A combination of pyrene excimer fluorescence (PEF), molecular mechanics optimizations (MMOs), intrinsic viscosity, and dynamic light scattering (DLS) was applied to gain structural information about the interior of amylopectin and nanosized amylopectin fragments (NAFs) in solution. Amylopectin and NAFs were randomly labeled with different amounts of 1-pyrenebutyric acid and their PEF was analyzed according to the Fluorescence Blob Model to yield the blob size Nblob representing the number of anhydroglucose monomers found in the volume probed by an excited pyrene. Regardless of the pyrene-labeled NAF or amylopectin sample, Nblob equaled 20. The larger Nblob value of 20 compared to an Nblob value of 10 for helical amylose implied that an excited pyrene attached to a side chain of amylopectin could sense a ground-state pyrene attached to another side chain. MMOs confirmed that an Nblob value of 20 would be compatible with helical oligosaccharide side chains separated by 2-3 nm in the interior of amylopectin or NAFs dispersed in DMSO. These combination of PEF and MMO measurements provided a measure of the density of amylopectin in DMSO which could be compared to that determined from a combination of intrinsic viscosity and DLS measurements. The discrepancy between the two methods led to the proposal that the interior of amylopectin is arranged according to the Solution-Cluster Model (Sol-CL). The Sol-CL model predicts that inside amylopectin, dense clusters of helices are held together by loose linear stretches of oligosaccharide that results in the excluded volume effect. The Sol-CL model predicted that a large NAF would hold more free volume than a small NAF. These predictions were validated by conducting osmotic pressure measurements on the pyrene-labeled NAFs.

Interior of amylopectin in solution probed by fluorescence, molecular mechanics optimizations, dynamic light scattering, and viscosity

2019 Fall National Meeting

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