha nge results from the torque generated by the glycosidic bonds when a force is applied to the pectin molecule. Hence, the glycosidic bonds act as mecha nical levers, driving the conformational transitions of the pyranose ring. When the glycosidic bonds are equatorial (e), the torque is zero, causing n o conformational change. However, when the glycosidic bond is axial (a), to rque is generated, causing a rotation around C-C bonds and a conformational change; This hypothesis readily predicts the number of transitions observe d in pyranose monomers with 1a-4a linkages (two), 1a-4e (one), and 1e-4e (n one). Our results demonstrate single-molecule mechanochemistry with the cap ability of resolving complex conformational transitions.