Studies of the conformation of amylose in solution

Abstract

It has been suggested previously that water is a nearly indifferent solvent for amylose on the basis that the exponent a of the Mark-Houwink equation is very nearly 0.50 and the second virial coefficient A₂ is extremely small. It is shown that such solutions do not show the expected precipitation on cooling and the temperature coefficient of the intrinsic viscosity is small and negative. It is suggested that water is both an indifferent and a nearly athermal solvent for this polymer. Various lines of evidence suggest the polymer to exist as a very stiff coil in this and in other solvents. Variations in the intrinsic viscosity and radius of gyration in various solvents show the importance of skeletal effects in addition to the usual osmotic effects. Application of the Porod-Kratky wormlike coil model leads to unreasonably short persistence lengths if the contour length is based on a fully extended conformation but large and reasonable values of the order 40-70 A. if it is assumed that the basic conformation is the helical one observed in various amylose complexes in the crystalline state. As was previously known, the specific optical rotation decreases sharply in aqueous solutions above approximately pH 12. Accompanying this change in rotation there is first a decrease, then an increase in intrinsic viscosity. This later behavior is strikingly reminiscent of the known behavior of polyglutamic acid in the pH-dependent transition from helix to coil. It is concluded that the conformation is that of a stiff coil with essentially helical backbone contour in neutral solution, and the decrease in rotation and in intrinsic viscosity at high pH results from a partial breakdown of the helical structure with a concomitant increase in backbone flexibility. The increase in viscosity and radius of gyration in extreme alkaline solution is probably due to a polyelectrolyte expansion of the polymer coil. Some other miscellaneous observations which are in accord with this model are also pointed out.