Binding-induced conformational transition of sodium poly-L-glutamate by iron(III) complex ions in aqueous solution

Abstract

The binding of pseudo-octahedral trans- and cis-Fe^III complex ions by sodium poly-L-glutamate (PLG) and dextransulphate (DS) in aqueous solution at about pH 7 has been studied. Equilibrium dialysis and “phase-separation” data show that the affinity for the complex counterions by both polyelectrolytes follows the order trans-[Fe(tetpy)(OH)₂]⁺ > cis-[Fe(pmen)(OH)₂]⁺. Evidence of a specific site binding, leading to a marked “renaturation” effect on the charged polypeptide, is produced in the case of the trans-Fe^III-quaterpyridine compound. The binding isotherm of the trans-complex + PLG system and the circular dichroism patterns of the polypeptide as a function of the bound-trans-complex to polymer-residue ratio were successfully treated by a two-state model for the polyelectrolyte and a preferential association of the complex ions to the helical conformation of the polymeric matrix. All these features are examined in the light of the structural characteristics of the interacting species. The implications of the different stereochemistry of the other complex ions studied on the binding process with PLG are also discussed.