Metastable, partially depolymerized xanthans and rearrangements toward perfectly matched duplex structures

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.