Further computational studies on the conformation of 1,5-dihydrolumiflavin

The optimized geometry of 1,5-dihydrolumiflavin has been calculated using density functional theory (DFT). Reduced lumiflavin was found to be bent along the N5-N10 axis, 25 degrees from planarity, which is nearly the same as previously reported restricted Hartree-Fock (RHF) calculations, which predict a bending angle of 27 degrees. The major difference in the DFT calculation is that the N10 methyl group adopts a more pseudoequatorial disposition and is only bent 13 degrees above the plane of the isoalloxazine ring system as opposed to 59 degrees in the RHF calculations. These computational results are compared with x-ray crystal structures of flavin models and flavoproteins. DFT calculation of 1,5-dihydroisoalloxazine resulted in a more modestly bent geometry of 17 degrees. This indicates that both electronic and steric interactions of the N10 methyl group of reduced lumiflavin contribute to the bent geometry.