1. <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