Aqua pyridine complex of protoporphyrinato IX iron(III): Effect of ligand binding, surfactant concentration and proton coupling on redox potential

Abstract

Binding of one molecule of pyridine to hemin with WJter as the other ligand gives a very large positive shift in midpoint potential of the Fe(II)/Fe(III) couple. The mid-point potential of the aqua pyridine complex are -0.076 V, -0.046 V,-0.012 V and +0.008 V (versus Ag-AgCl electrode) in water, SDS, TX-100 and CTAB micelles, respectively. The anodic

shift in potential on binding one molecule of pyridine to hemin is 0.290 V in SDS and TX-100 and 0.250<i> </i>V in CTAB. The high value of mid-point potential is due to the formation of the highly stable <i>bis </i>pyridine complex of hemin in reduced state. The average anodic shift in mid-point potential of the complex in aqueous micelles with respect to water is +80 mV in

CTAB, +64 mV in TX-100 and +30mV in SDS. The aqua pyridine complex, also shows <i>p</i>H dependence of redox potential. There are two proton coupled electron transfer sites detected involving one carboxylate of the heme and the coordinated water molecule.