Iron complexes, ferrous-ferric potentials

In addition to being an indirect measurement of the formation constant (Ki) of the iron complex, the reduction potential of the ferric siderophore complex is an important factor in developing the iron-release mechanism for siderophore-mediated iron transport. Under standard conditions, the reduction potentials for most known siderophores (ferric enterobactin —750 mV NHE V" ferriferrioxamine B 450mV NHE ) seem to preclude the use of biological reduc-tants (NAD(P)H/NAD(P)+ —324mV NHE ) to reduce the ferric ion to the ferrous ion and therefore prompt release of the ion from the siderophore. However, this potential is highly sensitive to the ratio of [Fe +]/[Fe +], as predicted by the Nernst equation. 

Ferric-ion complexes are important in acid-sulfate leaching because ferric ion can be generated from fenous ion using air or oxygen in situ. The reduction of ferric iron to ferrous occurs as the ferric-ion complex diffuses through fluid-filled pores and channels in the rock matrix and encounters reactive metals or sulfides. In most instances, as already discussed, the rate of ferric ion reduction is a diffusion-limited process. The oxidation of ferrous iron to ferric in aqueous solution becomes of primary importance because of its in situ regeneration capacity under appropriate oxidation potentials. 

The sequence of reactions in which the cytochromes participate is a mechanism for transferring electrons to molecular oxygen via iron complexes that are alternately in ferric and ferrous states. The order of the transfer has been deduced from studies with inhibitors, in which the electron-transport chain is broken so that components below the break are reduced, those above are oxidized from studies with poised potentials, in which the relative degrees of oxidation and reduction define the oxidation-reduction potentials of the various components and from rapid kinetic measurements, in which the order of reduction or oxidation can be seen. These methods agree on the following sequence ... 

Baron " and by Clark " in their studies on the potentials of the hemo-chromogens. The hemochromogens are thermodynamically reversible univalent redox systems, that is, they require a negligible activation energy to accept an electron in the oxidized state and to release an electron in the reduced state. The redox level of the iron in the heme may be varied by varying the N compounds that complex with the heme, and one may obtain E, values at pH 7 (Table III) as negative as —0.183 in the case of the ferrous-ferric protoporphyrin.2CN system and as high as... 

Iron or copper complexes will catalyse Fenton chemistry only if two conditions are met simultaneously, namely that the ferric complex can be reduced and that the ferrous complex has an oxidation potential such that it can transfer an electron to H2O2. However, we must also add that this reasoning supposes that we are under standard conditions and at equilibrium, which is rarely the case for biological systems. A simple example will illustrate the problem whereas under standard conditions reaction (2) has a redox potential of —330 mV (at an O2 concentration of 1 atmosphere), in vivo with [O2] = 3.5 x 10 5 M and [O2 ] = 10 11 M the redox potential is +230 mV (Pierre and Fontecave, 1999). 

The implications of these mechanistic studies for our understanding of environmental iron sequestration by siderophores is as follows. The hydroxyl containing aqua ferric ions will tend to form ferri-siderophore complexes more rapidly than the hexaaqua ion and ferrous ion will be sequestered more rapidly than the ferric ion. However, once in a siderophore binding site the ferrous ion will be air oxidized to the ferric ion, due to the negative redox potentials (see Section III.D). This also means that Fe dissolution from rocks will be influenced by mineral composition (other donors in the first coordination shell) as well as surface reductases in contact with the rock, and of course surface area (4,13). 

In the process, the iron is reduced to the ferrous form. Ferric cytochrome c is reduced by nitric oxide through a nitrosyl intermediate to produce ferrous cytochrome c and nitrite (Orii and Shimada, 1978). The nitrosyl cytochrome c absorbs at 560 nm, which is slightly higher than the 550-nm peak observed for reduced cytochrome c. Nitric oxide may be an interference in the assay of superoxide from cultured cells by the cytochrome c method. When nitric oxide reacts with cytochrome c, there is an initial decrease in absorbance at 550 nm as the nitrosyl complex is formed followed by a rise in absorbance as the complex decomposes to nitrite and reduced cytochrome c. This is a potential artifact in studies measuring the release of superoxide from cultured endothelial cells or other cells that make nitric oxide. 

It has been generally assumed that iron is transported across biological membranes in the ferrous form and that ferric iron would have to be reduced before it can be used by the organism. Thus, based on nutritional studies it has long been recognized that Fe(II) is1 more effectively absorbed than Fe(III), and this has been attributed to differences in the thermodynamic and kinetic stability of the complexes and chelates formed by these cations (for review, see Ref. 2). The experimental proof of a transport in the ferrous form has, however, not been given until quite recently in studies of iron transport in isolated mitochondria (23) as well as in enterobacteria (33). In rat liver mitochondria we have found that Fe(III) donated from a metabolically inert water soluble complex of sucrose interacts with the respiratory chain at the level of cytochrome c (and possibly cytochrome a) (23, 32) (Figure 1 B), which has a oxidation-reduction potential of around +250 mV (34) and is localized to the outer phase of the mitochondrial inner membrane (35). 

One probable mechanism for the release of iron from siderophores to the agents which are directly involved in cell metabolism is enzymatic reduction to the ferrous state. Due to the very low affinity of hydrdxamate and catecholate siderophores for Fe(II), the reduction converts the tightly bound ferric ion to the ferrous complex, which is unstable with respect to protonation and dissociation at neutral pH or below. Therefore comparison of siderophore complex redox potentials with those of physiological reductants can be very useful for the clarification of the mechanism of iron metabolism. Table IV shows the redox potentials [obtained by cyclic voltammetry (see Fig. 18)) of the siderophores tested so far. The values of all of the hydroxamates are within the... 

Aqueous Chemistry. Aqueous solutions of iron(n), not containing other complexing agents, contain the pale blue-green hexaaquoiron(n) ion, [Fe(H20)6]2 +. The potential of the Fe3+-Fe2+ couple, 0.771 V, is such that molecular oxygen can convert ferrous into ferric ion in acid solution ... 

---