DEPOLYMERIZATION OF DOUBLE-STRANDED XANTHAN BY ACID-HYDROLYSIS - CHARACTERIZATION OF PARTIALLY DEGRADED DOUBLE STRANDS AND SINGLE-STRANDED OLIGOMERS RELEASED FROM THE ORDERED STRUCTURES

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.