A New Voltammetric Strategy for Investigating Reactive Iron-Sulfur Clusters

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The aim of this study is to quantify complex reactivities of Fe-S clusters in proteins. These clusters perform numerous, diverse functions in nature. A novel methodology will be used, exploiting the unmediated voltammetry displayed by ferredoxins (small proteins containing one or two clusters) at graphite electrodes in the presence of aminocyclitols. Ferredoxins co-adsorb with aminocyclitols forming electroactive films. This makes possible highly economical, detailed studies involving film transfer between electrolytes containing reagents that react with Fe-S clusters. Cluster oxidation levels are controlled, and reactions are monitored via voltammetric signals assigned to specific states by EPR and MCD spectroscopy. Some longstanding problems of studying reactions of Fe-S clusters - very low reduction potentials, poor spectroscopic signatures, instability and lability - are overcome. Dr. Armstrong will characterize an unusual property of (3Fe-4S) clusters, the ability to undergo reversible, multiple electron and proton uptake. This property represents a dramatic departure from the current belief that the protein environment permits only very conservative changes in cluster oxidation level. He will examine the equilibria and kinetics of reversible metal ion uptake by (3 Fe-4S) to give (M3Fe-4S) in various proteins. The likely existence of clusters containing such ions as Zn will be assessed. He will compare the redox and ligand binding properties of various (M3Fe- 4S) clusters. He will study the course of cluster assembly in proteins, from trace levels of Fe and S, and examine the fidelity of this process in the presence of competing ions such as Zn. %%% This study, exploring the characteristics of this important class of compounds, will help to enlarge our understanding of oxidation -reduction and electron transfer in life processes.