Electronic transitions in the isoalloxazine ring and orientation of flavins in model membranes studied by polarized light spectroscopy

The orientation of flavin mononucleotide (FMN) in model membranes and the directions of the transition moments of the first three bands in the electronic absorption spectrum of the oxidized form of the isoalloxazine ring have been determined by means of linear dichroism and polarized fluorescence spectroscopy. Measured counterclockwise relative to the axis connecting the two nitrogens in the central ring (considered positive when going in the direction from -CN less than to greater than or equal to N), these angles are 58 +/- 4 degrees (450-nm band), 97 +/- 3 degrees (350-nm band), and 119 +/- 2 degrees (260-nm band).