Fine Structure of Corn Amylose and Amylopectin Fractions with Various Molecular Weights

Characterization of starch molecular structure is vital to comprehending starch structure−function relationships. Four corn starches were fractionated by aqueous leaching into six amylose and five amylopectin fractions. Molecular weight (MW), branching, and chain lengths were determined by size exclusion chromatography and laser light scattering (SEC/LLS); percent crystallinity was determined by X-ray analysis. Amylose fractions had different weight-average molecular weights ( M w = 1.03−4.89E+05) and branching ratios (0.3−1.0), and branching was seemingly inversely correlated with M w . Amylose fractions had similar polydispersities (1.3−1.9) and branch points per chain of 1000 glucose units (0.6−3.0). Degree of polymerization (DP) increased (324−1014) proportionally to M w . For amylopectin, some fractions had different M w values (7.08−9.88E+07) but statistically similar polydispersities (1.1−1.2) and branching ratios (1.5−4.3). The highest M w amylopectin fraction had longer branched chains (long, intermediate, short) than those of its lower M w counterparts. DP for short branched chains was correlated with M w . High to low M w amylopectin fractions with branching values >1.5, and short branched chains (DP = 15−18), showed high crystalline ratios (>28%). Similar MW and branching trends were observed in a previous study using SEC/refractive index and wet chemistry, respectively. These studies suggest that MW characterization possesses the potential for predicting some molecular structures in starch. Keywords: Starch; corn; amylose; amylopectin; molecular weight; branching; size exclusion chromatography; laser light scattering