Electronic properties of flavins: Implications on the reactivity and absorption properties of flavoproteins

Electronic and structural properties of three protonated forms [HFl_ox, H₂2Fl_ox ⁺(N₅), and H₂Fl_ox ⁺(N₁)] of lumiflavin have been determined at the HF/3-21G level of approximation. Larger delocalization and basicity explain the favorable protonation of N₁ with respect to N₅ whereas N₁ protonation considerably activates the electrophilicity of N₅ with respect to C_4a. The relative position of the first electronic transition evaluated at the CNDO/CI level for the radicalar H₂Fl⁺(N₅) and H₂Fl⁺(N₁) structures further supports the classification of the flavoenzymes into two classes. Moreover, the π-π* transition has been correctly placed as the lowest energy transition for the neutral oxidized lumiflavin and predicts a blue shift of the low-lying electronic transition upon monoprotonation. Finally, from the analysis of the molecular complex between oxidized lumiflavin (HFl_ox) and hydroquinone, we have rationalized the complex formation in terms of the complementarity between the molecular electrostatic potentials as well as in terms of the overlap between the frontier orbitals involved in these charee transfer process.