Stereoselective electron transfer between chiral substrates and metal chelates anchored to polypeptides

Electron transfer from ortho-dihydroxy substrates, such as L(+)-ascorbic acid, L-adrenaline, and L-dopa, to iron(III) in [Fe(tetpy)(OH)2]+ ions anchored to sodium poly(L-glutamate) (FeTL) or poly(D-glutamate) (FeTD) was found to proceed stereoselectively when structurally ordered and partially shielded active sites prevent easy approach for redox partner. Oxidant-reductant interactions are then mediated by the polypeptide, whose conformational asymmetry ensures an efficient sterically discriminating environment. Evidence is produced that stereoselectivity chiefly arises from transition state effects, while thermodynamic discrimination is of minor importance. Theoretical models of the diastereomeric electron-transfer complexes were constructed by conformational energy calculations based on Coulombic, nonbonded, and hydrogen-bonded energy terms. The molecular parameters of the models enabled "differential" thermodynamic functions of the diastereomeric pairs and stereoselectivity to be evaluated and satisfactorily compared with those experimentally determined. The models give good insight into the observed topochemical phenomena and support the idea that stereoselectivity is coupled with a remote attack mechanism on the central metal ion where the peripheral tetpy ligand of the active sites acts as an electron-transfer agent.