Spin-coupled model

Kent, T.A., Huynh, B.H., and Miinck, E. 1980. Iron-sulfur proteins Spin-coupling model for three-iron clusters. Proceedings of the National Academy of Sciences of the USA 77 6574-6576. 

The Mossbauer spectra for these proteins are consistent with the spin-coupled model proposed by Gibson et al. (148) for the active site of these proteins. In the next section we shall discuss this model in detail. 

The g—1.94 EPR signal of the reduced proteins must be explained by any model for their active site. Using subscript 1 to specify the ferric-iron site and subscript 2 the ferrous-iron site, the spin-coupled model explains this EPR signal in the following way. The electron magnetic... 

Other Mossbauer data which indicate that the model is correct are the measured -values for the low-temperature, reduced-protein spectra. The measured a-values for the ferric iron (Table 6) are close to isotropic with an average value of —17 gauss. Remembering that this a-value is calculated for an electron spin= 1/2 situation, we now recalculate the a-value for the ferric site in terms of the 5/2 spin present at this site. For the ferric site in the spin-coupled model,... 

In the preceding section we presented the experimental evidence in support of the spin-coupled model proposed by Gibson et al. (148) and Thornley et at. (150) for the plant-type ferredoxins. However, the spin-coupled model does not provide a spatial or configurational model for the active center. Therefore we proceed to a more detailed analysis with the goal of asserting a proper chemical and structural model of the active center. The following properties of the active site of these proteins are well-substantiated experimentally. 

The protein sequence data in Table 2 show that the cysteine residues in all the proteins occur in identical positions (18, 39, 44, 47, 77) in the sequence. Thus, the ligand field produced by the cysteinyl-sulfur atoms is not likely to be different among these proteins unless there is a difference in protein conformation which causes a displacement in one or more of the cysteinyl sulfur atoms. Note that a displacement of any cysteinyl sulfur atom in the model in Fig. 15 results in rhombic distortion at the iron to which it is ligated. Since, according to the spin-coupled model, this rhombic distortion will manifest itself in the difference between gx and gx for a particular protein, the EPR data in Table 1 provide a measure of the rhombic distortion around the ferrous iron in the reduced proteins. In particular, the g-values of adrenodoxin are axially symmetric while the g-values of spinach ferredoxin show a rhombic distortion. Thus, the observation of Kimura et al. (168) that adrenodoxin and spinach ferredoxin have different protein conformations is consistent with the prediction of the above model. 

The "spin-coupled model also predicts a constant value of gz, numerically 2.04. Inspection of Table 1 shows some deviation from gz = 2.04 for the plant-type ferredoxins. With respect to the model shown... 

Assuming that the structure shown in Fig. 15 is valid, one can draw some conclusions as to the characters of the iron-orbitals in the "spin-coupled model. Since the symmetry around the iron is tetrahedral, the d and dx2-y2 orbital are more ionic than the tg orbitals which must be covalent as the ligands are sulfur atoms. There are several important consequences of this conclusion 1. The energy level scheme is based on crystal-field approximations and therefore can be considerably in error. 

We found that these more sophisticated spin-coupled calculations, which used larger basis sets with polarization functions on all of the atoms and which allowed the a orbitals to relax, produced a picture of bonding in the 7t-electron system of benzene which is practically identical to that described earlier. As before, we found six equivalent spin-coupled orbitals which are transformed into one another by successive C6 rotations. The overlaps between the orbitals, ordered cpa to cp6 around the ring, are reported in Table 1. In this case, the electron correlation effects incorporated in the spin-coupled model provide an energy improvement over the SCF description of 170 kJ mol - with a further lowering of 20 kJ mol -1 on including spin-coupled ionic structures. 

Aromatic systems play a central role in organic chemistry, and a great deal of this has been fruitfully interpreted in terms of molecular orbital theory that is, in terms of electrons moving more-or-less independently of one another in delocalized orbitals. The spin-coupled model provides a clear and simple picture of the motion of correlated electrons in such systems. The spin-coupled and classical VB descriptions of benzene are very similar, except for the small but crucial distortions of the orbitals. The localized character of the orbitals allows the electrons to avoid one another. Nonetheless, the electrons are still able to influence one another directly because of the non-orthogonality of the orbitals. 

An important outcome of all these spin-coupled calculations is the consistency of the descriptions. In particular, a simple and highly-visual model emerges for the behaviour of correlated n electrons in all of the aromatic molecules that we have studied. These 7t-electron systems are well described in terms of fairly localized, nonorthogonal, singly-occupied orbitals. The special stability of such systems arises in the spin-coupled model from a profoundly quantum mechanical... 

For a wide range of aromatic, antiaromatic and nonaromatic systems [1—26], the spin-coupled model provides highly visual, but accurate, descriptions of the motion of correlated Jt electrons in terms of nonorthogonal orbitals and the dominance of particular patterns of spin coupling. A striking feature is the simplicity and consistency of the descriptions that emerge. 

Fig. 22. Spin-coupling model used for a linear tetramer of Mn Mn Mn Mn" (129).

Fig. 24. Spin-coupling model used for pair-of-Mn fOlaMn" dimers (134,141).

Fig. 33. Spin-coupling model used for planar-parallelogram Cu 4 cluster (239).

Although the spin state of atom (or a group of atoms) A within a molecule is not observable, local spins are employed for the description of spin-spin interactions between magnetic centers, similar to the metal centers of transition-metal clusters, in terms of a Heisenberg spin-coupling model and led to considerable interest in the development of partitioning schemes of the total spin expectation value during the past decade [20, 112-128]. 

Farad square pyramidal coordination probably antiferromagnetic magnetism fitted using the spin-spin coupling model of Bleaney-Bowers based upon... 

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