Direct detection of the formation of V-amylose helix by single molecule force spectroscopy

An important polysaccharide, amylose crystallizes as a regular single left-handed helix from a propanol, butanol, or iodine solution. However, its solution structure remains elusive because amylose does not form molecular solutions in these solvents, and standard spectroscopic techniques cannot be exploited to determine its structure. Using AFM, we forced individual amylose chains adsorbed to a surface to enter these poor solvents and carried out stretch-release measurements on them in solution. In this manner, we directly captured the formation of individual amylose helices induced by butanol and iodine. With an accuracy approaching that of X-ray diffraction on amylose crystals, we determined that the pitch of the helix in solution is 1.3 angstrom/ring. We also directly measured the force driving the formation of the helix in solution to be 50 pN. SMD simulations in explicit butanol reproduced the AFM-measured force-extension curves and revealed that the long plateau feature is caused by the rupture of O(2)(n)-O(6)(n+6) and O(3)(n)-O(6)(n+6) hydrogen bonds and by the unwinding of the helix. We also found that amylose helices formed in iodine solution are more compliant and hysteretic as compared to helices in butanol, which extend/relax reversibly. In iodine solution, the formation of the helix is inhibited by force and limited by the slow kinetics of the amylose-iodine complex. By forcing individual molecules into poor solvents and performing force spectroscopy measurements in solution, our AFM approach uniquely supplements X-ray diffraction and NMR methods for investigating solution conformations of insoluble biopolymers.