Acid hydrolysis and molecular weights of various corn amylopectin and glycogen

Abstract

Amylopectin samples were obtained from mature and immature sweet, dent, and waxy varieties of corn starch samples by dispersing the starch samples with either 6 M LiBr or bufered amyl alcohol solutions. The degree of branching, molecular weights, and radii of gyration were obtained on these amylopectins. The results were explained assuming that Erlander's proposed mechanism for the synthesis of starch from glycogen is valid. The corn amylopectins and an immature sweet corn glycogen sample obtained from Schoch were hydrolyzed at a pH of 4.2 at the refluxing temperature. Samples were taken at various time intervals. The values of Mw obtained from these samples showed that glycogen and amylopectin behave as randomly formed polymers. The rates of hydrolysis obtained in these studies indicate that the large molecules in all corn endosperm amylopectins and glycogens exist in chemical aggregates of four. These chemical aggregates do not appreciably influence the statistical size distribution of the amylopectin and glycogen molecules. A comparison of the properties of a sweet corn glycogen sample and an immature sweet corn amylopectin sample indicates strongly that glycogen is a precursor to starch.

Starch Biosynthesis. II. The Statistical Model for Amylopectin and Its Precursor Plant Glycogen

“…1 and 3). Nevertheless, many structures are present as seen previously from the large ratio of Mw/Mn [9,10] for both aggregated and disaggregated amylopectins and also as seen from the diverse structures observed for clusters [30]. Thus it is the statistical model, modified as in the Meyer model with hidden inner A-chains, that best represents amylopectin.…”

“…Also the lower molecular weight values for the amylopectin could be associated with the conversion of glycogen into starch. That is, the change in extrapolated M w values from 4.58 ´10 6 for glycogen to 3.93 ´10 6 for the amylopectin (or a loss of about 14 % of the glycogen for the production of amylose) is in line with the removal of exterior A-chains to change the phytoglycogen precursor into amylopectin and amylose [10]. In contrast, dent corn amylopectin does not disaggregate during acid hydrolysis [36].…”

“…Thus again this suggests a difference in structures for waxy and dent corn amylopectins. In addition, it was observed [10] with immature sweet corn amylopectin and its plant glycogen that acid hydrolysis studies showed a dramatic drop in the molecular weights of this amylopectin and glycogen. That is, the ratios of the original to the extrapolated weight average molecular weight values were for 14th day (immature) sweet corn amylopectin 15.8 ´10 6 / 3.93 ´10 6 = 4.02 and for its glycogen were 18.5 10 6 / 4.58 ´10 6 = 4.04.…”

“…In the examination of the properties of amylopectin, it was previously concluded [9,10] that although the branching structure of amylopectin appeared to be random as in a statistical model, the size distribution of the linear chains of amylopectin did not follow the broad size distribution expected for an A-R-B 2 type condensation polymer. Hence, better agreement was obtained with experimental results when the size distribution of the linear chains of amylopectin were restricted by assuming rods of ten or so glucose units in length to represent the chains between branch points.…”

“…This much narrower size distribution was supported in the previous paper [11] by the examination of the polymodal behavior of debranched amylopectin chains, where it was shown that these linear chains can be correlated with several distinct polymers, each of which correlates with the very narrow Poisson size distribution. Thus the previously used [9,10] rod structures were actually a rough representation of the very narrow Poisson size distribution for a statistically branched polymer. In this paper an examination of the properties of the precursor glycogen and its amylopectin and amylose will be made in order to determine how starch is made in various plant tissues, how the statistical model can explain the relationship between the number of removed branches and yield of amylose in the conversion of glycogen to amylopectin, and how genetic variations can alter this mechanism of starch synthesis.…”