Stopped-Flow Kinetic Study of Reduction of Ferric Maltol Complex by Ascorbate

Abstract

Stopped-flow kinetic investigation of reduction of Fe^(III)-maltol complex is reported. The rates are dependent on pH in a complex way. On one hand at low pH there is a predominance of Fe^(III)(maltol)₂ which is easier to reduce compared to Fe^(III) (maltol)₃ which is more resistant to reduction. On the other hand ascorbate is a stronger reducing agent at higher pH. The rates are also found to be inversely dependent on the concentration of free ligand. These observations are explained by the following rate law:

Rate = ((k₀ +k₁[H⁺])k₂ [Asc^-]/ (k_-1[HMal] + k₂[Asc^-])) + k₃ [Asc^-] ) [Fe^III(Mal)₃]

 Here k₁ is the rate constant for acid hydrolysis of the Fe(maltol)₃ complex to Fe(maltol)₂ complex and is directly controlled by H⁺, k₀ is the rate constant for hydrolysis of the Fe(maltol)₃ complex to Fe(maltol)₂ complex and is an intrinsic process, k_-1 is the rate constant of reformation of the tris complex by reaction of the bis complex and the free ligand, k₂ is the rate constant for reduction of the bis complex by ascorbate and k₃ is the rate constant for the reduction of the tris complex by ascorbate.