Dihydroflavins planarity

Reduction of flavin by two electrons yields the 1,5-dihydroflavin (Scheme 3), often called reduced flavin . Since isomeric two-electron reduced flavin structures are known (cf. below), the term reduced flavin should be avoided unless defined to prevent misunderstanding. From all flavin species possible in a redox reaction the solution of 1,5-dihydroflavin is, in contrast to that of some isomeric compounds, devoid of a stront colour but not colourless as indicated by the term leucoflavin , which is still used (Table 4). The only true colourless species is (77). Because of the very high oxygen-sensitivity of 1,5-dihydroflavin its chemical and physical properties were investigated only recently Long before crystallographic data on flavins were available, conclusions were drawn from the molar extinction coefficient at 450 nm of 1,5-dihydroflavins with respect to the planarity of the molecules. From the data presented in Table 5 it was proposed that anionic... 

Next to isomerism, the planarity of a dihydroflavin is the most important structural problem (95). For 1,8- and 4a,5-isomers this question is relatively easy to answer in that these chromophores are planar, apart from the one tetrahedral carbon center where the hydride equivalent is localized. In the 1,5-isomer, or flavohydroquinone, however, the two electron equivalents of reduction are delocalised to an extent which depends on the pyramidality of the N-centers 5 and 10. And... 

Recent x-ray studies on reduced flavodoxins (109), on the other hand, reveal a dihydroflavin moiety which appears largely planar at 2.5 A resolution. But there is one main fact one should not forget in the context of flavodoxins These proteins do serve as ferredoxin substitutes and are unique in being involved exclusively with one-electron transfer, i.e. they are l/l-flavoproteins (see above). Furthermore, delocalisation of the N(5)-nonbonding electrons in flavohydroquinone can be inhibited in a largely planar flavin nucleus, if only the N(5)-H and N(10)-R bonds are kept out of the plane. Of course, apoproteins... 

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