Zusammenfassung
The depolymerization of double-stranded xanthan and triple-stranded
scleroglucan is simulated using a Monte Carlo method. Chain scissions
are introduced randomly in each strand to simulate chemical degradation.
The cooperative single- to double- or single- to triple-stranded
conformational change requires that the chain length exceeds a lower
critical value which is incorporated in the model. The simulations
show that degradation of such polymers deviates significantly from
that of single-stranded polymers. There is initially an apparently
enhanced stability where scission of the glycosidic linkages is effectively
masked because of interstrand stabilization. This period is followed
by a power law region, M(w) approximately t(-nu), where the exponent
nu for a monodisperse starting material is observed to be 1.0, 1.66
+/- 0.06, and 2.3 +/- 0.1 for the single-, double-, and triple-stranded
cases, respectively. Exponents larger than 1 originate from the unmasking
of preexisting broken glycosidic linkages when additional scissions
are introduced. Analysis of the data reported for the acid-catalyzed
degradation of xanthan in the ordered conformation yielded nu = 1.5
+/- 0.1, in good agreement with the prediction for a double-stranded
model.
Nutzer