Coupling between binding-induced conformational phenomena and stereospecific effects in asymmetric reactions

Abstract

The oxidation by H₂O₂ of L-(+)-ascorbate anion in the presence of 2,2′,2″,2‴-tetrapyridineiron(III) complex ions anchored to poly(L-glutamate)(FeL) or poly(D-glutamate)(FeD) has been studied at pH 7.0 and varying complex-to-polymer-residue ratio [C]/[P]. The reaction follows two parallel routes; one corresponds to an electron-transfer process within a substrate–catalyst adduct and the other refers to an uncatalysed pathway to the dehydroascorbic acid. Unusual phenomena are observed in the catalysis in the sense that only the conformational dissymmetry of the active sites, arising from the binding-induced coil-to-α-helix transition of polypeptide matrices by Fe^III complex counter-ions, is able to impart stereospecific effects in the reaction.

Evidence is produced to show that stereoselectivity is driven by activation entropy. The effect probably arises from the formation of a rather rigid precursor complex, with the optically active substrate molecules bound to the chiral residues of the ordered polymer surrounding the active sites. The stereochemistry of such an intermediate allows the reaction to proceed only by a remote electron-transfer pathway, through the quaterpyridine ligand of the metal chelate. Evidence suggests that the asymmetric [Fe(tetpy)(OH)₂]⁺–polyelectrolyte systems also behave as environmental controllers of the uncatalysed oxidation of the L-(+)-ascorbate anion. This effect is briefly discussed in terms of the role played by macroions in ionic reactions in solution.