Influence of Polypeptides on the Reactivity of Thiols toward Iron(III) Complex Ions

Oxidation of L-cysteine by quaterpyridineiron(III) complex ions (FeT) anchored to ordered poly(L-glutamate) or poly(D-glutamate) was studied in the pH range 7–8. Surprisingly, electron transfer from substrate to the central metal ion does not take place, despite the favorable thermodynamic driving force. Instead, a stable, polymer-supported FeIIIT-cysteinate complex forms, to which no stereoselectivity is associated. This finding contrasts the known lability of FeIII-thiolate complexes, ultimately yielding disulfides and iron(II) species, and is relevant in view of the current interest in model compounds of cytochrome P-450, where cysteine is axially bound to ferric porphyrin. The formation of this complex was studied by kinetics and low-temperature electron paramagnetic resonance spectroscopy. The stereochemical features of the diastereomeric adducts were investigated by theoretical conformational analysis and compared with those already computed for the encounter complexes with some L-dihydroxy substrates. The results collectively emphasize the environmental control of the polypeptide matrix on the stability of cysteinate sulfur axial ligation.