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

Abstract

The mechanism of the D-mannitol and fucoidan oxidation by •OH, peroxyl radicals, PRX• = , HOCH₂ , and CH₂(), inorganic radicals, , , and [Ni^III(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 [Ni^III(Me₆-[14]aneN₄)H₂O]³⁺. A rate constant k = 2.2 × 10⁷ (mol L⁻¹)⁻¹ s⁻¹ was estimated for the complexation of [Ni^III(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.

Keywords:

• D -mannitol
• Fucoidan
• Copper complexes
• Radicals
• Photolysis

Acknowledgements

The Radiation Laboratory is supported by the Office of Basic Energy Sciences of the US Department of Energy. This is contribution no. NDRL-4853 from the Notre Dame Radiation Laboratory. B. Matsuhiro acknowledges support from DICYT, Universidad de Santiago de Chile. S. Torres acknowledges support from Conicyt and scholarship from Mecesup and VRID (Universidad de Santiago).