Kinetic Studies of Redox reaction between L-tryptophan and oxo-bridged diiron(III,III) complex ion [Fe2(μ-O)(phen)4(H2O)2]4+

The title di-iron complex [FeIII2(μ-O)(phen)4(H2O)2]4+(1) (phen= 1,10-phenanthroline) acts a dibasic acid in aqueous media. However, at pH below 1.6, the amounts of its conjugate bases [FeIII2(μ-O)(phen)4(H2O)(OH)]3+(1a) and [FeIII2(μ-O)(phen)4(OH)2]2+(1b) are negligible (pKa1= 3.71 ± 0.03, pKa2= 5.28 ± 0.07). In aqueous acidic solutions (pH £1.6), complex 1 is reduced by L-tryptophan to produce [Fe(phen)3]2+ in the presence of externally added phen, following simple first order kinetics. However, at room temperature, the reaction is very slow. With 10 mM tryptophan the reaction takes more than 6 hours to complete. The method of initial rate is used to study the kinetics. The observed rate constants show first-order dependence on the concentrations of both the redox partners. In the pH range of this study, (0.36-1.60) the observed inverse dependence of rate on [H+] is attributed to the fact that the singly deprotonated form of the amino acid is the active reductant.