[Fe(tetpy)(OH) 2] + complex ions (tetpy = 2,2′,2′',2‴-tetrapyridyl) anchored to poly(L-glutamate) (FeL) or poly(D-glutamate) (FeD) through the γ-carboxylate groups of the polymers [1] exhibit stereoselective peroxidatic activity with a number of optically active substrates, according to the reaction (pH = 7.0, Tris buffer 0.05 M) [2, 3]: ▪ Progressive binding of complex ions was found to determine a coil-to-α-helix transition in the charged polypeptide matrices, as well as aggregation phenomena with a freezing of iron molecules inbetween helical chains. Under these conditions, electron transfer from catecholamines (L-adrenalin and L-dopa) to the central iron(III) ion, which is rate-determining, proceeds stereoselectively because extensive and possibly specific interactions between substrate molecules and the peptidic residues in the close environment of the active sites different steric constraints for the two diastereomeric precursor complexes. The rate constants of electron transfer processes can be expressed by nuclear and electronic factors which are highly sensitive to the separation of the redox centers [4]. Therefore, even small differences in steric hindrances between the diastereo-isomers would affect differently the optimal mutual orientation of the reacting OH group of the substrates and the peripheral tetrapyridyl ligand of the active sites, whose π-system very likely acts as an electron-transfer agent [5]. Preliminary results on conformational analysis of substrate-catalyst adducts support such conclusions. This is reflected in the rate constants of the electron-transfer step, as illustrated by the Lineweaver-Burk plot of the reaction catalyzed by FeL or FeD systems at a complex to polymer-residue ratio [C]/[P] = 0.20, reported in Fig. 1. From the results, the turnover numbers for the oxidation of L-adrenaline are 0.56±0.07 and 2.04±0.23 min −1, respectively, whereas those for the oxidation of L-dopa are 0.42±0.05 and 1.14±0.12 ▪ min −1 (25.9 °C). On the other hand, the Michaelis constants are K M = (1.30±0.18) × 10 −3 and (1.05±0.15) × 10 −3 M in the former case, and (8.0±1.12) × 10 −4 and (5.50±0.48) × 10 −4 M in the latter. According to Marcus theory [6], the ultimate rate of electron transfer depends on the rate of reorganization of the surrounding medium (precursor and successor complex formation). When the asymmetric polymers play an active role in the reaction, being involved in the formation of the precursor complex the polypeptide itself might experience local conformational changes. Since the time scale of these rearrangements is unknown, it is possible that the polymer reorganization retards the overall rate of the electron transfer process. This could explain the finding that stereoselectivity occurs at the expense of the efficiency of reaction, as shown in Fig. 2. Using catalysts at low [C]/[P] ratio, the active sites are exposed to the bulk solvent and a substrate-coordinated metal chelate forms without any ▪ assistance from the coiled polypeptides [3]. This accounts for the lack of stereoselectivity in the oxidation reaction, which takes place at much higher rate however (Fig. 2).