Studies on the redox characteristics of ferrioxamine E

Thermodynamic parameters for the reduction of ferrioxamine E as calculated from redox potentials determined at four different temperatures were found to be DeltaH( not equal)=7.1+/-3.4 kJ mol(-1) and DeltaS( not equal)=-146 J mol(-1) K(-1). The negative entropy value is large, because the decrease in the charge at the metal center and an increase in its ionic radius force the structure of the complex to become less rigid and resemble the desferrisiderophore. The hydrophilic groups of the system are now (relatively more) available for solvent interaction. Thus, a large negative entropy change accompanies the reduction of the complex. Kinetics of reduction of ferrioxamine by V(II), Cr(II), Eu(II), and dithionite were measured at different temperatures and by dithionite at different pH values. The Cr(II) and Eu(II) reactions proceed by an inner-sphere mechanism and have second-order rate constants at 25 degrees of 1.37x10(4) and 1.23x10(5) M(-1) s(-1), respectively. For the V(II) reduction, the corresponding rate constant was 1.89x10(3) M(-1) s(-1). The activation parameters for the V(II) reduction were DeltaH( not equal) = 8.3 kJ mol(-1); DeltaS( not equal) =-154 J mol(-1) K(-1). These values are indicative of an outer-sphere mechanism for V(II) reduction. The reduction by dithionite is half order in dithionite concentration indicating that SO(2)(-*) is the sole reducing species. log of reduction rate constants of different trihydroxamates by this reductant were correlated with their respective redox potentials, and the variation was found to be in approximate correspondence with the expectations of Marcus relationship.