X‐ray diffraction studies of addition compounds of amylose with inorganic salts

Abstract

AbstractMany salts in aqueous alcohol solution can be exchanged for the alkali in oriented filaments of alkali amylose. If the filaments are held taut during exchange, orientation is retained, and excellent fiber patterns are produced. By this method, compounds of amylose were obtained with the iodide, bromide, formate, acetate, and propionate of potassium and with sodium bromide and ammonium fluoride. In addition, potassium bicarbonate–amylose was formed by the action of carbon dioxide on moist filaments of potassium hydroxide–amylose. Data on the composition of the amylose–potassium salt filaments indicate stoichiometric compound formation between amylose and salt, the ratio of salt molecules to glucose residues being 1:2 for the iodide, bromide, formate, and tetragonal form of the acetate, and 1:1 for the propionate and the orthorhombic form of the acetate. As shown by x‐ray diffraction patterns, the potassium salts crystallized in two structures. One, based on a tetragonal unit cell, was common to the iodide, bromide, formate, acetate, and bicarbonate; the other, based on an orthorhombic unit, was the stable structure for the acetate and the propionate. Dimensions of the tetragonal unit cells of air‐dried filaments of the iodide and bromide were identical, with a0 = 10.7 and c0 (fiber axis) = 16.1 A. Corresponding dimensions for the formate and acetate were 10.8 and 16.1 A., and those for the bicarbonate were 10.8 and 15.8 A. The orthorhombic structures of the potassium acetate and propionate addition compounds with amylose appeared to be isomorphous, unit cells having a0 = 11.0, b0 = 18.1, c0 (fiber axis) = 17.9 A. for the acetate, and a0 = 11.4, b0 = 18.0, c0 = 17.6 A. for the propionate. The tetragonal structural modification is of particular interest. Observed diffraction maxima were consistent with the enantiomorphous space groups, P4121 and P4321, and this, together with composition and density data, demands that the amylose chains posses fourfold screw symmetry, with four glucose residues corresponding to the fiber repeat period of 16.1 A. Succesive residues in the amylose chain must, therefore, be identical. This is the first x‐ray evidence which has demonstrated the identity of successive residues in the amylose chain.From a Patterson projection of potassium bromide amylose, the positions of the inorganic ions were determined. The Fourier projection, ρz(x,y), computed from Fhk0 with signs determined by the bromide ion, confirmed the location of the inorganic ions and gave a general outline of the packing of the amylose chains.