ELECTRONIC-PROPERTIES OF FLAVINS - IMPLICATIONS ON THE REACTIVITY ANDABSORPTION PROPERTIES OF FLAVOPROTEINS

Electronic and structural properties of three protonated forms [HFl(ox ), H(2)Fl(ox)(+)(N-5), and H(2)Fl(ox)(+)(N-1)] of lumiflavin have been determined at the HF/3-21G level of approximation. Larger delocalizat ion and basicity explain the favorable protonation of N-1 with respect to N-5 whereas N-1 protonation considerably activates the electrophil icity of N-5 with respect to C-4a. The relative position of the first electronic transition evaluated at the CNDO/CI level for the radicalar H(2)Fl(.)(N-5) and H(2)Fl(.)(N-1) structures further supports the cla ssification of the flavoenzymes into two classes. Moreover, the pi-pi transition has been correctly placed as the lowest energy transition for the neutral oxidized lumiflavin and predicts a blue shift of the l ow-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 ter ms of the complementarity between the molecular electrostatic potentia ls as well as in terms of the overlap between the frontier orbitals in volved in these charge transfer process. (C) 1997 John Wiley & Sons, I nc.