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 t o 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 c hains, molecular weight (light scattering), molecular weight distribut ion (HPLC, gel filtration), intrinsic viscosity, and conformation (opt ical rotation). Additional information about molecular size and confor mation was obtained by electron microscopy. For each sample a fraction consisting of depolymerized, double-stranded species could be obtaine d. Their conformational properties were largely analogous to undegrade d 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 elec tron micrographs and from the [eta]-M(W) relationship using the wormli ke chain model. As the degradation proceeded, a second fraction, consi sting of shorter and conformationally disordered fragments, constitute d a progressively larger part of the population. This is in accordance with the model requiring the minimum chain length (DP(min)) to take p art in an ordered, double-stranded structure in a cooperative manner. Their release partly explains the increased flexibility of the remaini ng double-stranded species, since their release exposes local single-s tranded and hence more flexible, regions. The experimental M(W) data a re 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 fra gments which depends on DP(min). Comparison to experimental data gives a minimum estimate of DP(min) in the range of 10-15 glucose residues.