Molecular weight distribution of amylose fractions obtained by aqueous leaching of corn starch

Ultracentrifugation was used to clarify seven amylose fractions obtained by aqueous leaching of corn starch by 5 degrees C steps over a temperature range of 65-95 degrees C. Each fraction was characterized by its hydrodynamic radius <(R)over bar (H)>, as determined by dynamic light scattering - and by the macromolecular features (<(M)over bar (w)>, the weight-average molecular weight; <(R)over bar (G)>, the z-average gyration radius, MWD, the molecular weight distribution) determined by size-exclusion chromatography coupled on line with multi-angle laser light scattering using 0.1 M KOH as solvent. This procedure provided new insights into the leaching process. In particular, a key temperature (85 degrees C) was noted, after which amylose <(M)over bar (w)>, increased markedly from 8 x 10(4) -1.4 x 10(5) to 3 x 10(5) -3.5 x 10(5) g . mol(-1). Molecular weight dependences of macromolecular sizes were consistent with the behaviour of an expanded chain in good solvent with in particular a hydrodynamic coefficient v within the 0.6-0.7 range for the R(G) similar to M(v) relation. Finally, a method was developed to fit corresponding experimental molecular weight distributions with mathematical distributions of two populations. This enabled us to determine that the molecular weight distribution for each leached amylose fraction was at least bimodal and to calculate the corresponding <(M)over bar (w)> to each component.