Mixed ligand complexes of iron with cyanide and phenanthroline as new probes of metalloprotein electron transfer reactivity. Analysis of reactions involving rusticyanin from Thiobacillus ferrooxidans

A family of 12 different mixed ligand complexes of iron with cyanide and substituted 1,10-phenanthroline was prepared. The electron transfer properties of each reagent were systematically manipulated by varying the substituent(s) on the aromatic ring system and the stoichiometry of the two types of ligands in the complex. Values for the standard reduction potentials of each member of this family of electron transfer reagents were determined and spanned from 500 to 900 mV. The one-electron transfer reactions between each of these substitution-inert reagents and the high potential blue copper protein, rusticyanin, from Thiobacillus ferrooxidans were studied by stopped flow spectrophotometry under acidic conditions. For comparison with the protein results, the kinetics of electron transfer between each of these reagents and sulfatoiron were also investigated. The Marcus theory of electron transfer was successfully applied to this set of kinetic data to demonstrate that 10 of the 12 reagents had equal kinetic access to the redox center of the rusticyanin and utilized the same reaction pathway for electron transfer. The utility of these synthetic electron transfer reagents in characterizing the electron transfer properties of very high potential, redox-active metalloproteins is illustrated.