X-ray structures of two oxidation states of a flavin-nicotinamide biscoenzyme and models for flavin--nicotinamide interactions.

The flavin nicotinamide biscoenzymes Flox(-)--C3--Nic+ and H2Flred--C3--Nic+ assume extended conformations in the solid state. In both derivatives the nicotinamide and flavin groups associate through hydrogen bonding. The bending angle of the reduced flavin moiety is less than half that in any previously reported 1,5-dihydroflavin structure. This effect is apparently due to ring stacking interactions.