|Yokoyama, Wallace - Wally|
Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/7/2005
Publication Date: 2/22/2006
Citation: Zhong, F., Yokoyama, W.H., Wang, Q., Shoemaker, C.F. 2006. Rice Starch, Amylopectin, and Amylose: Molecular Weight and Solubility in Dimethyl Sulfoxide-Based Solvents. Journal of Agricultural and Food Chemistry. 10.1021:A-G. Interpretive Summary: Starch is the most abundant component of plant foods for humans and domesticated animals, and has been the object of basic and applied research for centuries. Amylopectin, the branched form of starch, is the predominant form of starch and makes up about 75% of total starch in most grains. The molecular weight of amylopectin has been reported to range from 50-500 million Daltons. The wide range of reported values is mainly due to the difficulty in solubilizing amylopectin while reducing entanglements of the branches. This study demonstrates that aqueous fractionation of starch followed by solubilization in a solution of dimethylsulfoxide containing 50 mM LiBr resulted in amylopectin of 40-50 million Daltons under mild conditions. The ability to accurately characterize amylopectin size may be useful to develop plants with a range of starch processing characteristics.
Technical Abstract: Dimethyl sulfoxide (DMSO), with either 50 mM LiBr, 10% water or both, was used as solvent for multi-angle laser-light scattering (MALLS) batch mode analysis of rice starch, and amylopectin and amylose weight-average molecular weight (Mw). DMSO/50 mM LiBr was a better solvent for these measurements than was DMSO/10% water, based on this solvent’s ability to dissolve starch and to reduce the size of starch aggregates. Starch concentration decreased and amylose:amylopectin ratio increased when starch suspended in DMSO was centrifuged or filtered prior to size-exclusion chromatography (SEC)-MALLS analysis. A higher amylose:amylopectin ratio made starch more soluble, and the higher this ratio, the lower the Mw of eluted amylopectin. For SEC analysis of Mw, fractions of starch amylopectin and amylose dispersed in DMSO-based solvents yielded better results than starch dispersed directly into the solvents, because dispersion of these fractions decreased starch aggregation. When these two starch components were fractionated and then dissolved separately in DMSO/50 mM LiBr, the Mw of dispersed amylopectin ranged from 40 to 50 million, and that of amylose was ca. 3 million, whereas starch from three rice varieties of varying amylose content ranged from 60 to 130 million. We recommend that SEC evaluation of amylopectin and amylose be accomplished with fractionated samples as in this study; such evaluations were superior to evaluations of natural mixtures of amylopectin and amylose.