Article,
DEPOLYMERIZATION OF DOUBLE-STRANDED XANTHAN BY ACID-HYDROLYSIS - CHARACTERIZATION OF PARTIALLY DEGRADED DOUBLE STRANDS AND SINGLE-STRANDED OLIGOMERS RELEASED FROM THE ORDERED STRUCTURES
Macromolecules, 26 (22): 6111 -- 6120 (October 1993)
Abstract
Double-stranded xanthan was depolymerized by partial acid hydrolysis
to produce samples where M(W) ranged from 6 x 10(6) to 7 x 10(4).
These were characterized with respect to chemical composition in
the side chains, molecular weight (light scattering), molecular weight
distribution (HPLC, gel filtration), intrinsic viscosity, and conformation
(optical rotation). Additional information about molecular size and
conformation was obtained by electron microscopy. For each sample
a fraction consisting of depolymerized, double-stranded species could
be obtained. Their conformational properties were largely analogous
to undegraded xanthan, despite the reduced M(W) and the changes in
the side chains. The chain flexibility increased with degradation
time, as indicated by a gradual reduction in persistence length (q),
calculated from electron micrographs and from the eta-M(W) relationship
using the wormlike chain model. As the degradation proceeded, a second
fraction, consisting of shorter and conformationally disordered fragments,
constituted a progressively larger part of the population. This is
in accordance with the model requiring the minimum chain length (DP(min))
to take part in an ordered, double-stranded structure in a cooperative
manner. Their release partly explains the increased flexibility of
the remaining double-stranded species, since their release exposes
local single-stranded and hence more flexible, regions. The experimental
M(W) data are in qualitative accordance with a Monte Carlo model
for the apparent depolymerization kinetics of double-stranded polymers.
The model also predicts a relationship between M(W) and the amount
of disordered fragments which depends on DP(min). Comparison to experimental
data gives a minimum estimate of DP(min) in the range of 10-15 glucose
residues.
Tags
- ;;
- conformation;
- dilute-solution;
- dissociation;
- electron-microscopy;
- gelation
- helix;
- intrinsic-viscosity;
- light-scattering;
- polysaccharide;
- scleroglucan;
- [isi:]
