Conformation of the Side Chains of Amylopectin in

Conformation of the Side Chains of Amylopectin in DMSO Determined by Pyrene Excimer Fluorescence and Molecular Mechanics Optimizations Lu Li, Jean Duhamel IPR, Waterloo Institute for Nanotechnology, Department of Chemistry Introduction Starch is composed of two polysaccharides, amylose and amylopectin. Both are complex -glucans whose molecular structures are strongly linked to their utility as energy storage units in plants, ingredients in food, raw material for paper coating additives. However, detailed structural information on polysaccharides is very difficult to obtain, and they are therefore often described either schematically of as “averaged” structures, obscuring the molecular detail. Steady-State Fluorescence Spectroscopy Py* + 1/t M Py A) Amylopectin Amylose <Nblob> Amylopectin 20± 3 Amylose 11± 2 Before Amylopectin B) 26 Å between the helices Good overlap 32 Å between the helices Poor overlap Amylose <kblob×Nblob> (ns-1) 0. 27 ± 0. 02 0. 17 ± 0. 01 Amylose chain conformation in DMSO Two pyrene groups separated by 1 Glucose Good overlap k diff h n + Py Amylopectin side chain conformation in DMSO FBM Results (Py. Py)* 1/t. E Two pyrene groups separated by 6 Glucose Poor overlap If amylose is a random coil in DMSO, <Nblob> must be 7 or less. That is contrary to the results of fluorescence analysis. Density calculated from blob model Density calculated from intrinsic viscosity ØFBM analysis indicates that the sides chains in amylopectin must be densely packed. ØIntrinsic viscosity indicates the whole macromolecule has a much lower density. Solution-Cluster Model for Amylopectin in DMSO Fluorescence Blob Model (FBM) FBM Parameters: –kblob: Excimer formation rate constant in a blob containing one excited and one ground-state pyrene –<n>: Average number of ground-state pyrenes per blob –kexch: Exchange rate constant of ground-state pyrenes between two blobs –Nblob: The blob size in number of monomer units –<kblob Nblob>: Excimer formation rate constant Two pyrene groups separated by 1 Glucose separated by 22 Glucose Good overlap Poor overlap An illustration of the Solution-Cluster (Sol-CL) model where dense clusters of double helices are connected to each other by linear polysaccharide If amylose adopts a helical conformation in DMSO, the more compact geometry leads to a blob size of 11 which is in excellent agreement with FBM analysis. ADVANCED POLYERS FROM PLANET EARTH Acknowledgements Department of Chemistry, 2013