Stereospecific oxidation of ascorbic acid by chiral association complexes between polypeptides and iron(III) chelate ions

Abstract 2,2,′,2″,2‴‐Tetrapyridineiron(III) complex ions anchored to poly(L‐glutamate) (FeL) or poly(D‐glutamate) (FeD) were used as catalysts for the H2O2 oxidation of L(+)‐ascorbic acid at pH 7 and varying complex:polymer‐residue molar ratios [C]/[P]. Evidence is produced that the reaction is a composite process reflecting contributions from parallel pathways, one of which corresponds to a catalytic route and is [H2O2]‐independent and the other to an uncatalyzed electron‐transfer process between the ascorbate anion and hydrogen peroxide. Stereospecific effects in the catalysis are observed on increasing the complex:polymer ratio, which corresponds to an increase of the amount of α‐helical fraction in the polypeptide supports (xa). Thus, at [C]/[P] = 0.01 (xa < 0.05), kFeD/kFeL = 1.0; but at [C]/[P] = 0.20 (xa ≈ 0.70), kFeD/kFeL = 4.0 ± 0.5, kFeD and kFeL being the second‐order rate constants of the electron‐transfer reaction between the FeD or FeL isomer of the asymmetric catalyst and the L‐ascorbate anion. The activation energies were found to increase markedly on going from the former to the latter complex:polymer ratio but, at the same time, to exhibit equal values with both enantiomeric catalysts. Stereoselectivity therefore appears to be an entropy‐controlled process, arising from the conformational rigidity of the precursor complex, which very likely sees the substrate molecules bound to the chiral residues of the ordered polymer surrounding the active sites. The implications of the stereochemical features of the substrate–catalyst adduct on the mechanism of electron transfer are also discussed. Evidence is presented that the asymmetric [Fe(tetpy)(OH)2]+–polyelectrolyte systems play the additional role of environmental controller of the uncatalyzed oxidation of the L‐ascorbate anion.