<kblob×Nblob> (ns-1) 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 Amylopectin side chain conformation in DMSO Introduction FBM Results 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. Good overlap Poor overlap 26 Å between the helices 32 Å between Amylopectin Amylose A) B) Before 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. <Nblob> <kblob×Nblob> (ns-1) Amylopectin 20±3 0.27 ± 0.02 Amylose 11±2 0.17 ± 0.01 FBM analysis indicates that the sides chains in amylopectin must be densely packed. Intrinsic viscosity indicates the whole macromolecule has a much lower density. Density calculated from blob model Steady-State Fluorescence Spectroscopy Amylose chain conformation in DMSO h n + Py Py* + (PyPy)* 1/t M E k diff Two pyrene groups separated by 1 Glucose Good overlap Density calculated from intrinsic viscosity Two pyrene groups separated by 6 Glucose Poor overlap Solution-Cluster Model for Amylopectin in DMSO If amylose is a random coil in DMSO, <Nblob> must be 7 or less. That is contrary to the results of fluorescence analysis. 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 An illustration of the Solution-Cluster (Sol-CL) model where dense clusters of double helices are connected to each other by linear polysaccharide Two pyrene groups separated by 1 Glucose Good overlap Two pyrene groups separated by 22 Glucose Poor overlap 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