OH-radical-induced chain scission of chitosan in the absence and presence of dioxygen

Hydroxyl radicals (and 15% H-atoms) were generated radiolytically in N(2)O-containing aqueous solutions of protonated chitosan (M(w) = 4.0 x 10(5) Da, degree of deacetylation 90.5%, pH similar to 3). The rate constant of H-abstraction from chitosan by OH radicals is k = 6.4 x 10(8) dm(3) mol(-1) s(-1), as measured by pulse radiolysis using thymine as competitor. With SCN(-) as competitor the apparent rate constant is found to be too high, because of a condensation of SCN(-) around the positively charged macromolecule. The radiation-chemical yields of chain scission are G = 3.4 x 10(-7) mol J(-1) in N(2)O-saturated and G = 2.1 x 10(-7) mol J(-1) in N(2)O-O(2)-saturated solutions. The kinetics have been followed by pulse radiolysis with conductometric detection. For each chain break, similar to 3.1 counterions (ClO(4)(-)) are released, on average, from the condensation zone into the bulk solution. In N(2)O-saturated solution the kinetics of chain scission are independent of dose rate, although more than one first-order process contributes (overall: k approximate to 95 s(-1) at pH 3.3). In the presence of dioxygen, the kinetics of chain scission depend on the dose rate, i.e. second-order processes play also a significant role.