Iron interconversions

The water complexes are high spin, whereas the cyanide and phen complexes are low spin. In the case of the cyanide and phen complexes, the interconversion of the 2g iron(ii) and /2g iron(iii) states simply involves the loss or gain of an electron from the 2g level. Since these are the orbitals oriented between the ligand donor atoms. 

In particular, the study of SRB ferredoxins enables us to survey the different properties of simple iron-sulfur proteins, including electron transfer, flexibility in coordination chemistry, and ability to undergo cluster interconversions. Most of the observations can be extrapolated to more complex situations. 

The mimicking of the iron-sulfur clusters by synthetic chemistry has been quite successful over the years. One of the last synthetic clusters to be obtained was the [3Fe-4S] cluster (109, 110). This new synthetic compound was useful for the demonstration of interconversion pathways, as well as for the formation of different heterometal clusters beyond those produced in proteins (111). The [3Fe-4S] core... 

Iron-sulfur centers can participate in regulation mechanisms either directly, when they control the activity of an enzyme, or at a more integrated level, when they modulate the expression of some genes. The regulation mechanisms that have been elucidated so far involve either a change in the redox state or the interconversion of iron—sulfur centers. 

The most significant results obtained for complexes of iron(II) are collected in Table 3. The data derive from laser Raman temperature-jump measurements, ultrasonic relaxation, and the application of the photoperturbation technique. Where the results of two or three methods are available, a gratifying agreement is found. The rate constants span the narrow range between 4 x 10 and 2 X 10 s which shows that the spin-state interconversion process for iron(II) complexes is less rapid than for complexes of iron(III) and cobalt(II). 

Table 3. Rate constants and activation parameters for the LS HS and HS LS unimolecular spin-state interconversions in iron(II) complexes...

During the lifetime of a root, considerable depletion of the available mineral nutrients (MN) in the rhizosphere is to be expected. This, in turn, will affect the equilibrium between available and unavailable forms of MN. For example, dissolution of insoluble calcium or iron phosphates may occur, clay-fixed ammonium or potassium may be released, and nonlabile forms of P associated with clay and sesquioxide surfaces may enter soil solution (10). Any or all of these conversions to available forms will act to buffer the soil solution concentrations and reduce the intensity of the depletion curves around the root. However, because they occur relatively slowly (e.g., over hours, days, or weeks), they cannot be accounted for in the buffer capacity term and have to be included as separate source (dCldl) terms in Eq. (8). Such source terms are likely to be highly soil specific and difficult to measure (11). Many rhizosphere modelers have chosen to ignore them altogether, either by dealing with soils in which they are of limited importance or by growing plants for relatively short periods of time, where their contribution is small. Where such terms have been included, it is common to find first-order kinetic equations being used to describe the rate of interconversion (12). 

Even though the iron atoms are separated in haemoglobin by about 25 A, communication between them is still able to occur and this has been postulated to involve a trigger mechanism (Perutz, 1971). The trigger is the movement of the proximal histidine as dioxygen binds to (or is released from) the Fe(n) and results in interconversion between the T and R structures. This movement causes a conformational change which is transmitted through the protein to the other iron sites. X-ray studies indicate that relative shifts of up to 6 A at subunit interfaces occur between the T and R states (Perutz, 1978). 

Scheme5.5 Interconversion of DNICs with an iron-sulfur nitrosyl.

This time it is conrotatory movement that results in a bonding situation, and formation of the irons dimethylcyclobutene (18). For the photochemical interconversion (which tends to lie over in favour of the cyclobutene), irradiation of the diene will result in the promotion of an electron into the orbital of next higher energy level, i.e. 2 3,... 

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