Observations on the mechanisms of the thermal and photoinduced oxidation of D-mannitol and fucoidan by transition metal complexes and inorganic radicals

The mechanism of the D-mannitol and fucoidan oxidation by •OH, peroxyl radicals, PRX• = , HOCH2 , and CH2(), inorganic radicals, , , and [NiIII(Me6-[14]aneN4)H2O]3+ was investigated by pulse radiolysis. Relative to the diffusion-controlled reaction of the •OH radical, the reactions of PRX•, , and are slow, with a 105–106 (mol L−1)−1 s−1 order of magnitude. The formation of saccharide–Ni(III) complexes also accounts for oxidation of the D-mannitol and fucoidan by [NiIII(Me6-[14]aneN4)H2O]3+. A rate constant k = 2.2 × 107 (mol L−1)−1 s−1 was estimated for the complexation of [NiIII(Me6-[14]aneN4)H2O]3+ by D-mannitol. In addition, both carbohydrates accelerate the decay of the Ni(III) complex which occurs with half life t 1/2 ∼ 102 ms. Consistent with the formation of fucoidan–Cu2+ complexes, the flash irradiations of these complexes at 351 nm produces transient spectra assigned to (n = 2, 3) species. The non-linear dependence of the product concentration on the flash intensity shows that formation of the species is a biphotonic process.