Article,
Metastable, partially depolymerized xanthans and rearrangements toward perfectly matched duplex structures
Macromolecules, 29 (8): 2939 -- 2944 (April 1996)
Abstract
The depolymerization of double-stranded xanthan by H2O2/Fe2+ (at 20
degrees C) leads to the formation of a metastable duplex stabilized
by partially overlapping chains. A heat treatment well above the
conformational melting temperature, T-m, followed by cooling below
T-m resulted in a relatively large decrease in the weight average
molecular weight, M(w), as shown by size exclusion chromatography.
A corresponding decrease in the contour length of the individual
duplexes was demonstrated by electron microscopy. The decrease is
primarily ascribed to a rearrangement of chain fragments which dissociate
from the duplexes (T > T-m) and reassociate to form more perfectly
matched duplexes upon cooling below T-m. The decrease in (M) over
bar(w) was most pronounced (factor 7-9) for slightly depolymerized
xanthan whereas this factor decreased for more extensively depolymerized
xanthan. A second heat treatment did not result in any further change
in the molecular weight distribution, indicating that depolymerization
did not occur to any significant extent. The experimental results
were fairly well reproduced by a Monte Carlo simulation, where the
duplex stability is governed by the degree of chain scission (alpha)
and the critical degree of polymerization (DPc) needed for maintaining
a stable duplex. For xanthan partially degraded by acid hydrolysis
at high temperatures we suggest that the relatively small decrease
in (M) over bar(w) (factor 1.3-1.5) upon heat treatment well above
T-m is because a certain rearrangement toward perfectly matched duplexes
occurred during the hydrolysis. This is partly ascribed to the larger
DPc prevailing under the depolymerization conditions and partly because
chain rearrangements are kinetically facilitated at high temperatures.
