|Millard, Merle - RETIRED ARS|
|Wolf, Walter - RETIRED ARS|
|Dintzis, Frederick - RETIRED ARS|
Submitted to: Carbohydrate Polymers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 11, 1999
Publication Date: N/A
Interpretive Summary: Starch is a mixture of two components known as amylose and amylopectin. The size of either type of molecule impacts the properties of the starch. It is, therefore, important to be able to accurately measure the molecular size of these materials. The size of amylopectin molecules is difficult to measure because they are very large, and there is controversery among various researchers as to its magnitude. We have employed a method which combines two independent techniques, ultracentrifugation and dynamic light scattering, to measure the molecular size of amylopectin from cornstarch. The value obtained agrees well with results from other techniques. The results will be useful to researchers who need to understand the size of undegraded amylopectin and how it changes during processes such as cooking.
Technical Abstract: Static light scattering of high amylopectin waxy maize starch gently dispersed in 90% dimethyl sulfoxide-water yielded a weight average molecular weight Mw and radius of gyration Rg of 560 x 10**6 g/mol and 342 nm, respectively. To obtain an independent hydrodynamic characterization of these solutions, we measured the sedimentation coefficient for the main component in an analytical ultracentrifuge. The value of s**o, the infinite dilution sedimentation coefficient, was 199 S. The translational diffusion coefficient D**o in very dilute solutions was measured by dynamic light scattering at 90 degrees and found to be 2.33 x 10**-9 cm**2/sec. An effective hydrodynamic radius Rh was calculated from this diffusion constant using the Stokes-Einstein equation and found to be 348 nm. The structure-related parameter Rho = Rg/Rh was calculated to be 0.98. The weight average molecular weight calculated from the Svedberg equation using the values measured for s**o and D**o was 593 x 10**6 g/mol This result is in reasonable agreement with the light scattering results. Because light scattering results are subject to experimental errors due to the possibility of dust contamination, the presence of microgel or aggregates and the questionable applicability of light scattering theory to interpret results for macromolecular sizes approaching the wave length of light used as a source for scattering, it is advisable to have corroborating hydrodynamic data when possible to further validate light scattering results in this very high molecular weight range.