Radical chemistry of glucosamine naphthalene acetic acid and naphthalene acetic acid: a pulse radiolysis study

摘要

Free radical-induced oxidation reactions of glucosamine naphthalene acetic acid (GNaa) and naphthalene acetic acid (Naa) have been studied using pulse radiolysis. GNaa was synthesized by covalently attaching Naa on glucosamine. Hydroxyl adduct (fromthe reaction of hydroxyl radicals (●OH) at the naphthalene ring) was identified as themajor transient intermediate (suggesting that the ●OHreaction is on the naphthalene ring) and is characterized by its absorption maxima of 340 and 400 nm. Both GNaa and Naa undergo similar reaction pattern. The bimolecular rate constants determined for the reactions are 4.8 × 10~9 and 8.9×10~9 dm~3 mol~(-1) s~(-1) forGNaa and Naa respectively. Themechanismof reactionof ●OHwithGNaawas further confirmed using steady-state method. Radical cation of GNaa was detected as an intermediate during the reaction of sulfate radical (SO_4~(●-)) with GNaa (k_2=4.52×10~9 dm~3 mol~(-1) s~(-1)). This radical cation transforms to a ●OH adduct at higher pH. The radical cation of GNaa is comparatively long lived, and a cyclic transition state by neighboring group participation accounts for its stability. The oxy radical anion (O~(●-)) reacts with GNaa (k_2=1.12×10~9 dm~3 mol~(-1) s~(-1)) mainly by one-electron transfer mechanism. The reduction potential values of Naa and GNaa were determined using cyclic voltammetric technique, and these are 1.39V versus NHE for Naa and 1.60V versus NHE for GNaa.