Towards an Understanding of the Fine Structure of Starch Molecules

Abstract

Molecular structures of amylose and amylopectin from several sources have been analyzed in detail by newly developed methods. Amylose content has been suggested to be determined based on the iodine affinities (or blue value) of amylose, amylopectin and starch of each specimen. The value was slightly lower than the value by assuming the iodine affinities of amylose and amylopectin as 18-20 and 0, respectively. The latter value was defined as apparent amylose content. Reducing and non-reducing residues were determined by the modified Park-Johnson's colorimetric assay and the rapid Smith degradation coupled with determination of glycerol by enzymic assay. Number-average molecular weight (or d. p. n) and number-average chain length (c.1. n) were calculated from these values and total carbohydrate measured by phenol-sulfuric acid method. Molar fractions of linear and branched amylose were determined by 1) (number of branch linkage of amylose)/(number of branch linkages of β-limit dextrin) or 2) radioactivities of H3-amylose and H3-limit dextrin. Molecular weight distribution and weight-average molecular weight (or i p, w) of amylose and debranched amylopectin (chain-length distribution) were analyzed by high-performance-gel-exclusion chromatography monitoring with a low-angle laser-light-scattering photometer and a differential refractometer. The mode of chain-linking of amylopectin was analyzed by β, i, p-degradation, which supported the irregular Meyer structure. It was found that the B chains of wheat and rice amylopectins linked increased number of A chains with increased chain length. Some B chains of wheat amylopectin were (37%) found to link no A chains but only B chains. A key factor for the cause of crystalline polymorphism of starch granules was found to be the chain length of amylopectin. The temperature and various materials appear to be secondary modulators. This suggests that molecular structure is the most basic factor on the functional properties of starch, but still we know only a part. We need more effort toward the full understanding of starch.