Interflavin contact

Independent support for interflavin o-contacts comes from recent chemical studies by Favaudon and Lhoste (13,14). The french authors describe, as already anticipated, a nearby diffusion-controlled dimer formation in aprotic polar medium as the first step in the interflavin contact between oxidized and reduced states, which would finally yield two flavin radicals. This dimer was shown to be not identical in any respect with the well known quinhydrone which can only be obtained in aqueous systems at high flavin concentrations. The long wave band in the absorption spectrum of the new dimer appears to be of charge transfer type, but with a highly reduced half width and better resolved shape than the flavoquinhydrone spectrum. 

Figure 5. Interflavin contact and v-electron transfer (based upon the data of Ref. 13. In the syn-conformation the flavin halves are at an unstrained angle of 50°, sufficient for efficient charge transfer.

Flavin-dependent le -transfer in enzymes and chemical model systems can he differentiated from 2e -transfer activities, i.e., (de)hydrogenation and oxygen activation, by chemical structure and dynamics. For le -transfer, two types of contacts are discussed, namely outer sphere for interflavin and flavin-heme and inner sphere for flavinr-fenedoxin contacts. Flavin is the indispensable mediator between 2e - and le -transfer in all biological redox chains, and there is a minimal requirement of three cooperating redox-active sites for this activity. The switch between 2e - and le -transfer is caused by apoprotein-dependent prototropy between flavin positions N(l)/0(2a) and N(5) or by N(5)-metal contact. 

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. 

In summary, it appears that interflavin o-contact is a quite common phenomenon which may occur both in artificial (mostly irreversible) as... 

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