Starch acetate molecular-weight determinations

Starch acetate solutions, because of their stability, are suited for osmotic pressure measurements and have been employed with considerable success for direct molecular weight determinations. 

Staudinger and Husemann determined the osmotic pressure of solutions of a potato starch acetate which had been fractionated into four parts by precipitation of its chloroform solution with ether. The molecular weights of the fractions ranged from 45,000 to 275,000. All of the fractions were soluble in chloroform, but fractions of low molecular weight were also soluble in acetone. For various concentrations of solute in either chloroform or acetone, the osmotic pressure did not increase in direct proportion to the solute concentration, but the deviation from van t Hoff s law was the smallest in the case of the acetone solutions. Osmotic pressure measurements on amylose and amylopectin tri-acetates dissolved in tetrachloroethane have been made by Meyer and co-workers, who have deduced molecular weights for these substances of approximately 78,000 and 300,000, respectively (see above discussion of the purity of these fractions). 

Diffusion measurements " have been made on starch acetates dissolved in such solvents as acetic acid and acetic anhydride, and molecular weight values up to 224,700 have been indicated. Unfortunately, diffusion constants of starch fractions have been investigated only to a very limited extent. Diffusion measurements probably would provide evidence of value in the determination of the physical shape of the molecules of the two starch fractions the rate of diffusion of the amylopectin acetate should conform most nearly to the normal hydrodynamic predictions for spherical macromolecules. 

---