Coupling between pH-induced conformational phenomena and stereospecific effects in electron-transfer 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 a complex-to-polymer-residue ratio of 0.10 and in the pH range 6–8. Evidence is produced that the reaction is a composite process reflecting contributions from parallel routes; one of these corresponds to a catalytic, [H₂O₂]-independent pathway and the other refers to an uncatalysed electron-transfer process between ascorbate anion and hydrogen peroxide. Stereospecific effects in the catalysis are observed with decreasing pH, accompanied by an increase in the amount of α-helix in the polypeptide supports (ƒ_h). Thus at pH 7.8 (ƒ_h≈ 0.13), k_FeD= 1382.3 ± 113.2 and k_FeL= 1034.4 ± 79.3 dm³ mol^–1 s^–1 and the activation energy is 3.8 ± 0.3 kcal mol^–1 with both enantiomeric catalysts, whereas at pH 6.3 (ƒ_h≈ 0.84), k_FeD= 70.9 ± 5.5 and k_FeL= 13.6 ± 1.1 dm³ mol^–1 s^–1 and the activation energy is 18.0 ± 1.3 kcal mol^–1 in both cases. The results indicate that stereoselectivity is an entropy-controlled phenomenon. The effect is probably caused by conformational rigidity of the precursor complex, which arises from interactions between the optically active substrate molecules and the chiral residues of the ordered polymer surrounding the active centres.

Effects of the stereochemical features of the substrate–catalyst adduct on the mechanism of electron transfer are discussed. The evidence suggests that the asymmetric[Fe(tetpy)(OH)₂]⁺–polyelectrolyte systems also behave as environmental controllers of the uncatalysed oxidation of ascorbate anion.