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-(H), as determined by dynamic light scattering--and by the macromolecular features (M-(W), the weight-average molecular weight; R-(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-(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) approximately M(v) relation. Finally, a method was developed to fit corresponding experimental molecular weight distributions with mathematical distributions of two populations. This enable us to determine that the molecular weight distribution for each leached amylose fraction was at least bimodal and to calculate the corresponding M(W) to each component.

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