Flavin iron contacts

In these systems, binary carrier contacts, whether of measurable lifetime or of shorter duration, implicate, therefore, two classes— interflavin and flavin—heme contacts and flavin—iron (sulfur) contacts. Figure 3 makes it clear that, even irrespective of whether one or two redox equivalents are being transferred, the geometry of these contacts (roughly spoken irora) must be asked for, and if it is o-, the site of contact must be discussed, whether inner sphere or outer sphere. 

The question remains open whether or not there is a direct contact between flavin and metal in a le -accepting binuclear iron cluster as, for example, in plant ferredoxin. Outer sphere le -transfer through flavin position 8 remains the alternative. On the other hand, there can no longer be a question about direct and reversible electron transfer between dihydronicotinamide and metal proteins. Any such claims (49) must be taken with greatest reservation and require careful independent confirmation. 

The electronic coupling has been calculated for each of the flavin electron transfer complexes described in this chapter and is described below, with the exceptions of CPR, TMADH and PDR. For these three proteins, the two redox cofactors are in direct van der Waals contact, either between the C-7 and C-8 methyl groups of two flavins (CPR), or between the flavin C-8 methyl and a cysteinyl sulfur ligand to the iron-sulfur center (TMADH and PDR). In these cases the coupling between the redox centers should be maximal and the electron transfer rates should depend only on the driving foree and reorganization energy for the electron transfer proeesses. 

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