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

Abstract

Hydroxyl radicals (and 15% H-atoms) were generated radiolytically in N₂O-containing aqueous solutions of protonated chitosan (M_w = 4.0 × 10⁵ Da, degree of deacetylation 90.5%, pH ≈ 3). The rate constant of H-abstraction from chitosan by OH radicals is k = 6.4 × 10⁸ dm³ 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 × 10⁻⁷ mol J⁻¹ in N₂O-saturated and G = 2.1 × 10⁻⁷ mol J⁻¹ in N₂O–O₂-saturated solutions. The kinetics have been followed by pulse radiolysis with conductometric detection. For each chain break, ≈3.1 counterions (ClO₄⁻) are released, on average, from the condensation zone into the bulk solution. In N₂O-saturated solution the kinetics of chain scission are independent of dose rate, although more than one first-order process contributes (overall: k ≈ 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.